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A Minor-Axis Surface Brightness Profile for M31
Abstract
We use data from the Isaac Newton Telescope Wide-Field Camera survey of M31 to determine the surface brightness profile of M31 along the southeast minor axis. We combine surface photometry and faint red giant branch star counts to trace the profile from the innermost regions out to a projected radius of 4° (≈55 kpc), where μV ~ 32 mag arcsec-2; this is the first time the M31 minor-axis profile has been mapped over such a large radial distance using a single data set. We confirm the finding by Pritchet & van den Bergh that the minor-axis profile can be described by a single de Vaucouleurs law out to a projected radius of 14 or ≈20 kpc. Beyond this, the surface brightness profile flattens considerably and is consistent with either a power law of index about -2.3 or an exponential of scale length 14 kpc. The fraction of the total M31 luminosity contained in this component is ≈2.5%. While it is tempting to associate this outer component with a true Population II halo in M31, we find that the mean color of the stellar population remains roughly constant at V - i ≈ 1.6 from 05 to 35 along the minor axis. This result suggests that the same metal-rich stellar population dominates both structural components.
... Despite not explicitly fitting for the detailed properties of the shelves, the Fardal et al. minor merger models have predicted the existence of the SE shelf Gilbert et al. 2007) and show excellent agreement with the kinematics of RGB stars in the W shelf . These successes appear to be natural consequences of the constraints placed on the merger scenario using spatial and kinematical observations of the GSS (McConnachie et al. 2003;Ibata et al. 2004;Gilbert et al. 2009) and stellar surface density maps of M31 (Ibata et al. 2001a;Ferguson et al. 2002;Irwin et al. 2005), where the orbital trajectory of the progenitor is only loosely required to pass through the location of the NE shelf. Moreover, Fardal et al. (2013) demonstrated that the predicted kinematical signature for the NE shelf partially overlaps with a small sample of PNs that form an apparent stream near the shelf (Merrett et al. 2003(Merrett et al. , 2006. ...
... The naming convention for the NE shelf fields indicates the order in which the masks were designed, rather than their radial distance from M31. We included reference DEIMOS fields from the Elemental Abundances in M31 survey (Gilbert et al. 2019;Escala et al. 2020aEscala et al. , 2020b and fields previously observed as part of the SPLASH survey Gilbert et al. 2007Gilbert et al. , 2009Fardal et al. 2012) targeting the SE shelf, W shelf, and GSS. Figure 1 also indicates the edges of the NE and W shelves defined by Fardal et al. (2007) via applying the Sobel operator to the Irwin et al. (2005) star map. ...
... We also show PARSEC isochrones (Marigo et al. 2017) for reference, Gilbert et al. 2019;Escala et al. 2020aEscala et al. , 2020b and fields previously observed as part of the SPLASH survey (open boxes; Kalirai et al. 2006;Gilbert et al. 2007Gilbert et al. , 2009Fardal et al. 2012) spanning the SE shelf (blue), W shelf (pink), and GSS (green). The black points are the edges of the NE and W shelves defined by Fardal et al. (2007) via applying an edge filter to the Irwin et al. (2005) star count map. ...
We obtained Keck/DEIMOS spectra of 556 individual red giant branch stars in four spectroscopic fields spanning 13−31 projected kpc along the northeast (NE) shelf of M31. We present the first detection of a complete wedge pattern in the space of projected M31-centric radial distance versus line-of-sight velocity for this feature, which includes the returning stream component of the shelf. This wedge pattern agrees with expectations of a tidal shell formed in a radial merger and provides strong evidence in favor of predictions of Giant Stellar Stream (GSS) formation models in which the NE shelf originates from the second orbital wrap of the tidal debris. The observed concentric wedge patterns of the NE, west (W), and southeast (SE) shelves corroborate this interpretation independently of the models. We do not detect a kinematical signature in the NE shelf region corresponding to an intact progenitor core, favoring GSS formation models in which the progenitor is completely disrupted. The shelf’s photometric metallicity ([Fe/H] phot ) distribution implies that it is dominated by tidal material, as opposed to the phase-mixed stellar halo or the disk. The metallicity distribution ([Fe/H] phot = −0.42 ± 0.01) also matches the GSS, and consequently the W and SE shelves, further supporting a direct physical association between the tidal features.
... dominates the minor axis density profile of M31 from a projected distance of ~8 kpc out to 25 kpc (refs 19,20 ), suggesting that the inner stellar halo in this radial range is built up primarily by the debris of M32p (Fig. 3b). ...
... There is good agreement between the median minor axis profile of the accreted stellar component of M31 analogues and the observed minor axis surface brightness profile (μ i ) 19,20 of M31's stellar halo (Fig. 3b). This comparison suggests that the minor axis profile beyond a projected distance of 10 kpc (fainter than μ i ~25 mag arcsec -2 ) appears to be predominantly accreted stellar material. ...
... 1. b,We also indicate the median stellar mass profile of the final accreted stellar component along the minor axis (green) along with the debris contributed by the massive dominant accreted progenitors (black). We compare this with the M31's minor axis stellar halo profile from the i-band surface brightness profile (blue circles19 and orange squares 20 ) using M/L i = 1.8. The horizontal dotted line indicates the Illustris mass resolution limit. ...
Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies, uncertainty remains about M31's most important mergers. Previous studies focused individually on the giant stellar stream or the impact of M32 on M31's disk, thereby suggesting many substantial satellite interactions. Yet models of M31's disk heating and the similarity between the stellar populations of different tidal substructures in M31's outskirts both suggested a single large merger. M31's stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites and provides information about its important mergers. Here we use cosmological models of galaxy formation to show that M31's massive and metal-rich stellar halo, containing intermediate-age stars, dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5 x 10^10 solar masses, the third largest member of the Local Group) about 2 Gyr ago. This single event explains many observations that were previously considered separately: M31's compact and metal-rich satellite M32 is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo contains most of the merger debris, and the giant stellar stream is likely to have been thrown out during the merger. This interaction may explain M31's global burst of star formation about 2 Gyr ago in which approximately a fifth of its stars were formed. Moreover, M31's disk and bulge were already in place, suggesting that mergers of this magnitude need not dramatically affect galaxy structure.
... In the case of M31 we appear to see an extension of the disk, which has been shown to possess a complex structure with considerable warping, both in the optical and in H i (e.g., Brinks & Burton 1984;Walterbos & Kennicutt 1988;Braun 1991;Morris et al. 1994;Corbelli et al. 2010), probably caused and modified by interactions (e.g., McConnachie et al. 2009;Richardson et al. 2011;Qu et al. 2011). In recent years the surroundings of M31 have been mapped using deep groundbased photometric surveys of the Andromeda galaxy (Ferguson et al. 2002;Irwin et al. 2005;Ibata et al. 2007;McConnachie et al. 2009) with the wide-field cameras of the Isaac Newton Telescope (INT) and the Canada-France-Hawaii Telescope. Ibata et al. (2005Ibata et al. ( , 2007 presented a surface brightness profile for M31 and concluded that along the minor axis, in the region 0. • 2 < R < 0. • 4, the classical inner (thin) disk of M31 contributes to the profile, but at 0. • 5 < R < 1. • 3 the extended disk component becomes dominant. ...
... In addition, all PNe from the dwarf elliptical galaxy NGC 205 were excluded. An aspect ratio of 3:5 was used for this region (Ferguson et al. 2002;Irwin et al. 2005;Ibata et al. 2007). PN densities were calculated within elliptical apertures with a step of 0. • 2. The PN data cover only part of the studied region and this geometrical incompleteness increases with radius. ...
... Then the extended disk becomes dominant, and our profile shows an exponential decline out to the R = 20 kpc, where our data, in principle, still trace the extended disk. Fitting the data for the region 8 < R < 20 kpc, we measure an exponential scale length of 3.21 ± 0.14 kpc, which is quite similar to what was found for the same region by Irwin et al. (2005) and Ibata et al. (2007), who found a scale length of 3.22 ± 0.02 kpc along the minor axis profile using photometric data from the INT Wide Field Camera survey of M31. This structure has a very low central surface brightness at a level of μ 0 ∼ 23 mag arcsec −2 . ...
We have developed a method to identify planetary nebula (PN) candidates in imaging data of the Sloan Digital Sky Survey (SDSS). This method exploits the SDSS's five-band sampling of emission lines in PN spectra, which results in a color signature distinct from that of other sources. Selection criteria based on this signature can be applied to nearby galaxies in which PNe appear as point sources. We applied these criteria to the whole area of M31 as scanned by the SDSS, selecting 167 PN candidates that are located in the outer regions of M31. The spectra of 80 selected candidates were then observed with the 2.2 m telescope at Calar Alto Observatory. These observations and cross-checks with literature data show that our method has a selection rate efficiency of about 90%, but the efficiency is different for the different groups of PN candidates. In the outer regions of M31, PNe trace different well-known morphological features like the Northern Spur, the NGC 205 Loop, the G1 Clump, etc. In general, the distribution of PNe in the outer region 8 < R < 20 kpc along the minor axis shows the "extended disk"—a rotationally supported low surface brightness structure with an exponential scale length of 3.21 ± 0.14 kpc and a total mass of ~1010M
☉, which is equivalent to the mass of M33. We report the discovery of three PN candidates with projected locations in the center of Andromeda NE, a very low surface brightness giant stellar structure in the outer halo of M31. Two of the PNe were spectroscopically confirmed as genuine PNe. These two PNe are located at projected distances along the major axis of ~48 Kpc and ~41 Kpc from the center of M31 and are the most distant PNe in M31 found up to now. With the new PN data at hand we see the obvious kinematic connection between the continuation of the Giant Stream and the Northern Spur. We suggest that 20%-30% of the stars in the Northern Spur area may belong to the Giant Stream. In our data we also see a possible kinematic connection between the Giant Stream and PNe in Andromeda NE, suggesting that Andromeda NE could be the core or remnant of the Giant Stream. Using PN data we estimate the total mass of the Giant Stream progenitor to be ≈109M
☉. About 90% of its stars appear to have been lost during the interaction with M31.
... But M31 is also quite different than the MW, providing a foil for comparison. It is a more massive, metalrich, evolved spiral galaxy (e.g., Irwin et al. 2005;Brown et al. 2006;Kalirai et al. 2006;Watkins et al. 2010;Fardal et al. 2013;Gilbert et al. 2014;Mackey et al. 2019), with a different accretion history than the MW, as highlighted by its wellknown prominent substructures (e.g., McConnachie et al. 2003McConnachie et al. , 2018Zucker et al. 2004;Martin et al. 2006Martin et al. , 2009Martin et al. , 2013bMartin et al. , 2013cMartin et al. , 2013aIbata et al. 2007;Irwin et al. 2008;Richardson et al. 2011;Bernard et al. 2015;Escala et al. 2022). ...
... There have been a number of substantial efforts to provide high-quality data on the M31 satellites, in order to compare them to their MW counterparts. Beyond mapping of the entire M31 area of the sky (e.g., Ibata et al. 2001;Ferguson et al. 2002;Irwin et al. 2005;Majewski et al. 2007;McConnachie et al. 2009), significant investments with spectroscopic Keck instruments have produced resolved stellar metallicities, including α-abundances in some cases, and velocity dispersions for most M31 satellites (e.g., Geha et al. 2006Geha et al. , 2010Guhathakurta et al. 2006;Kalirai et al. 2010;Tollerud et al. 2012;Collins et al. 2013;Vargas et al. 2014;Gilbert et al. 2019; Kirby et al. 2020;Wojno et al. 2020;Escala et al. 2021). Recently, deep Hubble Space Telescope (HST) imaging has produced high-fidelity SFHs for a small set of M31 satellites (e.g., Geha et al. 2015;Skillman et al. 2017) and coarse SFHs for a much larger sample (e.g., Weisz et al. 2019a). ...
We measure homogeneous distances to M31 and 38 associated stellar systems (−16.8 ≤ M V ≤ −6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From >700 orbits of new/archival Advanced Camera for Surveys imaging, we identify >4700 RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 day and 0.04 mag. Based on period–Wesenheit–metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of ∼20 kpc (3%) and ∼10 kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that ∼80% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms 7–23 kpc) planar “arc” of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess the physical proximity of notable associations such as the NGC 147/185 pair and M33/AND xxii ; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with M V > − 9.5, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.
