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![Fig. 5. Hα+[NII] (left) and [NII] (right) intensities from Ciardullo et...](profile/Anne-Laure-Melchior/publication/252136079/figure/fig2/AS:668977990742024@1536508190644/Ha-NII-left-and-NII-right-intensities-from-Ciardullo-et-al-1988-and-Boulesteix_Q320.jpg)
The study of the gas kinematic in the central 1.5 kpc × 1.5 kpc region of M 31 has revealed several surprises. The starting point of this investigation was the detection at the IRAM-30 m telescope of molecular gas with very large line splittings up to 260 km s-1 within the beam (~40 pc). In this region, which is known for its low gas content, we al...
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... nevertheless occurred in a few positions that some signal is de- tected in the OFF positions. Near transit, we had to use position- switching mode, taking an extinction-free OFF position located at a given position from the nucleus as indicated in the last col- umn of Table 1. During the second epoch of observations, in order to sample in CO(2-1) the CO(1-0) beam and to compute the CO(2-1)/CO(1-0) line ratio, little maps of 7 points with a spacing of 5" have been done for 4 positions (A, C, D and G). ...
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We present a new HI modelling tool called \textsc{Galactus}. The program has been designed to perform automated fits of disc-galaxy models to observations. It includes a treatment for the self-absorption of the gas. The software has been released into the public domain. We describe the design philosophy and inner workings of the program. After this...
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... The inclination even decreases considerably as the radius approaches the center. Melchior & Combes (2011) found that the line-of-sight velocities of CO show two components in the central 1.5 × 1.5 kpc region of M31. They attribute the main component to the rotation of the tilted inner disk, and they attribute the second component to the perturbation caused by a recent merger between M31 and M32 (Block et al. 2006). ...
The large-scale gaseous shocks in the bulge of M31 can be naturally explained by a rotating stellar bar. We use gas dynamical models to provide an independent measurement of the bar pattern speed in M31. The gravitational potentials of our simulations are from a set of made-to-measure models constrained by stellar photometry and kinematics. If the inclination of the gas disk is fixed at i = 77°, we find that a low pattern speed of 16–20 km s ⁻¹ kpc ⁻¹ is needed to match the observed position and amplitude of the shock features, as shock positions are too close to the bar major axis in high Ω b models. The pattern speed can increase to 20–30 km s ⁻¹ kpc ⁻¹ if the inner gas disk has a slightly smaller inclination angle compared with the outer one. Including subgrid physics such as star formation and stellar feedback has minor effects on the shock amplitude, and does not change the shock position significantly. If the inner gas disk is allowed to follow a varying inclination similar to the H i and ionized gas observations, the gas models with a pattern speed of 38 km s ⁻¹ kpc ⁻¹ , which is consistent with stellar-dynamical models, can match both the shock features and the central gas features.
... Several papers have attempted to reproduce the morphology and velocity field in the inner kiloparsec of M31, based on either N-body simulations (Block et al. 2006 and references therein) or analytical modeling (Melchior & Combes 2011 and references therein). The reported simulation models provide a snapshot of the filament morphology and velocity field based on specific assumptions, e.g., an interaction of M31 with its companion M32 (Block et al. 2006) or a geometric configuration of the different gas planes in the central region (Ciardullo et al. 1988;Melchior & Combes 2011). In this work, we go a step further by using hydrodynamical simulations to see under which initial conditions the flow of a streamer of gas particles injected from a given radius into the central potential of M31 moves and evolves throughout the inner kiloparsec of the interstellar medium (ISM) of the galaxy to produce both the observed filament morphology and kinematic properties. ...
... Predictions for the mass inflow rate and arrival time of the dust/gas filaments or streamers at the center of M31 are discussed. Various models in the literature that explain the large-scale structure of the M31 galaxy are reviewed in Combes (2011) andBlaña Díaz et al. (2018). In this work, a distance to M31 of 0.78 ± 0.04 Mpc (de Grijs & Bono 2014) is used, implying 1″ = 3.76 pc. ...
