No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Star Formation Histories of Dwarf Spheroidal and Dwarf Elliptical Galaxies in the Local Universe
Abstract
We present the star formation histories (SFHs) of early-type dwarf galaxies, dSphs and dEs, in the local universe within z = 0.01. The SFHs of early-type dwarf galaxies are characterized by pre-enriched, metal-poor old stellar populations, absence of moderately old stars that have ages of a few Gyr. There are some differences in the SFHs of dSphs and dEs. In particular, dSphs formed old (≳ 10 Gyr old) metal-poor stars ∼2 times more than dEs. The effects of reionization and feedback from supernova explosions are thought to be strong enough to remove the gas left, which prevent moderately old stellar populations in dSphs. In contrast, the ejected gas are not completely removed from dEs and fall back to ignite burst of star formation at a few Gyr after the first period of violent bursts of star formation, showing a suppression of star formation at lookback time ≈9.6 Gyr. The second peak of star formation at lookback time ≈4.5 Gyr in dEs produce moderately old stellar populations. Distinction between dSphs and dEs is useful to examine the SFHS of the early-type dwarfs since the cumulative SFHs are most closely related to their morphology. The stellar mass plays an important role in the SFHs of the early-type dwarfs as a driver of star formation, especially in galaxies with primordial origin.
Star formation histories (SFHs) are essential for understanding galaxy formation and evolution. We present the mean SFHs of 148 dwarf lenticular galaxies (dS0s) derived from the Sloan Digital Sky Survey (SDSS) spectra. These SFHs reveal two distinct periods of active star formation. The first period, concluding approximately 6 Gyr ago, witnessed the formation of predominantly old, metal-poor (Z=0.0004) stars, accounting for approximately 60 per cent of the stellar mass and 30 per cent of the luminosity. The scarcity of extremely metal-poor (Z=0.0001) stars suggests pre-enrichment during the re-ionization era. Star formation gradually waned during this initial phase. In contrast, the second period, ending around 1 Gyr ago, exhibited a peak in the middle of the period, contributing to the formation of moderately old stellar populations with intermediate metallicity. The SFHs of dS0 galaxies show a clear dependence on stellar mass, with more massive dS0s forming stars earlier. But, we find no significant correlation with morphological properties such as outer spiral arms and nucleation. The SFHs of dS0 galaxies share many similarities with those of dE galaxies, suggesting a common origin, mostly not primordial.
Halo inhabitants are individual stars, stellar streams, star, and globular clusters, and dwarf galaxies. Here we compare the two last categories that include objects of similar stellar mass, which are often studied as self-dynamical equilibrium systems. We discover that the half-light radius of globular clusters depends on their orbital pericentre and total energy, and that Milky Way (MW) tides may explain the observed correlation. We also suggest that the accretion epoch of stellar systems in the MW halo can be calibrated by the total orbital energy, and that such a relation is due to both the mass growth of the MW and dynamical friction affecting mostly satellites with numerous orbits. This calibration starts from the bulge to Kraken, Gaia Sausage Enceladus, Sagittarius stellar systems, and finally to the new coming dwarfs, either or not linked to the vast-polar structure. The most eccentric globular clusters, and dwarfs have their half-light radius scaling as the inverse of their binding energy, and this over more than two decades. This means that earlier arriving satellites are smaller due to the tidal effects of the MW. Therefore, most halo inhabitants appear to have their structural parameters shaped by MW tides, and also by ram-pressure for the most recent arrivals, the dwarf galaxies. The correlations found in this study can be used as tools to further investigate the origin of globular clusters and dwarfs, as well as the assembly history of our Galaxy.
We present measurements of black hole masses and Eddington ratios ( λ Edd ) for a sample of 38 bright ( M 1450 < −24.4 mag) quasars at 5.8 ≲ z ≲ 7.5, derived from Very Large Telescope/X–shooter near–IR spectroscopy of their broad C iv and Mg ii emission lines. The black hole masses (on average, M BH ∼ 4.6 × 10 ⁹ M ⊙ ) and accretion rates (0.1 ≲ λ Edd ≲ 1.0) are broadly consistent with that of similarly luminous 0.3 ≲ z ≲ 2.3 quasars, but there is evidence for a mild increase in the Eddington ratio above z ≳ 6. Combined with deep Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [C II ] 158 μ m line from the host galaxies and VLT/MUSE investigations of the extended Ly α halos, this study provides fundamental clues to models of the formation and growth of the first massive galaxies and black holes. Compared to local scaling relations, z ≳ 5.7 black holes appear to be over-massive relative to their hosts, with accretion properties that do not change with host galaxy morphologies. Assuming that the kinematics of the T ∼ 10 ⁴ K gas, traced by the extended Ly α halos, are dominated by the gravitational potential of the dark matter halo, we observe a similar relation between black hole mass and circular velocity as reported for z ∼ 0 galaxies. These results paint a picture where the first supermassive black holes reside in massive halos at z ≳ 6 and lead the first stages of galaxy formation by rapidly growing in mass with a duty cycle of order unity. The duty cycle needs to drastically drop toward lower redshifts, while the host galaxies continue forming stars at a rate of hundreds of solar masses per year, sustained by the large reservoirs of cool gas surrounding them.