... But M31 is also quite different than the MW, providing a foil for comparison. It is a more massive, metal-rich, evolved spiral galaxy (e.g., Irwin et al. 2005;Brown et al. 2006;Kalirai et al. 2006;Watkins et al. 2010;Fardal et al. 2013;Gilbert et al. 2014;Mackey et al. 2019), with a different accretion history than the MW, as highlighted by its well-known prominent substructures (e.g, McConnachie et al. 2003;Zucker et al. 2004;Martin et al. 2006;Ibata et al. 2007;Irwin et al. 2008;Martin et al. 2009;Richardson et al. 2011;Martin et al. 2013b,c,a;Bernard et al. 2015;McConnachie et al. 2018;Escala et al. 2022). ...
... There have been a number of substantial efforts to provide high-quality data on the M31 satellites, in order to compare them to their MW counterparts. Beyond mapping of the entire M31 area of the sky (e.g., Ibata et al. 2001;Ferguson et al. 2002;Irwin et al. 2005;Majewski et al. 2007;Mc-Connachie et al. 2009), significant investments with spectroscopic Keck instruments have produced resolved star metallicites, including α-abundances in some cases, and velocity dispersions for most M31 satellites (e.g., Geha et al. 2006;Guhathakurta et al. 2006;Geha et al. 2010;Kalirai et al. 2010;Tollerud et al. 2012;Collins et al. 2013;Vargas et al. 2014;Gilbert et al. 2019;Kirby et al. 2020;Wojno et al. 2020;Escala et al. 2021). Recently, deep Hubble Space Telescope (HST) imaging has produced high-fidelity SFHs for a small set of M31 satellites (e.g., Geha et al. 2015;Skillman et al. 2017) and coarse SFHs for a much larger sample (e.g., Weisz et al. 2019a). ...
We measure homogeneous distances to M31 and 38 associated stellar systems ($-$16.8$\le M_V \le$ $-$6.0), using time-series observations of RR Lyrae stars taken as part of the Hubble Space Telescope Treasury Survey of M31 Satellites. From $>700$ orbits of new/archival ACS imaging, we identify $>4700$ RR Lyrae stars and determine their periods and mean magnitudes to a typical precision of 0.01 days and 0.04 mag. Based on Period-Wesenheit-Metallicity relationships consistent with the Gaia eDR3 distance scale, we uniformly measure heliocentric and M31-centric distances to a typical precision of $\sim20$ kpc (3%) and $\sim10$ kpc (8%), respectively. We revise the 3D structure of the M31 galactic ecosystem and: (i) confirm a highly anisotropic spatial distribution such that $\sim80$% of M31's satellites reside on the near side of M31; this feature is not easily explained by observational effects; (ii) affirm the thin (rms $7-23$ kpc) planar "arc" of satellites that comprises roughly half (15) of the galaxies within 300 kpc from M31; (iii) reassess physical proximity of notable associations such as the NGC 147/185 pair and M33/AND XXII; and (iv) illustrate challenges in tip-of-the-red-giant branch distances for galaxies with $M_V > -9.5$, which can be biased by up to 35%. We emphasize the importance of RR Lyrae for accurate distances to faint galaxies that should be discovered by upcoming facilities (e.g., Rubin Observatory). We provide updated luminosities and sizes for our sample. Our distances will serve as the basis for future investigation of the star formation and orbital histories of the entire known M31 satellite system.
... As a case study, we focus on the nearby galaxy M31 (Andromeda galaxy). Its proximity to the Galaxy, at a distance of 780 kpc (McConnachie et al. 2003; 1 ≈ 4 pc and 1 • ≈ 14 kpc), allows detailed investigations into the faint stellar structures, showing that M31 is evidently in the process of the galaxy collision (Ibata et al. 2001;Irwin et al. 2005;Fardal et al. 2007;Mori & Rich 2008). Thus, it is a unique laboratory in which to examine how galaxy evolution and BH growth take place. ...
... As the satellite dwarf galaxy falls toward M31, it is elongated by the tidal force from M31, and eventually destroyed following a series of pericentric passages. Now, we see only the remnants of the satellite galaxy as a stellar stream (the so-called Andromeda stream) and two (western and northeastern) stellar shells (e.g., Ibata et al. 2001;Irwin et al. 2005). Detailed comparisons between observations and numerical simulations of the morphologies and the radial velocities of the stream and shells constrain the orbit, the mass and the concentration parameter of the satellite galaxy (Fardal et al. 2007;Mori & Rich 2008;Paper I). ...
Galaxies and massive black holes (BHs) presumably grow via galactic merging
events and subsequent BH coalescence. As a case study, we investigate the
merging event between the Andromeda galaxy (M31) and a satellite galaxy. We
compute the expected observational appearance of the massive BH that was at the
center of the satellite galaxy prior to the merger, and is currently wandering
in the M31 halo. We demonstrate that a radiatively inefficient accretion flow
with a bolometric luminosity of a few tens of solar luminosities develops when
Hoyle-Lyttleton accretion onto the BH is assumed. We compute the associated
broadband spectrum and show that the radio band (observable with EVLA, ALMA and
SKA) is the best frequency range to detect the emission. We also evaluate the
mass and the luminosity of the stars bound by the wandering BH and find that
such a star cluster is sufficiently luminous that it could correspond to one of
the star clusters found by the PAndAS survey. The discovery of a relic massive
BH wandering in a galactic halo will provide a direct means to investigate in
detail the coevolution of galaxies and BHs. It also means a new population of
BHs (off-center massive BHs), and offers targets for clean BH imaging that
avoids strong interstellar scattering in the center of galaxies.
... Our first task is to integrate the present results into complementary information about the outer regions of M31. We focus on studies of stellar populations since no significant reservoir of cold gas, dust, or ionized gas is found Irwin et al. 2005;Montalto et al. 2009;Azimlu et al. 2011), nor is there direct evidence of ongoing star formation in the disk of M31 beyond R g ≈ 20 kpc (Choi et al. 2002;Cour11). ...
... Their fields lie along the edge of the extended disk. This outer stellar disk is characterized by an exponential scale length of 14 kpc, a semimajor axis of 55 kpc (similar to the deprojected radii of M174 and M2496), and the general colors of a metalrich stellar population (Irwin et al. 2005). The CMDs at the edge of this extended disk constructed by Richardson et al. show that the metallicity of stars at the outer edge of has an irregular distribution at that projected radius. ...
Spectroscopic data of two relatively [O III]-luminous planetary nebulae (PNe) have been obtained with the 10.4 m Gran Telescopio Canarias. M174 and M2496 are each ~1° from the center of M31 along opposite sides of its minor axis. The ensemble of these 2 distant PNe plus 16 similarly luminous outer-disk PNe published previously by Kwitter et al. forms a homogeneous group in luminosity, metal content, progenitor mass, age, and kinematics. The main factual findings of our work are (1) O/H (and other low-mass α elements and their ratios to O) is uniformly solar-like in all 18 PNe (12 + log(O/H) = 8.62 ± 0.14); (2) the general sky distribution and kinematics of the ensemble much more closely resemble the rotation pattern of the classical disk of M31 than its halo or bulge; (3) the O/H gradient is surprisingly flat beyond Rg ~ 20 kpc. The PNe are too metal-rich to be bona fide members of M31's disk or halo, and (4) the abundance patterns of the sample are distinct from those in the spiral galaxies M33, M81, and NGC 300. Using standard PN age diagnostic methods, we suggest that all of the PNe formed ~2 Gyr ago in a starburst of metal-rich interstellar medium that followed an M31-M33 encounter about 3 Gyr ago. We review supporting evidence from stellar studies. Other more prosaic explanations, such as dwarf galaxy assimilation, are unlikely.
... Despite not explicitly fitting for the detailed properties of the shelves, the Fardal et al. minor merger models have predicted the existence of the SE shelf Gilbert et al. 2007) and show excellent agreement with the kinematics of RGB stars in the W shelf . These successes appear to be natural consequences of the constraints placed on the merger scenario using spatial and kinematical observations of the GSS (McConnachie et al. 2003;Ibata et al. 2004;Gilbert et al. 2009) and stellar surface density maps of M31 Ferguson et al. 2002;Irwin et al. 2005), where the orbital trajectory of the progenitor is only loosely required to pass through the location of the NE shelf. Moreover, Fardal et al. (2013) demonstrated that the predicted kinematical signature for the NE shelf partially overlaps with a small sample of PNe that form an apparent stream near the shelf (Merrett et al. 2003(Merrett et al. , 2006. ...
We obtained Keck/DEIMOS spectra of 556 individual red giant branch stars in 4 spectroscopic fields spanning $13-31$ projected kpc along the Northeast (NE) shelf of M31. We present the first detection of a complete wedge pattern in the space of projected M31-centric radial distance versus line-of-sight velocity for this feature, which includes the returning stream component of the shelf. This wedge pattern agrees with expectations of a tidal shell formed in a radial merger and provides strong evidence in favor of predictions of Giant Stellar Stream (GSS) formation models in which the NE shelf originates from the second orbital wrap of the tidal debris. The observed concentric wedge patterns of the NE, West (W), and Southeast (SE) shelves corroborate this interpretation independently of the models. We do not detect a kinematical signature in the NE shelf region corresponding to an intact progenitor core, favoring GSS formation models in which the progenitor is completely disrupted. The shelf's photometric metallicity distribution implies that it is dominated by tidal material, as opposed to the phase-mixed stellar halo or the disk. The metallicity distribution ([Fe/H]$_{\rm phot}$ = $-0.42$ $\pm$ $0.01$) also matches the GSS, and consequently the W and SE shelves, further supporting a direct physical association between the tidal features.
... Studies of M31ʼs stellar halo have made tremendous progress, from the discovery of M31ʼs extended stellar halo ( Guhathakurta et al. 2005;Irwin et al. 2005;Gilbert et al. 2006) to characterization of its global properties ( Kalirai et al. 2006a;Ibata et al. 2007;Koch et al. 2008;McConnachie et al. 2009;Gilbert et al. 2012Gilbert et al. , 2014Ibata et al. 2014). Spectroscopy of stars in M31ʼs halo has determined the fraction of stars in tidal streams in individual fields and characterized the kinematical properties of tidal debris features and the halo as a whole (e.g., Ibata et al. 2004;Gilbert et al. 2007;Chapman et al. 2008;Gilbert et al. 2009bGilbert et al. , 2009aGilbert et al. , 2012Gilbert et al. , 2018Dorman et al. 2012). ...
We present the first measurements of [Fe/H] and [α/Fe] abundances, obtained using spectral synthesis modeling, for red giant branch stars in M31's giant stellar stream (GSS). The spectroscopic observations, obtained at a projected distance of 17 kpc from M31's center, yielded 61 stars with [Fe/H] measurements, including 21 stars with [α/Fe] measurements, from 112 targets identified as M31 stars. The [Fe/H] measurements confirm the expectation from photometric metallicity estimates that stars in this region of M31's halo are relatively metal rich compared to stars in the Milky Way's inner halo: more than half the stars in the field, including those not associated with kinematically identified substructure, have [Fe/H] abundances >-1.0. The stars in this field are α-enhanced at lower metallicities, while [α/Fe] decreases with increasing [Fe/H] above metallicities of [Fe/H] ≳ −0.9. Three kinematical components have been previously identified in this field: the GSS, a second kinematically cold feature of unknown origin, and M31's kinematically hot halo. We compare probabilistic [Fe/H] and [α/Fe] distribution functions for each of the components. The GSS and the second kinematically cold feature have very similar abundance distributions, while the halo component is more metal poor. Although the current sample sizes are small, a comparison of the abundances of stars in the GSS field with abundances of M31 halo and dSph stars from the literature indicate that the progenitor of the stream was likely more massive, and experienced a higher efficiency of star formation, than M31's existing dSphs or the dEs NGC 147 and NGC 185.
... Considering the stellar halo as a whole, it is well established that the system belonging to M31 contains a higher fraction of the overall galaxy luminosity, is significantly more metal-rich, and is apparently more heavily substructured than that of the Milky Way (e.g. Mould & Kristian 1986;Pritchet & van den Bergh 1988;Ibata et al. 2001Ibata et al. , 2007Ibata et al. , 2014Ferguson et al. 2002;Irwin et al. 2005;McConnachie et al. 2009;Gilbert et al. 2009Gilbert et al. , 2012Gilbert et al. , 2014, while the M31 globular cluster population is more numerous than that of the Milky Way by at least a factor of 3 (e.g. Galleti et al. 2006Galleti et al. , 2007Huxor et al. 2008Huxor et al. , 2014Caldwell & Romanowsky 2016). ...