... The west side of the ring is not well defined. An equivalent dust map is shown in Melchior & Combes (2011). ...
The inner kiloparsec regions surrounding sub-Eddington (luminosity less than 10 ⁻³ in Eddington units, L Edd ) supermassive black holes (BHs) often show a characteristic network of dust filaments that terminate in a nuclear spiral in the central parsecs. Here we study the role and fate of these filaments in one of the least accreting BHs known, M31 (10 ⁻⁷ L Edd ) using hydrodynamical simulations. The evolution of a streamer of gas particles moving under the barred potential of M31 is followed from kiloparsec distance to the central parsecs. After an exploratory study of initial conditions, a compelling fit to the observed dust/ionized gas morphologies and line-of-sight velocities in the inner hundreds of parsecs is produced. After several million years of streamer evolution, during which friction, thermal dissipation, and self-collisions have taken place, the gas settles into a disk tens of parsecs wide. This is fed by numerous filaments that arise from an outer circumnuclear ring and spiral toward the center. The final configuration is tightly constrained by a critical input mass in the streamer of several 10 ³ M ☉ (at an injection rate of 10 ⁻⁴ M ⊙ yr − 1 ); values above or below this lead to filament fragmentation or dispersion respectively, which are not observed. The creation of a hot gas atmosphere in the region of ∼10 ⁶ K is key to the development of a nuclear spiral during the simulation. The final inflow rate at 1 pc from the center is ∼1.7 × 10 ⁻⁷ M ☉ yr ⁻¹ , consistent with the quiescent state of the M31 BH.
... Previous surveys of CO(1-0) (Nieten et al. 2006) and HI (Braun et al. 2009) emission have revealed the global structure and kinematics of cold gas in M31. Observations of CO(2-1) lines towards several selected regions of M31 found the CO(2-1)/CO(1-0) intensity ratios to be 0. 5-0.7 in the arms (Nieten et al. 2006) and close to unity in the central region (Melchior & Combes 2011), similar to the trend found in other disc galaxies. Despite the widespread use of CO(1-0) spectroscopy to trace the molecular gas, higher CO rotational transitions are needed to better constrain the molecular gas properties, in particular its excitation. ...
We present a CO(3–2) survey of selected regions in the M31 disc as part of the JCMT large programme, HARP and SCUBA-2 High-Resolution Terahertz Andromeda Galaxy Survey (HASHTAG). The 12 CO(3–2) fields in this survey cover a total area of 60 arcmin2, spanning a deprojected radial range of 2–14 kpc across the M31 disc. Combining these observations with existing IRAM 30 m CO(1–0) observations and JCMT CO(3–2) maps of the nuclear region of M31, as well as dust temperature and star formation rate surface density maps, we are able to explore the radial distribution of the CO(3–2)/CO(1–0) integrated intensity ratio (R31) and its relationship with dust temperature and star formation. We find that the value of R31 between 2 and 9 kpc galactocentric radius is 0.14, significantly lower than what is seen in the nuclear ring at 1 kpc (R31 ∼ 0.8), only to rise again to 0.27 for the fields centred on the 10 kpc star forming ring. We also found that R31 is positively correlated with dust temperature, with Spearman’s rank correlation coefficient ρ = 0.55. The correlation between star formation rate surface density and CO(3–2) intensity is much stronger than with CO(1–0), with ρ = 0.54 compared to –0.05, suggesting that the CO(3–2) line traces warmer and denser star forming gas better. We also find that R31 correlates well with star formation rate surface density, with ρ = 0.69.
... The disc of the galaxy is inclined at 77 • to the plane of the sky, with a tilt axis E-mail: Ken.Marsh@astro.cf.ac.uk at a position angle 38 • (Fritz et al. 2012). There is also an inner ring, of approximate radius 0.7 kpc, which is less inclined than the main structure (Melchior & Combes 2011 and which may represent the aftermath of a collision with the M32 dwarf galaxy (Block et al. 2006). Studies of the inner isophotes at optical and near-infrared wavelengths have provided evidence of a large-scale bar of length ∼10 kpc [Athanassoula & Beaton 2006;Beaton et al. 2007;Opitsch 2016;Diaz, Wegg & Gerhard 2017, although the existence of such a bar has been questioned by Melchior & Combes (2011)]. ...
... There is also an inner ring, of approximate radius 0.7 kpc, which is less inclined than the main structure (Melchior & Combes 2011 and which may represent the aftermath of a collision with the M32 dwarf galaxy (Block et al. 2006). Studies of the inner isophotes at optical and near-infrared wavelengths have provided evidence of a large-scale bar of length ∼10 kpc [Athanassoula & Beaton 2006;Beaton et al. 2007;Opitsch 2016;Diaz, Wegg & Gerhard 2017, although the existence of such a bar has been questioned by Melchior & Combes (2011)]. ...
... Such a comparison is facilitated by panel (d) of Fig. 8, which represents a superposition of the [N II] radial velocities on the dust column density distribution. The general trend of the radial velocities (red-shifted at the upper left and blue-shifted at the lower right) is consistent with the general rotational motion in the kinematic model presented by Melchior & Combes (2011), involving an inner disc tilted at a position angle of 70 • and an inclination of 43 • to the plane of the sky. For the reasons discussed above, we now believe that this disc is actually a compact spiral containing a classical bar. ...
We present new Herschel-derived images of warm dust in the Andromeda galaxy, M31, with unprecedented spatial resolution (~30 pc), column density accuracy, and constraints on the three-dimensional distributions of dust temperature and dust opacity index (hence grain size and composition), based on the newPPMAP Bayesian analysis procedure.We confirm the overall radial variation of dust opacity index reported by other recent studies, including the central decrease within ~3 kpc of the nucleus. We also investigate the detailed distribution of dust in the nuclear region, a prominent feature of which is an ~500 pc bar-like structure seen previously in H α. The nature of this feature has been the subject of some debate. Our maps show it to be the site of the warmest dust, with a mean line-of-sight temperature ~30 K. A comparison with the stellar distribution, based on 2MASS data, provides strong evidence that it is a gravitationally induced bar. A comparison with radial velocity maps suggests the presence of an inflow towards the nucleus from opposite directions along this bar, fed presumably by the nuclear spiral with which it appears to connect. Such behaviour is common in large-scale bars in spiral galaxies, as is the phenomenon of nested bars whereby a subkiloparsec nuclear bar exists within a large-scale primary bar. We suggest that M31 represents an example of such nesting.
... This is an asymmetry in the M31 observations that is not reproduced by our dust modelling. CO observations in this region (Melchior & Combes 2011) indicate that the molecular gas kinematics is complex and maybe tilted in this region, and so it may be that our dust modelling is too simple here. As the bar major axis υ los profile shows (Figure 26), the BPB velocities match the observations well, suggesting that CB light contribution could be much weaker in this particular region. ...
The Andromeda galaxy (M31) contains a box/peanut bulge (BPB) entangled with a classical bulge (CB) requiring a triaxial modelling to determine the dynamics, stellar and dark matter mass. We construct made-to-measure models fitting new VIRUS-W IFU bulge stellar kinematic observations, the IRAC-3.6$\mu$m photometry, and the disc's HI rotation curve. We explore the parameter space for the 3.6$\mu$m mass-to-light ratio $(\Upsilon_{3.6})$, the bar pattern speed ($\Omega_p$), and the dark matter mass in the composite bulge ($M^B_{DM}$) within 3.2kpc. Considering Einasto dark matter profiles, we find the best models for $\Upsilon_{3.6}=0.72\pm0.02\,M_\odot/L_\odot$, $M^B_{DM}=1.2^{+0.2}_{-0.4}\times10^{10}M_\odot$ and $\Omega_p=40\pm5\,km/s/kpc$. These models have a dynamical bulge mass of $M_{dyn}^B=4.25^{+0.10}_{-0.29}\times10^{10}M_{\odot}$ including a stellar mass of $M^B=3.09^{+0.10}_{-0.12}\times10^{10}M_\odot$(73%), of which the CB has $M^{CB}=1.18^{+0.06}_{-0.07}\times10^{10}M_\odot$(28%) and the BPB $M^{BPB}=1.91\pm0.06\times10^{10}M_\odot$(45%). We also explore models with NFW haloes finding that, while the Einasto models better fit the stellar kinematics, the obtained parameters agree within the errors. The $M^B_{DM}$ values agree with adiabatically contracted cosmological NFW haloes with M31's virial mass and radius. The best model has two bulge components with completely different kinematics that only together successfully reproduce the observations ($\mu_{3.6},\upsilon_{los},\sigma_{los},h3,h4$). The modelling includes dust absorption which reproduces the observed kinematic asymmetries. Our results provide new constraints for the early formation of M31 given the lower mass found for the classical bulge and the shallow dark matter profile, as well as the secular evolution of M31 implied by the bar and its resonant interactions with the classical bulge, stellar halo and disc.