We utilize the final catalogue from the Pan-Andromeda Archaeological Survey to investigate the links between the globular cluster system and field halo in M31 at projected radii R proj = 25-150 kpc. In this region the cluster radial density profile exhibits a power-law decline with index = −2.37 ± 0.17, matching that for the stellar halo component with [Fe/H] < −1.1. Spatial density maps reveal a striking correspondence between the most luminous substructures in the metal-poor field halo and the positions of many globular clusters. By comparing the density of metal-poor halo stars local to each cluster with the azimuthal distribution at commensurate radius, we reject the possibility of no correlation between clusters and field overdensities at 99.95 per cent significance. We use our stellar density measurements and previous kinematic data to demonstrate that ≈35-60 per cent of clusters exhibit properties consistent with having been accreted into the outskirts of M31 at late times with their parent dwarfs. Conversely, at least ∼40 per cent of remote clusters show no evidence for a link with halo substructure. The radial density profile for this subgroup is featureless and closely mirrors that observed for the apparently smooth component of the metal-poor stellar halo. We speculate that these clusters are associated with the smooth halo; if so, their properties appear consistent with a scenario where the smooth halo was built up at early times via the destruction of primitive satellites. In this picture the entire M31 globular cluster system outside R proj = 25 kpc comprises objects accumulated from external galaxies over a Hubble time of growth.
... Furthermore, we keep the logarithmic power-law slope α of the tracer density distribution free in the range between 2 and 3.5, in line with recent literature (e.g. Irwin et al. 2005;Ibata et al. 2007;Courteau et al. 2011;Gilbert et al. 2012;Ibata et al. 2014). ...
Our nearest large cosmological neighbour, the Andromeda galaxy (M31), is a dynamical system, and an accurate measurement of its total mass is central to our understanding of its assembly history, the life-cycles of its satellite galaxies, and its role in shaping the Local Group environment. Here, we apply a novel approach to determine the dynamical mass of M31 using high velocity Planetary Nebulae (PNe), establishing a hierarchical Bayesian model united with a scheme to capture potential outliers and marginalize over tracers unknown distances. With this, we derive the escape velocity run of M31 as a function of galacto-centric distance, with both parametric and non-parametric approaches. We determine the escape velocity of M31 to be $470\pm{40}$ km s$^{-1}$ at a galacto-centric distance of 15 kpc, and also, derive the total potential of M31, estimating the virial mass and radius of the galaxy to be $0.8\pm{0.1}\times10^{12}\,M_\odot$ and $240\pm{10}$ kpc, respectively. Our M31 mass is on the low-side of the measured range, this supports the lower expected mass of the M31-Milky Way system from the timing and momentum arguments, satisfying the HI constraint on circular velocity between $10\lesssim R/\textrm{kpc}<35$, and agreeing with the stellar mass Tully-Fisher relation. To place these results in a broader context, we compare them to the key predictions of the $\Lambda{\rm CDM}$ cosmological paradigm, including the stellar-mass-halo-mass and the dark matter halo concentration-virial mass correlation, and finding it to be an outlier to this relation.
... Since confirmed as a shell system through the prediction and discovery of the southeast shelf [25,42], observations of the western shelf (shown here), in conjunction with previous observations of the GSS and southeast shelf, were used to determine the mass and disruption time of the progenitor, and to constrain the mass of M31 [44]. (a) Star count map of M31's inner halo, derived from the INT survey of M31 (image created by M. Irwin [45]), with the location of the SPLASH spectroscopic slitmasks covering the western shelf region overlaid in red (Figure 1 of Fardal et al. [43]). The green curve shows the progenitor's path in the model of Fardal et al. [43]. ...
The SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) Survey has observed fields throughout M31's stellar halo, dwarf satellites, and stellar disk. The observations and derived measurements have either been compared to predictions from simulations of stellar halo formation or modeled directly in order to derive inferences about the formation and evolution of M31's stellar halo. We summarize some of the major results from the SPLASH survey and the resulting implications for our understanding of the build-up of M31's stellar halo.
... The observational characterization of stellar halo profiles has followed different approaches. Several authors fit the outer (stellar halo-dominated) surface brightness region with some analytical profile, for instance, exponential (e.g., Irwin et al 2005;Ibata et al 2007;Trujillo and Fliri 2016) or power-law (with logarithmic slopes ∼2.5; e.g., Tanaka et al 2010;Courteau et al 2011;Gilbert et al 2012). This approach has the disadvantage of assuming a particular shape of the stellar halo towards the inner region of galaxies, motivated by numerical simulations. ...
Deep imaging is a fundamental tool in the study of the outermost structures of galaxies. We review recent developments in ultra-deep imaging of galaxy disks and haloes, highlighting the technical advances as well as the challenges and summarizing observational results in the context of modern theory and simulations. The deepest modern galaxy imaging comes from three main sources: (1) surveys such as the Sloan Digital Sky Survey’s Stripe 82 project; (2) very long exposures on small telescopes, including by amateurs; and (3) long exposures on the largest professional telescopes. The technical challenges faced are common in all these approaches and include the treatment of light scattered by atmosphere and telescope/instrument, correct flat fielding and the subtraction of non-galaxy light in the images. We review scientific results on galaxy disks and haloes obtained with deep imaging, including the detection and characterization of stellar haloes, tidal features and stellar streams, disk truncations and thick disks. The area of ultra-deep imaging is still very much unexplored territory, and future work in this area promises significant advances in our understanding of galaxy formation and evolution.
... The stellar volume density of M31 stellar halo is found to be a continuous single power-law i.e., ρ ∝ r −α with α ∈ [2.5, 4] (e.g. Irwin et al. 2005;Ibata et al. 2007;Gilbert et al. 2007;Courteau et al. 2011;Deason et al. 2013;Ibata et al. 2014, etc). We generate particles following this power-law density distribution with α = 3.5. ...
We present a quantitative measurement of the amount of clustering present in the inner $\sim30$ kpc of the stellar halo of the Andromeda galaxy (M31). For this we analyse the angular positions and radial velocities of the carefully selected Planetary Nebulae (PNe) in the M31 stellar halo. We study the cumulative distribution of pair-wise distances in angular position and line of sight velocity space, and find that the M31 stellar halo contains substantially more stars in the form of close pairs as compared to that of a featureless smooth halo. In comparison to a smoothed/scrambled distribution we estimate that the clustering excess in the M31 inner halo is roughly $40\%$ at maximum and on average $\sim 20\%$. Importantly, comparing against the 11 stellar halo models of \cite{2005ApJ...635..931B}, which were simulated within the context of the $\Lambda{\rm CDM}$ cosmological paradigm, we find that the amount of substructures in the M31 stellar halo closely resembles that of a typical $\Lambda{\rm CDM}$ halo.
... The two-dimensional surface brightness of M32 is modeled as a series of concentric ellipses based on the Choi et al. (2002) measurements of I-band surface brightness, ellipticity, and position angle as a function of radius. The two-dimensional surface brightness of M31 is modeled as the sum of a Sersíc bulge and an exponential disk based on published V-band surface brightness data and a mean color V − I = 1.6 (Walterbos & Kennicutt 1987;Pritchet & van den Bergh 1994;Guhathakurta et al. 2005;Irwin et al. 2005). The 5% and 50% M32/(M31 + M32) I-band surface brightness ratios are shown in Figure 1 (for a more detailed mapping see Figure 5). ...
As part of the SPLASH survey of the Andromeda (M31) system, we have obtained Keck/DEIMOS spectra of the compact elliptical (cE) satellite M32. This is the first resolved-star kinematical study of any cE galaxy. In contrast to most previous kinematical studies that extended out to r 30'' ~ 1 r
effI
~ 100 pc, we measure the rotation curve and velocity dispersion profile out to r ~ 250'' and higher order Gauss-Hermite moments out to r ~ 70''. We achieve this by combining integrated-light spectroscopy at small radii (where crowding/blending are severe) with resolved stellar spectroscopy at larger radii, using spatial and kinematical information to account statistically for M31 contamination. The rotation curve and velocity dispersion profile extend well beyond the radius (r ~ 150'') where the isophotes are distorted. Unlike NGC 205, another close dwarf companion of M31, M32's kinematics appear regular and symmetric and do not show obvious sharp gradients across the region of isophotal elongation and twists. We interpret M31's kinematics using three-integral axisymmetric dynamical equilibrium models constructed using Schwarzschild's orbit superposition technique. Models with a constant mass-to-light ratio can fit the data remarkably well. However, since such a model requires an increasing tangential anisotropy with radius, invoking the presence of an extended dark halo may be more plausible. Such an extended dark halo is definitely required to bind a half-dozen fast-moving stars observed at the largest radii, but these stars may not be an equilibrium component of M32.
We present spectroscopic chemical abundances of red giant branch stars in Andromeda (M31), using medium-resolution ( R ∼ 6000) spectra obtained via the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo survey. In addition to individual chemical abundances, we coadd low signal-to-noise ratio spectra of stars to obtain a high enough signal to measure average [Fe/H] and [ α /Fe] abundances. We obtain individual and coadded measurements for [Fe/H] and [ α /Fe] for M31 halo stars, covering a range of 9–180 kpc in projected radius from the center of M31. With these measurements, we greatly increase the number of outer halo ( R proj > 50 kpc) M31 stars with spectroscopic [Fe/H] and [ α /Fe], adding abundance measurements for 45 individual stars and 33 coadds from a pool of an additional 174 stars. We measure the spectroscopic metallicity ([Fe/H]) gradient, finding a negative radial gradient of −0.0084 ± 0.0008 for all stars in the halo, consistent with gradient measurements obtained using photometric metallicities. Using the first measurements of [ α /Fe] for M31 halo stars covering a large range of projected radii, we find a positive gradient (+0.0027 ± 0.0005) in [ α /Fe] as a function of projected radius. We also explore the distribution in [Fe/H]–[ α /Fe] space as a function of projected radius for both individual and coadded measurements in the smooth halo, and compare these measurements to those stars potentially associated with substructure. These spectroscopic abundance distributions add to existing evidence that M31 has had an appreciably different formation and merger history compared to our own Galaxy.
We present spectroscopic chemical abundances of red giant branch (RGB) stars in Andromeda (M31), using medium resolution ($R\sim6000$) spectra obtained via the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo (SPLASH) survey. In addition to individual chemical abundances, we coadd low signal-to-noise ratio (S/N) spectra of stars to obtain a high enough to measure average [Fe/H] and [$\alpha$/Fe] abundances. We obtain individual and coadded measurements for [Fe/H] and [$\alpha$/Fe] for M31 halo stars, covering a range of 9--180 kpc in projected radius from the center of M31. With these measurements, we greatly increase the number of outer halo ($R_{\mathrm{proj}} > 50$ kpc) M31 stars with spectroscopic [Fe/H] and [$\alpha$/Fe], adding abundance measurements for 45 individual stars and 33 coadds from a pool of an additional 174 stars. We measure the spectroscopic metallicity ([Fe/H]) gradient, finding a negative radial gradient of $-0.0050\pm0.0003$ for all stars in the halo, consistent with gradient measurements obtained using photometric metallicities. Using the first measurements of [$\alpha$/Fe] for M31 halo stars covering a large range of projected radii, we find a positive gradient ($+0.0026\pm0.0004$) in [$\alpha$/Fe] as a function of projected radius. We also explore the distribution in [Fe/H]--[$\alpha$/Fe] space as a function of projected radius for both individual and coadded measurements in the smooth halo, and compare these measurements to those stars potentially associated with substructure. These spectroscopic abundance distributions highlight the substantial evidence that M31 has had an appreciably different formation and merger history compared to our own Galaxy.
We analyze existing measurements of [Fe/H] and [α/Fe] for individual red giant branch (RGB) stars in the Giant Stellar Stream (GSS) of M31 to determine whether spatial abundance gradients are present. These measurements were obtained from low- (R ~ 3000) and moderate- (R ~ 6000) resolution Keck/DEIMOS spectroscopy using spectral synthesis techniques as part of the Elemental Abundances in M31 survey. From a sample of 62 RGB stars spanning the GSS at 17, 22, and 33 projected kpc, we measure a [Fe/H] gradient of −0.018 ± 0.003 dex kpc⁻¹ and negligible [α/Fe] gradient with M31-centric radius. We investigate GSS abundance patterns in the outer halo using additional [Fe/H] and [α/Fe] measurements for six RGB stars located along the stream at 45 and 58 projected kpc. These abundances provide tentative evidence that the trends in [Fe/H] and [α/Fe] beyond 40 kpc in the GSS are consistent with those within 33 kpc. We also compare the GSS abundances to 65 RGB stars located along the possibly related Southeast (SE) shelf substructure at 12 and 18 projected kpc. The abundances of the GSS and SE shelf are consistent, supporting a common origin hypothesis, although this interpretation may be complicated by the presence of [Fe/H] gradients in the GSS. We discuss the abundance patterns in the context of photometric studies from the literature and explore implications for the properties of the GSS progenitor, suggesting that the high 〈[α/Fe]〉 of the GSS (+0.40 ± 0.05 dex) favors a major merger scenario for its formation.