The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position–velocity diagram of [O III ] and H i , we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km s ⁻¹ in the central region of M31 (4.6 kpc × 2.3 kpc, or 20 ′ × 10 ′ ). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar-induced shocks can produce velocity jumps similar to those in [O III ]. The identified shock features in both [O III ] and H i are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as to obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.
Context. Magnetic fields play an important role in the dynamics and evolution of galaxies; however, the amplification and ordering of the initial seed fields are not fully understood. The nearby spiral galaxy M 31 is an ideal laboratory for extensive studies of magnetic fields.
Aims. Our aim was to measure the intrinsic structure of the magnetic fields in M 31 and compare them with dynamo models of field amplification.
Methods. The intensity of polarized synchrotron emission and its orientation are used to measure the orientations of the magnetic field components in the plane of the sky. The Faraday rotation measure gives information about the field components along the line of sight. With the Effelsberg 100-m telescope three deep radio continuum surveys of the Andromeda galaxy, M 31, were performed at 2.645, 4.85, and 8.35 GHz (wavelengths of 11.3, 6.2, and 3.6 cm). The λ 3.6 cm survey is the first radio survey of M 31 at such small wavelengths. Maps of the Faraday rotation measures (RMs) are calculated from the distributions of the polarization angle.
Results. At all wavelengths the total and polarized emission is concentrated in a ring-like structure of about 7–13 kpc in radius from the centre. Propagation of cosmic rays away from the star-forming regions is evident. The ring of synchrotron emission is wider than the ring of the thermal radio emission, and the radial scale length of synchrotron emission is larger than that of thermal emission. The polarized intensity from the ring in the plane of the sky varies double-periodically with azimuthal angle, indicating that the ordered magnetic field is oriented almost along the ring, with a pitch angle of −14 ° ±2° at λ 6.2 cm. The RM varies systematically along the ring. The analysis shows a large-scale sinusoidal variation with azimuthal angle, signature of an axisymmetric spiral (ASS) regular magnetic field, plus a superimposed double-periodic variation of a bisymmetric spiral (BSS) regular field with about six times smaller amplitude. The RM amplitude of (118 ± 3) rad m ⁻² between λ 6.2 cm and λ 3.6 cm is about 50% larger than between λ 11.3 cm and λ 6.2 cm, indicating that Faraday depolarization at λ 11.3 cm is stronger (i.e. with a larger Faraday thickness) than at λ 6.2 cm and λ 3.6 cm. The phase of the sinusoidal RM variation of −7 ° ±1° is interpreted as the average spiral pitch angle of the regular field. The average pitch angle of the ordered field, as derived from the intrinsic orientation of the polarized emission (corrected for Faraday rotation), is significantly smaller: −26 ° ±3°.
Conclusions. The dominating ASS plus the weaker BSS field of M 31 is the most compelling case so far of a field generated by the action of a mean-field dynamo. The difference in pitch angle of the regular and the ordered fields indicates that the ordered field contains a significant fraction of an anisotropic turbulent field that has a different pattern than the regular (ASS + BSS) magnetic field.