We present measurements of [Fe/H] and [α/Fe] for 128 individual red giant branch stars (RGB) in the stellar halo of M31, including its Giant Stellar Stream (GSS), obtained using spectral synthesis of low- and medium-resolution Keck/DEIMOS spectroscopy (R∼3000 and 6000, respectively). We observed four fields in M31's stellar halo (at projected radii of 9, 18, 23, and 31 kpc), as well as two fields in the GSS (at 33 kpc). In combination with existing literature measurements, we have increased the sample size of [Fe/H] and [α/Fe] measurements from 101 to a total of 229 individual M31 RGB stars. From this sample, we investigate the chemical abundance properties of M31's inner halo, finding ⟨[Fe/H]⟩ = −1.08 ± 0.04 and ⟨[α/Fe]⟩ = 0.40 ± 0.03. Between 8--34 kpc, the inner halo has a steep [Fe/H] gradient (−0.025 ± 0.002 dex kpc⁻¹) and negligible [α/Fe] gradient, where substructure in the inner halo is systematically more metal-rich than the smooth component of the halo at a given projected distance. Although the chemical abundances of the inner stellar halo are largely inconsistent with that of present-day dwarf spheroidal (dSph) satellite galaxies of M31, we identified 22 RGB stars kinematically associated with the smooth component of the stellar halo that have chemical abundance patterns similar to M31 dSphs. We discuss formation scenarios for M31's halo, concluding that these dSph-like stars may have been accreted from galaxies of similar stellar mass and star formation history, or of higher stellar mass and similar star formation efficiency.
We present [Fe/H] and [α/Fe] abundances, derived using spectral synthesis techniques, for stars in M31's outer stellar halo. The 21 [Fe/H] measurements and 7 [α/Fe] measurements are drawn from fields ranging from 43 to 165 kpc in projected distance from M31. We combine our measurements with existing literature measurements, and compare the resulting sample of 23 stars with [Fe/H] and 9 stars with [α/Fe] measurements in M31's outer halo with [α/Fe] and [Fe/H] measurements, also derived from spectral synthesis, in M31's inner stellar halo (r < 26 kpc) and dSph galaxies. The stars in M31's outer halo have [α/Fe] patterns that are consistent with the largest of M31's dSph satellites (And I and And VII). These abundances provide tentative evidence that the [α/Fe] abundances of stars in M31's outer halo are more similar to the abundances of Milky Way halo stars than to the abundances of stars in M31's inner halo. We also compare the spectral synthesis–based [Fe/H] measurements of stars in M31's halo with previous photometric [Fe/H] estimates, as a function of projected distance from M31. The spectral synthesis–based [Fe/H] measurements are consistent with a large-scale metallicity gradient previously observed in M31's stellar halo to projected distances as large as 100 kpc.
Context. The Andromeda (M 31) galaxy subtends nearly 100 square degrees on the sky. Any study of its halo must therefore account for the severe contamination from the Milky Way halo stars whose surface density displays a steep gradient across the entire M 31 field of view.
Aims. Our goal is to identify a population of stars firmly associated with the M 31 galaxy. Planetary nebulae (PNe) are one such population that are excellent tracers of light, chemistry, and motion in galaxies. We present a 16 square degree survey of the disc and inner halo of M 31 with the MegaCam wide-field imager at the CFHT to identify PNe, and characterise the luminosity-specific PN number and PN luminosity function (PNLF) in M 31.
Methods. PNe were identified via automated detection techniques based on their bright [O III ] 5007 Å emission and absence of a continuum. Subsamples of the faint PNe were independently confirmed by matching with resolved Hubble Space Telescope sources from the Panchromatic Hubble Andromeda Treasury and spectroscopic follow-up observations with HectoSpec at the MMT.
Results. The current survey reaches two magnitudes fainter than the previous most sensitive survey. We thus identify 4289 PNe, of which only 1099 were previously known. By comparing the PN number density with the surface brightness profile of M 31 out to ∼30 kpc along the minor axis, we find that the stellar population in the inner halo has a luminosity-specific PN number value that is seven times higher than that of the disc. We measure the luminosity function of the PN population and find a bright cut-off and a slope consistent with previous determinations. Interestingly, it shows a significant rise at the faint end, present in all radial bins covered by the survey. This rise in the M 31 PNLF is much steeper than that observed for the Magellanic clouds and Milky Way bulge.
Conclusions. The significant radial variation of the PN specific frequency value indicates that the stellar population at deprojected minor-axis radii larger than ∼10 kpc is different from that in the disc of M 31. The rise at the faint end of the PNLF is a property of the late phases of the stellar population. M 31 shows two major episodes of star formation and the rise at the faint end of the PNLF is possibly associated with the older stellar population. It may also be a result of varying opacity of the PNe.
We have measured the radial profiles of isophotal ellipticity ($\varepsilon$) and disky/boxy parameter A$_4$ out to radii of about three times the semi-major axes for $\sim4,600$ star-forming galaxies (SFGs) at intermediate redshifts $0.5<z<1.8$ in the CANDELS/GOODS-S and UDS fields. Based on the average size versus stellar-mass relation in each redshift bin, we divide our galaxies into Small SFGs (SSFGs), i.e., smaller than average for its mass, and Large SFGs (LSFGs), i.e., larger than average. We find that, at low masses ($M_{\ast} < 10^{10}M_{\odot}$), the SSFGs generally have nearly flat $\varepsilon$ and A$_4$ profiles for both edge-on and face-on views, especially at redshifts $z>1$. Moreover, the median A$_4$ values at all radii are almost zero. In contrast, the highly-inclined, low-mass LSFGs in the same mass-redshift bins generally have monotonically increasing $\varepsilon$ with radius and are dominated by disky values at intermediate radii. These findings at intermediate redshifts imply that low-mass SSFGs are not disk-like, while low-mass LSFGs appear to harbour disk-like components flattened by significant rotation. At high masses ($M_{\ast} > 10^{10}M_{\odot}$), highly-inclined SSFGs and LSFGs both exhibit a general, distinct trend for both $\varepsilon$ and A$_4$ profiles: increasing values with radius at lower radii, reaching maxima at intermediate radii, and then decreasing values at larger radii. Such a trend is more prevalent for more massive ($M_{\ast} > 10^{10.5}M_{\odot}$) galaxies or those at lower redshifts ($z<1.4$). The distinct trend in $\varepsilon$ and A$_4$ can be simply explained if galaxies possess all three components: central bulges, disks in the intermediate regions, and halo-like stellar components in the outskirts.
We report deep spectroscopy of ten planetary nebulae (PNe) in the Andromeda Galaxy (M31) using the 10.4m GTC. Our targets reside in different regions of M31, including halo streams and dwarf satellite M32, and kinematically deviate from the extended disk. The temperature-sensitive [O III] 4363 auroral line is observed in all targets. For four PNe, the GTC spectra extend beyond 1 micron, enabling explicit detection of the [S III] 6312 and 9069,9531 lines and thus determination of the [S III] temperature. Abundance ratios are derived and generally consistent with AGB model predictions. Our PNe probably all evolved from low-mass (<2 M_sun) stars, as analyzed with the most up-to-date post-AGB evolutionary models, and their main-sequence ages are mostly ~2-5 Gyr. Compared to the underlying, smooth, metal-poor halo of M31, our targets are uniformly metal-rich ([O/H]> -0.4), and seem to resemble the younger population in the stream. We thus speculate that our halo PNe formed in the Giant Stream's progenitor through extended star formation. Alternatively, they might have formed from the same metal-rich gas as did the outer-disk PNe, but was displaced into their present locations as a result of galactic interactions. These interpretations are, although speculative, qualitatively in line with the current picture, as inferred from previous wide-field photometric surveys, that M31's halo is the result of complex interactions and merger processes. The behavior of N/O of the combined sample of the outer-disk and our halo/substructure PNe signifies that hot bottom burning might actually occur at <3 M_sun, but careful assessment is needed.
Galaxy haloes contain fundamental clues about the galaxy formation and evolution process: hierarchical cosmological models predict haloes to be ubiquitous and to be (at least in part) the product of past merger and/or accretion events. The advent of wide-field surveys in the last two decades has revolutionized our view of our own Galaxy and its closest “sister”, Andromeda, revealing copious tidal streams from past and ongoing accretion episodes, as well as doubling the number of their known faint satellites. The focus shall now be shifted to galaxy haloes beyond the Local Group: resolving individual stars over significant areas of galaxy haloes will enable estimates of their ages, metallicities and gradients. The valuable information collected for galaxies with a range of masses, morphologies and within diverse environments will ultimately test and quantitatively inform theoretical models of galaxy formation and shed light onto the many challenges faced by simulations on galactic scales.
The Universe is almost totally unexplored at low surface brightness levels. In spite of great progress in the construction of large telescopes and improvements in the sensitivity of detectors, the limiting surface brightness of imaging observations has remained static for about 40 years. Recent technical advances have at last begun to erode the barriers preventing progress. In this chapter, we describe the technical challenges to low surface brightness imaging, describe some solutions and highlight some relevant observations that have been undertaken recently with both large and small telescopes. Our main focus will be on discoveries made with the Dragonfly Telephoto Array (Dragonfly), which is a new telescope concept designed to probe the Universe down to hitherto unprecedented low surface brightness levels. We conclude by arguing that these discoveries are probably only scratching the surface of interesting phenomena that are observable when the Universe is explored at low surface brightness levels.
We describe the challenges inherent to low surface brightness imaging and present some early results from the Dragonfly Nearby Galaxies survey. Wide field, ultra-low surface brightness imaging (μ g > 31 mag arcsec ⁻² ) of the first eight galaxies in the survey reveals a rich variety in the distribution of stars in the outskirts of luminous nearby galaxies. The mean stellar halo mass fraction is 0.009 ± 0.005 with a peak-to-peak scatter of a factor of > 100. Some galaxies in the sample feature strongly structured halos resembling that of M31, but three of the eight galaxies have halos that are completely undetected in our data. We conclude that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. While the outskirts of some galaxies are dominated by halos with the rich substructures predicted by numerical simulations, in other cases the outermost parts of galaxies are simply the extrapolated smooth starlight from enormous stellar disks that closely trace neutral gas morphology out to around 20 scale lengths.
Galaxies are thought to grow through accretion; as less massive galaxies are disrupted and merge over time, their debris results in diffuse, clumpy stellar halos enveloping the central galaxy. Here we present a study of the variation in the stellar halos of galaxies, using data from the Dragonfly Nearby Galaxies Survey (DNGS). The survey consists of wide field, deep ($\mu_{g} > 31$ mag arcsec$^{-2}$) optical imaging of nearby galaxies using the Dragonfly Telephoto Array. Our sample includes eight spiral galaxies with stellar masses similar to that of the Milky Way, inclinations of $16-90$ degrees and distances between $7-18$ Mpc. We construct stellar mass surface density profiles from the observed $g$-band surface brightness in combination with the $g-r$ color as a function of radius, and compute the halo fractions from the excess stellar mass (relative to a disk$+$bulge fit) beyond $5$ half-mass radii. We find a mean halo fraction of $0.009 \pm 0.005$ and a large RMS scatter of $1.01^{+0.9}_{-0.26}$ dex. The peak-to-peak scatter is a factor of $>100$ -- while some galaxies feature strongly structured halos resembling that of M31, three of the eight have halos that are completely undetected in our data. We conclude that spiral galaxies as a class exhibit a rich variety in stellar halo properties, implying that their assembly histories have been highly non-uniform. We find no convincing evidence for an environmental or stellar mass dependence of the halo fraction in the sample.
To unveil a progenitor of the Andromeda Giant Stellar Stream, we investigate the interaction between an accreting satellite galaxy and the Andromeda Galaxy using an $N$-body simulation. A comprehensive parameter study with 247 models is performed by varying size and mass distribution of the progenitor dwarf galaxy. We show that the binding energy of the progenitor is the crucial parameter in reproducing the Andromeda Giant Stellar Stream and the shell-like structures surrounding the Andromeda Galaxy. As a result of the simulations, the progenitor must satisfy a simple scaling relation between the core radius, the total mass and the tidal radius. Using this relation, we successfully constrain the physical properties of the progenitors to have mass ranging from $5\times10^8 M_\odot$ to $5\times10^9 M_\odot$ and central surface density around $10^3\, M_\odot\, \mathrm{pc}^{-2}$. A detailed comparison between our result and the nearby observed galaxies indicates that possible progenitors of the Andromeda Giant Stellar Stream include a dwarf elliptical galaxy, a dwarf irregular galaxy, and a small spiral galaxy.
Our Galaxy, the Milky Way, is a benchmark for understanding disk galaxies. It is the only galaxy whose formation history can be studied using the full distribution of stars from white dwarfs to supergiants. The oldest components provide us with unique insight into how galaxies form and evolve over billions of years. The Galaxy is a luminous (L-star) barred spiral with a central box/peanut bulge, a dominant disk, and a diffuse stellar halo. Based on global properties, it falls in the sparsely populated "green valley" region of the galaxy colour-magnitude diagram. Here we review the key integrated, structural and kinematic parameters of the Galaxy, and point to uncertainties as well as directions for future progress. Galactic studies will continue to play a fundamental role far into the future because there are measurements that can only be made in the near field and much of contemporary astrophysics depends on such observations.
Tidal streams from existing and destroyed satellite galaxies populate the outer regions of the Andromeda galaxy (M31). This inhomogeneous debris can be studied without of many of the obstacles that plague Milky Way research. We review the history of tidal stream research in M31, and in its main satellite galaxies. We highlight the numerous tidal streams observed around M31, some of which reside at projected distances of up to \(\sim 120\) kpc from the center of this galaxy. Most notable is the Giant Stellar Stream, a signature of the most recent significant accretion event in the M31 system. This event involved an early-type progenitor of mass \(\sim 10^{9}\) M\(_{\odot }\) that came within a few kpc of M31’s center roughly a gigayear ago; almost all of the inner halo (R ≤ 50 kpc) debris in M31 can be tied either directly or indirectly to this event. We draw attention to the fact that most of M31’s outer halo globular clusters lie preferentially on tidal streams and discuss the potential this offers to use these systems as probes of the accretion history. Tidal features observed around M33, M32, NGC 205 and NGC 147 are also reviewed. We conclude by discussing future prospects for this field.
We present the first results from an extended survey of the Andromeda galaxy (M31) using 41.1 hours of observations by Spitzer-IRAC at 3.6 and 4.5 μm. This survey extends previous observations to the outer disc and halo, covering total lengths of 4.4° and 6.6° along the minor and major axes, respectively. We have produced surface brightness profiles by combining the integrated light from background-corrected maps with stellar counts from a new catalogue of point sources. Using auxiliary catalogues we have carried out a statistical analysis in colour-magnitude space to discriminate M31 objects from foreground Milky Way stars and background galaxies. The catalogue includes 426,529 sources, of which 66 per cent have been assigned probability values to identify M31 objects with magnitude depths of [3.6] = 19.0 ± 0.2, [4.5] = 18.7 ± 0.2. We discuss applications of our data for constraining the stellar mass and characterising point sources in the outer radii.
We present deep spectroscopy of planetary nebulae (PNe) that are associated
with the substructures of the Andromeda Galaxy (M31). The spectra were obtained
with the OSIRIS spectrograph on the 10.4 m GTC. Seven targets were selected for
the observations, three in the Northern Spur and four associated with the Giant
Stream. The most distant target in our sample, with a rectified galactocentric
distance >100 kpc, was the first PN discovered in the outer streams of M31. The
[O III] 4363 auroral line was well detected in the spectra of all targets,
enabling electron temperature determination. Ionic abundances are derived based
on the [O III] temperatures, and elemental abundances of helium, nitrogen,
oxygen, neon, sulfur, and argon are estimated. The relatively low N/O and He/H
ratios as well as abundance ratios of alpha-elements indicate that our target
PNe might belong to populations as old as ~2 Gyr. Our PN sample, including the
current seven and the previous three observed by Fang et al., have rather
homogeneous oxygen abundances. The study of abundances and the spatial and
kinematical properties of our sample leads to the tempting conclusion that
their progenitors might belong to the same stellar population, which hints at a
possibility that the Northern Spur and the Giant Stream have the same origin.
This may be explained by the stellar orbit proposed by Merrett et al. Judging
from the position and kinematics, we emphasize that M32 might be responsible
for the two substructures. Deep spectroscopy of PNe in M32 will help to assess
this hypothesis.
The detection of structures in the sky with optical surface brightnesses fainter than 30 mag arcsec−2 (3σ in 10 × 10 arcsec boxes; r-band) has remained elusive in current photometric deep surveys. Here we show how present-day telescopes of 10 m class can provide broadband imaging 1.5–2 mag deeper than most previous results within a reasonable amount of time (i.e., <10 hr on-source integration). In particular, we illustrate the ability of the 10.4 m Gran Telescopio de Canarias telescope to produce imaging with a limiting surface brightness of 31.5 mag arcsec−2 (3σ in 10 × 10 arcsec boxes; r-band) using 8.1 hr on source. We apply this power to explore the stellar halo of the galaxy UGC 00180, a galaxy analogous to M31 located at ~150 Mpc, by obtaining a radial profile of surface brightness down to μ
r
~ 33 mag arcsec−2. This depth is similar to that obtained using the star-counts techniques for Local Group galaxies, but is achieved at a distance where this technique is unfeasible. We find that the mass of the stellar halo of this galaxy is ~4 × 109M
⊙, i.e., (3 ± 1)% of the total stellar mass of the whole system. This amount of mass in the stellar halo is in agreement with current theoretical expectations for galaxies of this kind.
NGC5128 is the nearest, easily observable giant elliptical galaxy, and the only one in which red giant branch (RGB) stars can be resolved from the ground. The only way to derive the surface brightness profile in these outer regions is to resolve and count individual halo stars. The authors derive photometry result of two field image data in the outer stellar halo of NGC5128. CCD imaging data is obtained using the Magellan/IMACS f/4 camera in the V and I bands. The RGB stars candidates are selected by color and magnitude, and 4719 and 980 RGB star candidates are obtained in the inner and outer field respectively. The outer field is used as a control field to measure the level of contamination from unresolved background galaxies. By applying a Sersic law to the surface brightness profile in the inner field, major axis profile is derived with the Sersic parameters: re = 6.21 arcmin, μe = 22.05 mag/arcsec2, n = 1.90, with fitting error 0.37 mag for V-band; re = 6.57 arcmin, μe = 20.79 mag/arcsec2, n = 1.90, with fitting error 0.41 mag for I-band. The result shows consistent results between the V and I bands within the limits of the data, and no evidence of a break in a profile that would signify a transition to a halo population.
We present deep Hubble Space Telescope Advanced Camera for Surveys observations of the stellar populations in two fields lying at 20 and 23 kpc from the centre
of M31 along the south-west semimajor axis. These data enable the construction of colour–magnitude diagrams reaching the oldest
main-sequence turn-offs (∼13 Gyr) which, when combined with another field at 25 kpc from our previous work, we use to derive
the first precision constraints on the spatially-resolved star formation history of the M31 disc. The star formation rates
exhibit temporal as well as field-to-field variations, but are generally always within a factor of 2 of their time average.
There is no evidence of inside-out growth over the radial range probed. We find a median age of ∼7.5 Gyr, indicating that
roughly half of the stellar mass in the M31 outer disc was formed before z ∼ 1. We also find that the age–metallicity relations (AMRs) are smoothly increasing from [Fe/H] ≃ −0.4 to solar metallicity
between 10 and 3 Gyr ago, contrary to the flat AMR of the Milky Way disc at a similar number of scalelengths. Our findings
provide insight on the roles of stellar feedback and radial migration in the formation and evolution of large disc galaxies.
We propose that luminous transients, including novae and supernovae, can be
used to detect the faintest galaxies in the universe. Beyond a few Mpc, dwarf
galaxies with stellar masses $<10^6 M_{\odot}$ will likely be too faint and/or
too low in surface brightness to be directly detected in upcoming large area
ground-based photometric surveys. However, single epoch LSST photometry will be
able to detect novae to distances of $\sim30$ Mpc and SNe to Gpc-scale
distances. Depending on the form of the stellar mass-halo mass relation and the
underlying star formation histories of low mass dwarfs, the expected nova rates
will be a few to $\sim100$ yr$^{-1}$ and the expected SN rates (including both
type Ia and core-collapse) will be $\sim10^2-10^4$ within the observable
($4\pi$ sr) volume. The transient rate associated with intrahalo stars will be
comparably large, but these transients will be located close to bright
galaxies, in contrast to the dwarfs, which should trace the underlying large
scale structure of the cosmic web. Aggressive follow-up of hostless transients
has the potential to uncover the predicted enormous population of low mass
field dwarf galaxies.
We present the metallicity distribution of red giant branch (RGB) stars in
M31's stellar halo, derived from photometric metallicity estimates for over
1500 spectroscopically confirmed RGB halo stars. The stellar sample comes from
38 halo fields observed with the Keck/DEIMOS spectrograph, ranging from 9 to
175 kpc in projected distance from M31's center, and includes 52 confirmed M31
halo stars beyond 100 kpc. While a wide range of metallicities is seen
throughout the halo, the metal-rich peak of the metallicity distribution
function becomes significantly less prominent with increasing radius. The
metallicity profile of M31's stellar halo shows a continuous gradient from 9 to
~100 kpc, with a magnitude of -0.01 dex/kpc. The stellar velocity distributions
in each field are used to identify stars that are likely associated with tidal
debris features. The removal of tidal debris features does not significantly
alter the metallicity gradient in M31's halo: a gradient is maintained in
fields spanning 10 to 90 kpc. We analyze the halo metallicity profile, as well
as the relative metallicities of stars associated with tidal debris features
and the underlying halo population, in the context of current simulations of
stellar halo formation. We argue that the large scale gradient in M31's halo
implies M31 accreted at least one relatively massive progenitor in the past,
while the field to field variation seen in the metallicity profile indicates
that multiple smaller progenitors are likely to have contributed substantially
to M31's outer halo.
The cold dark matter (CDM) cosmology, which is the standard theory of the structure formation in the universe, predicts that
the outer density profile of dark matter halos decreases with the cube of distance from the center. However, so far not much
effort has been expended in examining this hypothesis. In the halo of the Andromeda galaxy (M 31), large-scale stellar structures
detected by the recent observations provide a potentially suitable window to investigate the mass-density distribution of
the dark matter halo. We explore the density structure of the dark matter halo in M 31 using an N-body simulation of the interaction between an accreting satellite galaxy and M 31. To reproduce the Andromeda Giant Southern
Stream and the stellar shells at the east and west sides of M 31, we find the sufficient condition for the power-law index
α of the outer density distribution of the dark matter halo. The best-fitting parameter is α = −3.7, which is steeper than
the CDM prediction.
The Galaxy has built up through a process of accretion and merging over billions of years which continues to this day. Astronomers are now embarking on a new era of massive stellar surveys over the coming decade.
While wide-field surveys of M31 have revealed much substructure at large radii, understanding the nature and origin of this
material is not straightforward from morphology alone. Using deep Hubble Space Telescope/Advanced Camera for Surveys data, we have derived further constraints in the form of quantitative star formation histories
(SFHs) for 14 inner halo fields which sample diverse substructures. In agreement with our previous analysis of colour–magnitude
diagram morphologies, we find the resultant behaviours can be broadly separated into two categories. The SFHs of ‘disc-like’
fields indicate that most of their mass has formed since z ∼ 1, with one quarter of the mass formed in the last 5 Gyr. We find ‘stream-like’ fields to be on average 1.5 Gyr older,
with ≲ 10 per cent of their stellar mass formed within the last 5 Gyr. These fields are also characterized by an age–metallicity
relation showing rapid chemical enrichment to solar metallicity by z = 1, suggestive of an early-type progenitor. We confirm a significant burst of star formation 2 Gyr ago, discovered in our
previous work, in all the fields studied here. The presence of these young stars in our most remote fields suggests that they
have not formed in situ but have been kicked-out from the thin disc through disc heating in the recent past.
We use the Illustris simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround
galaxies. We characterize the stellar haloes by means of the logarithmic slope of the spherically averaged stellar density
profiles, αSTARS at z = 0, and we relate these slopes to the properties of the underlying dark matter (DM) haloes, their central galaxies, and
their assembly histories. We analyse a sample of ∼5000 galaxies resolved with more than 5 × 104 particles each, and spanning a variety of morphologies and halo masses (3 × 1011 ≤ Mvir ≲ 1014 M⊙). We find a strong trend between stellar halo slope and total halo mass, where more massive objects have shallower stellar
haloes than the less massive ones (−5.5 ± 0.5 < αSTARS < −3.5 ± 0.2 in the studied mass range). At fixed halo mass, we show that disc-like, blue, young, and more massive galaxies
are surrounded by significantly steeper stellar haloes than elliptical, red, older, and less massive galaxies. Overall, the
stellar density profiles fall off much more steeply than the underlying DM, and no clear trend holds between stellar slope
and DM halo concentration. However, DM haloes which formed more recently, or which accreted larger fractions of stellar mass
from infalling satellites, exhibit shallower stellar haloes than their older analogues with similar masses, by up to ΔαSTARS ∼ 0.5–0.7. Our findings, combined with the most recent measurements of the strikingly different stellar power-law indices
for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterized by a much quieter accretion history
over the past 10 Gyr than its companion.