We analysed molecular observations performed at IRAM interferometer in CO(1-0) of the circum-nuclear region (within 250 pc) of Andromeda with 2.9″ = 11 pc resolution. We detected 12 molecular clumps in this region, corresponding to a total molecular mass of (8.4 ± 0.4)×10 ⁴ M⊙ . These clumps follow Larson’s mass-size relation, but lie well above the velocity-size relation. We discuss the possibility that these clumps are probably not virialised, but are transient agglomerations of smaller entities that might be virialised. Three of these clumps have been detected in CO(2-1) in a previous work, and we find a temperature line ratio below 0.5 in this work. With a radiative transfer analysis, we show that this gas is in non-local thermal equilibrium with a low excitation temperature ( Tex = 5 − 9 K ). We find a surface beam filling factor of order 5% and a gas density in the range 60 − 650 cm ⁻³ , which is well below the critical density. With a gas-to-stellar mass fraction of 4 × 10 ⁻⁴ and dust-to-gas ratio of 0.01, this quiescent region has exhausted its gas budget. Its spectral energy distribution is compatible with passive templates assembled from elliptical galaxies. While weak dust emission is present in the region, we show that no star formation is present and support the previous results that the dust is heated by the old and intermediate stellar population. We study the possibility that this region lies formally in the low-density part of the Kennicutt-Schmidt law in a regime where the star formation rate estimators are not completely reliable. We confirm the quiescence of the inner part of this galaxy known to lie on the green valley.
We measure the Balmer decrements of 23 of the brightest planetary nebulae (PNe) in the inner bulge (r ≲ 3′) of M31 and deredden the bright end of the region's [O iii] λ5007 PN luminosity function. We show that the most luminous PNe produce ≳1200 L o of power in their [O iii] λ5007 line, implying central star luminosities of at least ∼11,000 L o. Even with the most recent accelerated-evolution post-AGB models, such luminosities require central star masses in excess of 0.66 M o and main-sequence progenitors of at least ∼2.5 M o. Since M31's bulge has very few intermediate-age stars, we conclude that conventional single-star evolution cannot be responsible for these extremely luminous objects. We also present the circumstellar extinctions for the region's bright PNe and demonstrate that the distribution is similar to that found for PNe in the Large Magellanic Cloud, with a median value of A 5007 = 0.71. Finally, we compare our results to extinction measurements made for PNe in the E6 elliptical NGC 4697 and the interacting lenticular NGC 5128. We show that such extinctions are not unusual and that the existence of very high-mass PN central stars is a general feature of old stellar populations. Our results suggest that single-star population synthesis models significantly underestimate the maximum luminosities and total integrated light of AGB stars. © 2018. The American Astronomical Society. All rights reserved.
We present the study of stellar populations in the central 5.5 arcmin (~1.2 kpc) of the M31 bulge by using the optical colour magnitude diagram derived from the observations taken by HST Advanced Camera for Surveys observations. In order to enhance image quality and then obtain deeper photometry, we construct Nyquist-sampled images and use a deconvolution method to detect sources and measure their photometry. We demonstrate that our method performs better than DOLPHOT in the extremely crowded region. The resolved stars in the M31 bulge have been divided into nine annuli and the colour magnitude diagram fitting is performed for each of them. We confirm that the majority of stars (> 70 per cent) in the M31 bulge are indeed very old (> 5 Gyr) and metal rich ([Fe/H] ~ 0.3). At later times, the star formation rate decreased and then experienced a significant rise around 1 Gyr ago, which pervaded the entire M31 bulge. After that, stars formed at less than 500 Myr ago in the central 130 arcsec. Through simulation, we find that these intermediate-age stars cannot be the artefacts introduced by the blending effect. Our results suggest that although the majority of theM31 bulge are very old, the secular evolutionary process still continuously builds up the M31 bulge slowly. We compare our star formation history with an older analysis derived from the spectral energy distribution fitting, which suggests that the latter one is still a reasonable tool for the study of stellar populations in remote galaxies. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