We present the largest and most detailed survey to date of the stellar
populations in the outskirts of M31 based on the analysis of 14 deep
HST/ACS pointings spanning the range 11.5-45.0 kpc. We conduct a
quantitative comparison of the resolved stellar populations in these
fields and identify several striking trends. The color-magnitude
diagrams (CMDs), which reach ~3 magnitudes below the red clump, can be
classified into two main categories based on their morphologies.
`Stream-like' fields, so named for their similarity to the CMD of the
giant stellar stream, are characterized by a red clump that slants
bluewards at fainter magnitudes and an extended horizontal branch. On
the other hand, `disk-like' fields exhibit rounder red clumps with
significant luminosity width, lack an obvious horizontal branch and show
evidence for recent star formation (~0.25 - 2.0 Gyr ago). We compare the
spatial and line-of-sight distribution of stream-like fields with a
recent simulation of the giant stream progenitor orbit and find an
excellent agreement. These fields, found across the face of M31, attest
to the high degree of pollution caused by this event. Disk-like material
resides in the extended disk structure of M31 and is detected out to 44
kpc (projected); the uniform populations in these fields, including the
ubiquitous presence of young populations, and the strong rotation
reported elsewhere are most consistent with a scenario in which this
structure has formed through heating and disruption of the existing thin
disk, perhaps due to the impact of the giant stream progenitor. Our
comparative analysis sheds new light on the likely composition of two of
the ultra-deep pointings formerly presented as pure outer disk and pure
halo in the literature.
In the hierarchical structure formation scenario, galaxies have experienced many mergers with less massive galaxies and have grown larger and larger. On the other hand, the observations indicate that almost all galaxies have a central massive black hole (MBH) whose mass is ~ 10−3 of its spheroidal component. Consequently, MBHs of satellite galaxies are expected to be moving in the halo of their host galaxy after a galaxy collision, although we have not found such MBHs yet. We investigate the current-plausible position of an MBH of the infalling galaxy in the halo of the Andromeda galaxy (M31). Many substructures are found in the M31 halo, and some of them are shown to be remnants of a minor merger about 1 Gyr ago based on theoretical studies using N-body simulations. We calculate possible orbits of the MBH within the progenitor dwarf galaxy using N-body simulations. Our results show that the MBH is within the halo, about 30 kpc away from the center of M31.
In addition, further simulations are necessary to restrict the area in which the MBH exists, and hence to determine the observational field for the future observational detection. The most uncertainty of the current MBH position is caused by uncertainty about the infalling orbit of the progenitor dwarf galaxy. Therefore, we have performed a large (a few 104 realizations) set of parameter study to constrain the orbit in the six-dimensional phase space. For such purpose, we have already investigated in detail a few ten thousand orbit models using HA-PACS, a recently installed GPU cluster in University of Tsukuba.
We use a new telescope concept, the Dragonfly Telephoto Array, to study the low surface brightness outskirts of the spiral galaxy M101. The radial surface brightness profile is measured down to μg
~ 32 mag arcsec–2, a depth that approaches the sensitivity of star count studies in the Local Group. We convert surface brightness to surface mass density using the radial g – r color profile. The mass density profile shows no significant upturn at large radius and is well-approximated by a simple bulge + disk model out to R = 70 kpc, corresponding to 18 disk scale lengths. Fitting a bulge + disk + halo model we find that the best-fitting halo mass M
☉. The total stellar mass of M101 is M
☉, and we infer that the halo mass fraction . This mass fraction is lower than that of the Milky Way (f
halo ~ 0.02) and M31 (f
halo ~ 0.04). All three galaxies fall below the f
halo-M
tot, * relation predicted by recent cosmological simulations that trace the light of disrupted satellites, with M101's halo mass a factor of ~10 below the median expectation. However, the predicted scatter in this relation is large, and more galaxies are needed to better quantify this possible tension with galaxy formation models. Dragonfly is well suited for this project: as integrated-light surface brightness is independent of distance, large numbers of galaxies can be studied in a uniform way.
In the hierarchical structure formation scenario, galaxies enlarge through
multiple merging events with less massive galaxies. In addition, the Magorrian
relation indicates that almost all galaxies are occupied by a central
supermassive black hole (SMBH) of mass $10^{-3}$ of its spheroidal component.
Consequently, SMBHs are expected to wander in the halos of their host galaxies
following a galaxy collision, although evidence of this activity is currently
lacking. We investigate a current plausible location of an SMBH wandering in
the halo of the Andromeda galaxy (M31). According to theoretical studies of
$N$-body simulations, some of the many substructures in the M31 halo are
remnants of a minor merger occurring about 1 Gyr ago. First, to evaluate the
possible parameter space of the infalling orbit of the progenitor, we perform
numerous parameter studies using a Graphics Processing Unit (GPU) cluster. To
reduce uncertainties in the predicted position of the expected SMBH, we then
calculate the time evolution of the SMBH in the progenitor dwarf galaxy from
$N$-body simulations using the plausible parameter sets. Our results show that
the SMBH lies within the halo ($\sim$20--50 kpc from the M31 center), closer to
the Milky Way than the M31 disk. Furthermore, the predicted current positions
of the SMBH were restricted to an observational field of $0\degr.6 \times
0\degr.7$ in the northeast region of the M31 halo. We also discuss the origin
of the infalling orbit of the satellite galaxy and its relationships with the
recently discovered vast thin disk plane of satellite galaxies around M31.
The absence of stellar disc truncations in low-inclined spiral galaxies has
been a matter of debate in the last decade. Disc truncations are often observed
in highly inclined galaxies but no obvious detection of this feature has so far
been made in face-on spirals. Here we show, using a simple exponential disc
plus stellar halo model based on current observational constraints, that
truncations in face-on projections occur at surface brightness levels
comparable to the brightness of stellar haloes at the same radial distance. In
this sense, stellar haloes outshine the galaxy disc at the expected position of
the truncations, forcing their studies only in highly inclined (edge-on)
orientations.
We present wide-field near-infrared J and Ks images of the Andromeda
Galaxy taken with WIRCam on the Canada-France-Hawaii Telescope (CFHT) as
part of the Andromeda Optical and Infrared Disk Survey (ANDROIDS). This
data set allows simultaneous observations of resolved stars and NIR
surface brightness across M31's entire bulge and disk (within R=22 kpc).
The primary concern of this work is the development of NIR observation
and reduction methods to recover a uniform surface brightness map across
the 3x1 degree disk of M31. This necessitates sky-target nodding across
27 WIRCam fields. Two sky-target nodding strategies were tested, and we
find that strictly minimizing sky sampling latency does not maximize sky
subtraction accuracy, which is at best 2% of the sky level. The mean
surface brightness difference between blocks in our mosaic can be
reduced from 1% to 0.1% of the sky brightness by introducing scalar sky
offsets to each image. The true surface brightness of M31 can be known
to within a statistical zeropoint of 0.15% of the sky level (0.2 mag
arcsec sq. uncertainty at R=15 kpc). Surface brightness stability across
individual WIRCam frames is limited by both WIRCam flat field evolution
and residual sky background shapes. To overcome flat field variability
of order 1% over 30 minutes, we find that WIRCam data should be
calibrated with real-time sky flats. Due either to atmospheric or
instrumental variations, the individual WIRCam frames have typical
residual shapes with amplitudes of 0.2% of the sky after real-time flat
fielding and median sky subtraction. We present our WIRCam reduction
pipeline and performance analysis here as a template for future
near-infrared observers needing wide-area surface brightness maps with
sky-target nodding, and give specific recommendations for improving
photometry of all CFHT/WIRCam programs. (Abridged)
Extensive photometric and spectroscopic surveys of the Andromeda galaxy (M31) have discovered tidal debris features throughout M31's stellar halo. We present stellar kinematics and metallicities in fields with identified substructure from our on-going SPLASH survey of M31 red giant branch stars with the DEIMOS spectrograph on the Keck II 10 m telescope. Radial velocity criteria are used to isolate members of the kinematically cold substructures. The substructures are shown to be metal-rich relative to the rest of the dynamically hot stellar population in the fields in which they are found. We calculate the mean metallicity and average surface brightness of the various kinematical components in each field, and show that, on average, higher surface brightness features tend to be more metal-rich than lower surface brightness features. Simulations of stellar halo formation via accretion in a cosmological context are used to illustrate that the observed trend can be explained as a natural consequence of the observed dwarf galaxy mass-metallicity relation. A significant spread in metallicity at a given surface brightness is seen in the data; we show that this is due to time effects, namely, the variation in the time since accretion of the tidal streams' progenitor onto the host halo. We show that in this theoretical framework a relationship between the alpha-enhancement and surface brightness of tidal streams is expected, which arises from the varying times of accretion of the progenitor satellites onto the host halo. Thus, measurements of the alpha-enrichment, metallicity, and surface brightness of tidal debris can be used to reconstruct the luminosity and time of accretion onto the host halo of the progenitors of tidal streams.
We characterize the bulge, disk, and halo subcomponents in the Andromeda galaxy (M31) over the radial range 4 kpc < R
proj < 225 kpc. The cospatial nature of these subcomponents renders them difficult to disentangle using surface brightness (SB) information alone, especially interior to ~20 kpc. Our new decomposition technique combines information from the luminosity function (LF) of over 1.5 million bright (20 < m
814W < 22) stars from the Panchromatic Hubble Andromeda Treasury survey, radial velocities of over 5000 red giant branch stars in the same magnitude range from the Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo survey, and integrated I-band SB profiles from various sources. We use an affine-invariant Markov chain Monte Carlo algorithm to fit an appropriate toy model to these three data sets. The bulge, disk, and halo SB profiles are modeled as a Sérsic, exponential, and cored power law, respectively, and the LFs are modeled as broken power laws. We present probability distributions for each of 32 parameters describing the SB profiles and LFs of the three subcomponents. We find that the number of stars with a disk-like LF is 5.2% ± 2.1% larger than the number with disk-like (dynamically cold) kinematics, suggesting that some stars born in the disk have been dynamically heated to the point that they are kinematically indistinguishable from halo members. This is the first kinematical evidence for a "kicked-up disk" halo population in M31. The fraction of kicked-up disk stars is consistent with that found in simulations. We also find evidence for a radially varying disk LF, consistent with a negative metallicity gradient in the stellar disk.
The objective of this work is to obtain the disk thickness and spiral arm structure of M31. The method to estimate the thickness of galactic disk is based on using Poisson’s equation for the logarithmical matter density in three-dimensional disk galaxies. We use new infrared image of M31 at 24 μm taken with the Multiband Imaging Photometer for Spitzer (MIPS). Far infrared images would be free of blurring by the bright bulge, so that the spiral arms can be traced inwards to smaller radii (the forbidden radii). We investigate the fitting of a two-armed (a- and b-arm) spiral structure of M31 by logarithmical spirals. From a-arm structure, we acquire the disk inclination γa=74.5±1.5°γa=74.5±1.5°, and from b-arm structure, γb=70.3±2.0°γb=70.3±2.0°. The mean values for the forbidden radius r0¯, the winding parameter Λ¯, and the disk thickness H¯ are 2.82±0.232.82±0.23 kpc, 12.49±1.5012.49±1.50, and 0.45±0.090.45±0.09 kpc, respectively.
We present a sample of 148 candidate RR Lyrae stars selected from Sloan Digital Sky Survey (SDSS) commissioning data for about 100 deg2 of sky surveyed twice with Δt = 1.9946 days. Although the faint-magnitude limit of the SDSS allows us to detect RR Lyrae stars to large Galactocentric distances (~100 kpc, or r* ~ 21), we find no candidates fainter than r* ~ 20, i.e., farther than ~65 kpc from the Galactic center. On the assumption that all 148 candidates are indeed RR Lyrae stars (contamination by other species of variable star is probably less than 10%), we find that their volume density has roughly a power-law dependence on Galactocentric radius, R-2.7±0.2, between 10 and 50 kpc and drops abruptly at R ~ 50–60 kpc, possibly indicating a sharp edge to the stellar halo as traced by RR Lyrae stars. The Galactic distribution of stars in this sample is very inhomogeneous and shows a clump of over 70 stars at about 45 kpc from the Galactic center. This clump is also detected in the distribution of nonvariable objects with RR Lyrae star colors. When sources in the clump are excluded, the best power-law fit becomes consistent with the R-3 distribution found from surveys of bright RR Lyrae stars. These results imply that the halo contains clumpy overdensities inhomogeneously distributed within a smooth R-3 background, with a possible cutoff at ~50 kpc.
We have used the Gemini Multi-Object Spectrograph (GMOS) on the Gemini South 8 m telescope in exceptional conditions (06 FWHM seeing) to observe the outer stellar disk of the Sculptor Group galaxy NGC 300 at two locations. At our point-source detection threshold of r' = 27.0 (3 σ) mag, we trace the stellar disk out to a radius of 24', or 2.2R25, where R25 is the 25 mag arcsec-2 isophotal radius. This corresponds to about 10 scale lengths in this low-luminosity spiral galaxy (MB = -18.6), or about 14.4 kpc at a Cepheid distance of 2.0 ± 0.07 Mpc. The background galaxy counts are derived in the outermost field, and these are within 10% of the mean survey counts from both Hubble Deep Fields. The luminosity profile is well described by a nucleus plus a simple exponential profile out to 10 optical scale lengths. We reach an effective surface brightness of μ = 30.5 mag arcsec-2 (2 σ) at 55% completeness, which doubles the known radial extent of the optical disk. These levels are exceedingly faint in the sense that the equivalent surface brightness in B or V is about 32 mag arcsec-2. We find no evidence for truncation of the stellar disk. Only star counts can be used to reliably trace the disk to such faint levels, since surface photometry is ultimately limited by nonstellar sources of radiation. In the Appendix, we derive the expected surface brightness of one such source: dust scattering of starlight in the outer disk.
We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of the halo mass, the initial density fluctuation spectrum, and the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fitted over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology, the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation that reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results with previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, that allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.
The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a "noninteracting" warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array H I 21 cm line maps. The visible material in the ring has an integrated luminosity ≤108 L☉, and its brightest part has a color R-I~0.9. All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (ΔV=45 km s-1) to the giant spiral galaxy. This dwarf is seen at the tip of the H I warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies.
We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M-giant star tracers detected in the complete Two Micron All Sky Survey (2MASS). Near-infrared photometry of Sgr's prominent M-giant population permits an unprecedentedly clear view of the center of Sgr. The main body is fitted with a King profile of limiting major-axis radius 30°-substantially larger than previously found or assumed-beyond which is a prominent break in the density profile from stars in the Sgr tidal tails; thus the Sgr radial profile resembles that of Galactic dwarf speroidal (dSph) satellites. Adopting traditional methods for analyzing dSph light profiles, we determine the brightness of the main body of Sgr to be MV=-13.27 (the brightest of the known Galactic dSph galaxies) and the total Sgr mass-to-light ratio to be 25 in solar units. However, we regard the latter result with suspicion and argue that much of the observed structure beyond the King-fit core radius (224') may be outside the actual Sgr tidal radius as the former dwarf spiral/irregular satellite undergoes catastrophic disruption during its last orbits. The M-giant distribution of Sgr exhibits a central density cusp at the same location as, but not due to, the old stars constituting the globular cluster M54. A striking trailing tidal tail is found to extend from the Sgr center and arc across the south Galactic hemisphere with approximately constant density and mean distance varying from ~20 to 40 kpc. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an apogalacticon ~45 kpc from the Sun and then turns toward the north Galactic cap (NGC), from where it descends back toward the Galactic plane, becomes foreshortened, and, at brighter magnitudes, covers the NGC. The leading and trailing Sgr tails lie along a well-defined orbital plane about the Galactic center. The Sun lies within a kiloparsec of that plane and near the path of leading Sgr debris; thus, it is possible that former Sgr stars are near or in the solar neighborhood. We discuss the implications of this new view of the Sgr galaxy and its entrails for the character of the Sgr orbit, mass, mass-loss rate, and contribution of stars to the Milky Way halo. The minimal precession displayed by the Sgr tidal debris along its inclined orbit supports the notion of a nearly spherical Galactic potential. The number of M giants in the Sgr tails is at least 15% that contained within the King limiting radius of the main Sgr body. The fact that M giants, presumably formed within the past few gigayears in the Sgr nucleus, are nevertheless so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but also poses a timing problem that bears on the recent binding energy of the Sgr core and that is most naturally explained by recent and catastrophic mass loss. Sgr appears to contribute more than 75% of the high-latitude, halo M giants, despite substantial reservoirs of M giants in the Magellanic Clouds. No evidence of extended M-giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M-giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris, with the exception of Sgr carbon stars, which must be subluminous compared with counterparts in other Galactic satellites in order to resolve the discrepancy.
We develop an explicit model for the formation of the stellar halo from tidally disrupted, accreted dwarf satellites in the cold dark matter (CDM) framework, focusing on predictions testable with the Sloan Digital Sky Survey (SDSS) and other wide-field surveys. Subhalo accretion and orbital evolution are calculated using a semi-analytic approach within the Press-Schechter formalism. Motivated by our previous work, we assume that low-mass subhalos (v < 30 km/s) can form significant populations of stars only if they accreted a substantial fraction of their mass before the epoch of reionization. With this assumption, the model reproduces the observed velocity function of galactic satellites in the Local Group, solving the ``dwarf satellite problem'' without modifying the popular LCDM cosmology. The disrupted satellites yield a stellar distribution with a total mass and radial density profile consistent with those observed for the Milky Way stellar halo. Most significantly, the model predicts the presence of many large-scale, coherent substructures in the outer halo. These substructures are remnants of individual, tidally disrupted dwarf satellite galaxies. Substructure is more pronounced at large galactocentric radii because of the smaller number density of tidal streams and the longer orbital times. This model provides a natural explanation for the coherent structures in the outer stellar halo found in the SDSS commissioning data, and it predicts that many more such structures should be found as the survey covers more of the sky. The detection (or non-detection) and characterization of such structures could eventually test variants of the CDM scenario, especially those that aim to solve the dwarf satellite problem by enhancing satellite disruption. Comment: 12 pages, 8 figures, Submitted to ApJ
(abridged) We present an analysis of B-R and R-K_s color maps for 47 late-type, edge-on, unwarped, bulgeless disk galaxies spanning a wide range of mass. The color maps show that the thin disks of these galaxies are embedded within a low surface brightness red envelope that is substantially thicker than the thin disk (a/b~4:1 vs a/b>8:1), extends to at least 5 vertical disk scale heights above the galaxy midplane, and has a radial scale length that appears to be uncorrelated with that of the embedded thin disk. The color of the red envelope is similar from galaxy to galaxy and is consistent with a relatively old (>6Gyr) stellar population that is not particularly metal-poor. The color difference between the thin disk and the envelope varies systematically with rotation speed, indicating a younger thin disk relative to the red envelope in lower mass galaxies. The red stellar envelopes are similar to the MW thick disk, having common surface brightnesses, spatial distributions, mean ages, and metallicities. The ubiquity of the red stellar envelopes implies that the formation of the thick disk is a nearly universal feature of disk formation and need not be associated with bulge formation. Furthermore, our data suggest that the thick disk forms early, even in cases where the majority of star formation was recent. Finally, we find that our data and the observed properties of the MW thick disk argue in favor of a merger origin for the thick disk population. If so, then the age of the thick disk marks the end of the epoch of major merging, and the age difference between the thin and thick disks can become a strong constraint on cosmological constants and models of galaxy and/or structure formation. Comment: accepted to the September 2002 Astronomical Journal, LaTeX, 31 pages including figures
Understanding galaxy formation involves look-back and fossil-record studies of distant and nearby galaxies, respectively. Debris trails in our Galaxy's spheroidal halo of old stars provide evidence of "bottom-up" formation via tidal disruption/merging of dwarf satellite galaxies, but it is difficult to study our Galaxy's large-scale structure from within. Studies of our neighbouring Andromeda galaxy have concluded that its spheroid contains chemically enriched ("metal-rich") stars out to a radius of 30 kiloparsecs with an exponential r^1/4 fall-off in density thereby resembling a galactic "bulge". Were Andromeda's true halo to be found, our detailed yet global view of its stellar dynamics, substructure, chemical abundance, and age distribution would directly constrain hierarchical halo formation models. We report here on the discovery of a hitherto elusive halo of metal-poor Andromeda stars, distinct from its bulge, with a power-law brightness profile extending beyond r = 160 kiloparsecs. This is 3 - 5 times larger than any previously mapped Andromeda spheroidal/disk component. Together, the Galactic and Andromeda halos span >1/3 of the distance between them, suggesting that stars occupy a substantial volume fraction of our Local Group, and possibly most galaxy groups.
The existence of a stream of tidally stripped stars from the Sagittarius Dwarf galaxy demonstrates that the Milky Way is still in the process of accreting mass. More recently, an extensive stream of stars has been uncovered in the halo of the Andromeda galaxy (M31), revealing that it too is cannibalizing a small companion. This paper reports the recent observations of this stream, determining it spatial and kinematic properties, and tracing its three-dimensional structure, as well as describing future observations and what we may learn about the Andromeda galaxy from this giant tidal stream. Comment: 3 Pages. Refereed contribution to the 5th Galacto Chemodynamics conference held in Swinburne, July 2003. Accepted for publication in PASA
We give a brief overview of the INT Wide Field Camera (WFC) together with the automated pipeline processing developed specifically for the Wide Field Survey (WFS). The importance of accurate and complete FITS header information is stressed. Data processing products output from the complete pipeline are discussed.
A complete sample of blue horizontal branch (BHB) stars in the magnitude
range 13.0 less than or equal to V less than or equal to 16.5 is
isolated in two Galactic fields that have previously been searched for
RR Lyrae variables: SA 57 in the Northern Polar Cap and the Lick
Astrograph field RR 7 in the Anticenter (l = 183 deg, b = +37 deg).
These BHB stars are a subset of the AF stars found in the Case
Low-Dispersion Northern Survey; lists of these AF stars were made
available by the late Nick Sanduleak. In the color range 0.00 less than
or equal to (B - V)0 less than or equal to +0.20, we can
distinguish the BHB stars among these AF stars by comparing them both
with well known local field horizontal branch (FHB) stars and also the
BHB members of the halo globular clusters M3 and M92. The criteria for
this comparison include (1) a (u - B)K color index (derived
from photoelectric observations using the Stroemgren u filter and the
Johnson B and V filters) that measures the size of the Balmer jump, (2)
a spectrophotometric index lambda that measures the steepness of the
Balmer jump, and (3) a parameter D0.2 that is the mean width
of the H-delta and H-gamma Balmer lines measured at 20 percent of the
continuum level. These criteria give consistent results in separating
BHB stars from higher gravity main sequence AF stars in the color range
0.00 less than or equal to (B - V)0 less than or equal to
+0.20. All three photometric and spectrophotometric criteria were
measured for 35 stars in the SA 57 field and 37 stars in the RR 7 field
that are in the color range (B - V)0 less than or equal to
+0.23 and in the magnitude range 13.0 less than or equal to V less than
or equal to 16.5. For a small number of additional stars only (u -
B)K was obtained. Among the AF stars that are fainter than B
= 13 and bluer than (B - V)0 = +0.23, about half of those in
the SA 57 field and about one third of those in the lower latitude RR 7
field are BHB stars. Isoabundance contours were located empirically in
plots of the pseudoequivalent width versus (B - V)0 for the
lines of Mg II lambda-4481 A, Ca II lambda-3933 A, and Fe I lambda-4272
A. Solar abundances were defined by the data from main sequence stars in
the Pleiades and Coma open clusters. Data from the BHB stars in M3 and
M92 defined the (Fe/H) = -1.5 and -2.2 isoabundance contours,
respectively. Metallicities of all stars were estimated by interpolating
the measured pseudoequivalent widths in these diagrams at the observed
(B - V)0. The distribution of (Fe/H) found for the BHB stars
in this way is very similar to that which we found for the RR Lyrae
stars in the same fields using the Preston Delta-S method. The space
densities of these BHB stars were analyzed both separately and together
with earlier observations of field BHB stars. This analysis supports a
two-component model for the halo of our Galaxy that is similar in many
respects to that proposed by Hartwick although our discussion refers
only to the region outside the solar circle.
Counts of stars near the tip of the red giant branch have been used to
estimate the surface brightness of the halo of M31 down to a level of
muV approximately equals 30 mag arcsec-2 (R
approximately equals 20 kpc). Over the range 0.2 approximately less than
R(kpc) approximately less than 20 the surface brightness distribution in
the halo of the Andromeda galaxy is well-represented by a single de
Vaucouleurs law. Alternatively, the outer halo of M31 can also be
modelled by a power-law density distribution of the form rho (R) varies
as R-5. This result suggests that the globular cluster
component of the halo of M31 (for which rho varies as R-3) is
more extended than the stellar halo of this galaxy. At muV
approximately equals 28 mag arcsec-2 (R approximately equals
10 kpc), the axial ratio of the halo of M31 is found to be b/a = 0.55
+/- 0.05.
The authors report star counts, as a function of position and apparent
magnitude, in the rich, relatively open southern globular cluster NGC
6809 (M55). Three AAO 150arcsec plates were scanned by the Automatic
Plate Measuring System (APM) at the Institute of Astronomy, Cambridge,
and 20825 images were counted by its associated software. Previously
known features of rich globular clusters which appear in the raw counts
include a flattening of the luminosity function below MB
≡ +5.2, increased central concentration of bright stars relative
to faint ones (normally interpreted as mass segregation), and mild
deviations in radial profile from King models. Crowding of the field,
which causes the counting procedure to miss faint stars preferentially
near the cluster center, contributes to all of these, and may be
responsible for all of the apparent mass segregation, but not for all of
the other two effects.
From analysis of a large sample of field blue horizontal-branch stars we
find that the mean unreddened color, w, in the
color window -0.02 <(B-V)0 < +0.18 increases outward in
the Galaxy by ˜0.025 on 2 kpc < R < 12 kpc. On the basis of
several tests of the sample, we conclude that this result is due neither
to errors in reddening estimates nor to contamination of the sample by
stars of the young disk or by the population described by Lance (1988b).
Calibration of the horizontal-branch (HB) parameter B/(B + R) with two
theoretical, age-dependent isochrone parameters and an empirical
correlation between w and B/(B + R) suggest that
the mean age of the field horizontal branch decreases outward in the
halo systematically by a few Gyr. HB models appropriate for standard red
giant evolution lead to a similar conclusion, but HB theory also permits
an interpretation of the color gradient as a consequence of a systematic
outward decrease in HB core mass at constant age, for which there is no
plausible reason.
We calculate space densities of blue horizontal-branch (BHB) stars from
our sample and recalculate space densities for RR Lyrae stars from
published data. The RR Lyrae densities are well represented by a
spheroidal power-law model in which flattening decreases outward. The
power-law exponent for the RR Lyrae stars is n = -3.2±0.1. BHB
stars in a more restricted distance range yield n = -3.5, as would be
expected from the sign of the color gradient.
Predicted densities of BHB and RR Lyrae stars in the solar neighborhood,
their ratio (6.5/1), and the abundance distribution and
horizontal-branch morphologies of globular clusters are used to estimate
the local density of red HB stars and the total local HB number density,
42 kpc-3.
Total HB populations for 44 globular clusters, derived from published
photometry and the use of King models, establish that the visual
luminosity of a globular cluster is closely proportional to the number
of HB stars that it contains. The ratio is Lv/NHB
= 540±40 uncorrected for incompleteness of the counts that would
decrease it, or possible radial stratification of HB stars in clusters
that would increase it.
From the local field HB number density, Lv/NHB,
and an assumed M/Lv = 2.5, we estimate the local halo mass
density to be 5.7 × 104 Msun
kpc-3, a value within the range estimated by other methods.
The local observed density of metal-poor RR Lyrae stars unaccountably
exceeds the number predicted by our model by a factor of at least 2.
We present a statistical analysis of halo emission for a sample of 1047 edge-on disc galaxies imaged in five bands by the Sloan Digital Sky Survey (SDSS). Stacking the homogeneously rescaled images of the galaxies, we can measure surface brightnesses as deep as mur~ 31 mag arcsec-2. The results strongly support the almost ubiquitous presence of stellar haloes around disc galaxies, whose spatial distribution is well described by a power law rho~r-3, in a moderately flattened spheroid (c/a~ 0.6). The colour estimates in g-r and r-i, although uncertain, give a clear indication for extremely red stellar populations, hinting at old ages and/or non-negligible metal enrichment. These results support the idea of haloes being assembled via early merging of satellite galaxies.
McKay et al. have recently used measurements of weak galaxy- galaxy lensing in the Sloan Digital Sky Survey to estimate the cross-correlation between galaxies and the projected dark matter density field. They have derived a relation between aperture mass within a radius of 260 h (-1) kpc, M (260), and lens galaxy luminosity that does not depend on galaxy luminosity, type or environment. In this paper, we study the cross-correlation between galaxies and dark matter using galaxy catalogues constructed from a high-resolution N -body simulation of a LambdaCDM universe. We show that the simulation reproduces the observational results reasonably well, except that the predicted mass-to-light ratio is about a factor of 2 too high. In the simulation, M (260) is approximately equal to the halo virial mass for L * galaxies. M (260) overestimates the virial mass for fainter galaxies and underestimates it for brighter galaxies. We use the simulations to show that under certain circumstances the halo virial mass may be recovered by fitting a Navarro-Frenk-White (NFW) profile to the projected galaxy-mass correlation function. If we apply our method to the observations, we find that L * galaxies typically reside in haloes of similar to2 x 10(12) h (-1) M., consistent with halo masses estimated from the observed Tully-Fisher relation. In the simulations, the halo virial mass scales with galaxy luminosity as L (1.5) for central galaxies in haloes and for galaxies in low-density regions. For satellite galaxies, and for galaxies in high-density regions, there is no simple relation between galaxy luminosity and halo mass and care must be exercised when interpreting the lensing results.
We present extremely deep charge coupled device (CCD) surface photometry of the edge-on Sc galaxy NGC 5907. Our data reach reliably to a surface brightness of R=27 mag/sq arcsec, some two magnitudes fainter than any previous work. We obtained this improvement using a 2048X2048 CCD with a wide (approximately 24 min) field, which made it possible to sky subtract directly from the galaxy frame, and by taking many dark sky flatfields. Our analysis of these data, using a full 2D model fitting procedure with a detailed error model, confirm the thin disk parameters of van der Kruit & Searle (1981). In particular, we confirm that the galaxy's disk has a radial cutoff and a constant scale height with radius. We find evidence for a stellar warp in this system, which has the same orientation as the H I warp. Our deep data also confirm that NGC 5907 has no thick disk. This suggests that theories of thick disk formation from star formation in the early stages of disk collapse, or by secular heating mechanisms, are unlikely to be correct, because they would predict that every galaxy would have a thick disk. Thick disk formation from the accretion of satellite galaxies is more likely.
Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.
We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top three magnitudes of the M31 halo are strongly present. Photometry of a more distant control field to subtract field contamination is used to derive the `cleaned' luminosity function and metallicity distribution function (MDF) of the M31 halo field. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47 +/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated by a moderately high-metallicity population ([m/H]~ -0.5) found previously in more interior M31 halo/bulge fields, and is much more metal-rich than the [m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending on AGB star contribution) metal-poor population is also present. To first order, the shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y=0.0055. It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars; the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems. Comment: to appear in the Astronomical Journal; 43 pages, including 15 figures
We have obtained Johnson V and Gunn i photometry for a large number of Local Group galaxies using the Isaac Newton Telescope Wide Field Camera (INT WFC). The majority of these galaxies are members of the M31 subgroup and the observations are deep enough to study the top few magnitudes of the red giant branch in each system. We previously measured the location of the tip of the red giant branch (TRGB) for Andromeda I, Andromeda II and M33 to within systematic uncertainties of typically < 0.05 mags (McConnachie et al. 2004). As the TRGB acts as a standard candle in old, metal poor stellar populations, we were able to derive distances to each of these galaxies. Here we derive TRGB distances to the giant spiral galaxy M31 and 13 additional dwarf galaxies - NGC205, NGC185, NGC147, Pegasus, WLM, LGS3, Cetus, Aquarius, AndIII, AndV, AndVI, AndVII and the newly discovered dwarf spheroidal AndIX. The observations for each of the dwarf galaxies were intentionally taken in photometric conditions. In addition to the distances, we also self-consistently derive the median metallicity of each system from the colour of their red giant branches. This allows us to take into account the small metallicity variation of the absolute I magnitude of the TRGB. The homogeneous nature of our data and the identical analysis applied to each of the 17 Local Group galaxies ensures that these estimates form a reliable set of distance and metallicity determinations that are ideal for comparative studies of Local Group galaxy properties.
We report the discovery of significant stellar substructure in the halo and
outer disk of our nearest large galactic neighbour, M31. Our deep panoramic
survey with the Isaac Newton Telescope Wide Field Camera currently maps out an
area of ~25 square degrees around M31, extending along the semi-major axis to
55 kpc, and is the first to allow an uninterrupted study of the density and
color distribution of individual red giant branch stars across a large fraction
of the halo of an external spiral galaxy. We find evidence for both spatial
density and metallicity (as inferred from colour information) variations, which
are often, but not always, correlated. In addition to the previously reported
giant stellar stream (Ibata 2001b), the data reveal the presence of significant
stellar overdensities at large radii close to the south-western major axis, in
the proximity of the very luminous globular cluster G1, and near the
north-eastern major axis, coinciding with and extending beyond the
previously-known `northern spur'. The most prominent metallicity variations are
found in the southern half of the halo, where two large structures with above
average metallicites are apparent; one of these coincides with the giant
stellar stream while the other corresponds to a much lower-level stellar
enhancement. Our findings contrast with, but do not conflict with, past studies
of the M31 halo and outer disk which have suggested a rather homogeneous
stellar population at large radius: the bulk of our newly-detected substructure
lies in the previously-uncharted far outer regions of the galaxy. We discuss
the possible origin of the substructure observed and the implications it has
for constraining the galaxy assembly process.
We analyse the detailed structure of a highly-inclined (i≳ 80°) disc galaxy that lies within the Hubble Ultra Deep Field (UDF). The unprecedented depth of the UDF data allow disc and extraplanar emission to be traced reliably to surface brightness
levels of μV,i,z∼ 29–30 mag arcsec−2 (corresponding to rest-frame equivalents of μg,r,i∼ 28–29 mag arcsec−2) in this redshift z= 0.32 system. We detect excess emission above the disc, which is characterized by a moderately flattened (b/a∼ 0.6) power law (I∝R−2.6). The structure and colour of this component are very similar to the stellar halo detected in an SDSS stacking analysis of
local disc galaxies and lend support to the idea that we have detected a stellar halo in this distant system. Although the
peculiar colours of the halo are difficult to understand in terms of normal stellar populations, the consistency found between
the UDF and SDSS analyses suggests that they cannot be easily discounted.
We present the results of a wide-field (V,I) photometric study of the red-giant branch (RGB) stars in the outer halo of M31, in a field located 30 to 35 kpc from the center of the galaxy along the southeast minor axis. At this remote location, we find that RGB stars belonging to M31 are sparsely but definitely present, after statistical subtraction of field contamination. We derive the metallicity distribution (MDF) for the halo stars using interpolation within a standard (I,V-I) grid of RGB evolutionary tracks. The halo MDF is quite broad but dominated by a moderately high-metallicity population peaking at [m/H] ~ -0.5, strikingly different from the [m/H] ~ -1.3 level which characterizes the outer halo of the Milky Way. However,the shape and peak metallicity for this region are entirely similar to those found in other studies for the inner regions of the M31 halo, particularly our previous study of a 20-kpc region (Durrell, Harris, & Pritchet 2001) employing similar data. In summary, we find no evidence for a metallicity gradient or systematic change in the MDF out to quite large distances in the M31 halo: it appears to be a homogeneous and moderately metal-rich subsystem of the galaxy at all locations. The star counts in the 30-kpc field are also consistent with the r^1/4 law that fits the interior regions of the M31 spheroid surface brightness profile. The metal-rich MDF and the r^1/4 spheroid suggests M31 more strongly resembles a giant elliptical galaxy than other, Milky-Way-like, spirals. Comment: 28 pages, including 9 figures; accepted for publication in the Astronomical Journal
If the favored hierarchical cosmological model is correct, then the Milky Way system should have accreted and subsequently tidally destroyed ~100 low-mass galaxies in the past ~12 Gyr. We model this process using a hybrid semi-analytic plus N-body approach and show that the disrupted systems lead naturally to stellar halos with masses and density profiles much like the stellar halo of our own Galaxy. We present predictions for the properties of stellar halos and show that ours is likely dominated by substructure beyond \~50kpc and more spatially smooth within that radius. The average stellar halo density profile is expected to drop off with radius more quickly than that of the dark matter because the stellar halo is formed from the most tightly bound material in infalling systems, while the majority of the accreted dark matter is stripped and deposited at larger radii. We argue that stars associated with the inner halo should be quite different chemically from stars in surviving satellites and also different from stars in the outer halo or those liberated in recent disruption events. We discuss how deep halo surveys and chemical probes may be useful tools for uncovering evidence of accretion. Searches of this kind offer a direct test of whether cosmology is indeed hierarchical on small scales.
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