We present the structural parameters of $\sim910$ dwarf elliptical-like galaxies in the local universe ($z\lesssim0.01$) derived from the $r-$band images of the Sloan Digital Sky Survey (SDSS). We examine the dependence of structural parameters on the morphological types (dS0, dE,dE$_{bc}$, dSph, and dE$_{blue}$). There is a significant difference in the structural parameters among the five sub-types if we properly treat the light excess due to nucleation in dSph and dE galaxies. The mean surface brightness within the effective radius ($<\mu_{e}>$) of dSph galaxies is also clearly different from that of other sub-types. The frequency of disk features such as spiral arms depends on the morphology of dwarf galaxies. The most pronounced difference between dSph galaxies and other sub-types of early-type dwarf galaxies is the absence of disk feature which is thought to be closely related to their origin.
We have examined the star formation history (SFH) of Andromeda VII (And VII), the brightest and most massive dwarf spheroidal (dSph) satellite of the Andromeda galaxy (M 31). Although M 31 is surrounded by several dSph companions with old stellar populations and low metallicity, it has a metal-rich stellar halo with an age of 6−8 Gyr. This indicates that any evolutionary association between the stellar halo of M 31 and its dSph system is frail. Therefore, the question is whether And VII (a high-metallicity dSph located ∼220 kpc from M 31), can be associated with M 31's young, metal-rich halo. Here, we perform the first reconstruction of the SFH of And VII employing long-period variable (LPV) stars. As the most-evolved asymptotic giant branch (AGB) and red supergiant (RSG) stars, the birth mass of LPVs can be determined by connecting their near-infrared photometry to theoretical evolutionary tracks. We found 55 LPV candidates within two half-light radii, using multi-epoch imaging with the Isaac Newton Telescope in the i and V bands. Based on their birth mass function, the star-formation rate (SFR) of And VII was obtained as a function of cosmic time. The main epoch of star formation occurred ≃6.2 Gyr ago with a SFR of 0.006±0.002 M⊙/yr. Over the past 6 Gyr, we find slow star formation, which continued until 500 Myr ago with a SFR ∼0.0005±0.0002 M⊙ /yr. We determined And VII's stellar mass M=(13.3±5.3)×10^6 M⊙ within a half-light radius r_0.5=3.8±0.3 arcmin and metallicity Z=0.0007, and also derived its distance modulus of μ=24.38 mag.
Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at ∼50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315−45 is an ultra-diffuse galaxy with an effective radius of ∼2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics. Surprisingly, in the dwarf KK 203 we find an extended H α ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30 and 300 Myr. The H α emission is most likely linked to a ∼40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out.
The predominantly ancient stellar populations observed in the lowest mass galaxies (i.e. ultra-faint dwarfs) suggest that their star formation was suppressed by reionization. Most of the well-studied ultra-faint dwarfs, however, are within the central half of the Milky Way dark matter halo, such that they are consistent with a population that was accreted at early times and thus potentially quenched via environmental processes. To study the potential role of environment in suppressing star formation on the smallest scales, we utilize the Exploring the Local Volume in Simulations suite of N-body simulations to constrain the distribution of infall times for low-mass subhaloes likely to host the ultra-faint population. For the ultra-faint satellites of the Milky Way with star formation histories inferred from Hubble Space Telescopeimaging, we find that environment is highly unlikely to play a dominant role in quenching their star formation. Even when including the potential effects of pre-processing, there is a ≲0.1 per cent probability that environmental processes quenched all of the known ultra-faint dwarfs early enough to explain their observed star formation histories. Instead, we argue for a mass floor in the effectiveness of satellite quenching at roughly M⋆∼10^5M⊙, below which star formation in surviving galaxies is globally suppressed by reionization. We predict a large population of quenched ultra-faint dwarfs in the Local Field (1 < R/R_(vir) < 2), with as many as ∼250 to be discovered by future wide-field imaging surveys.
The environmental dependence of the morphology of dwarf galaxies in isolated satellite systems is analyzed to understand the origin of the dwarf galaxy morphology using the visually classified morphological types of 5836 local galaxies with z ≲ 0.01. We consider six sub-types of dwarf galaxies, dS0, dE, dEbc, dSph, dEblue, and dI, of which the first four sub-types are considered as early-type and the last two as late-type. The environmental parameters we consider are the projected distance from the host galaxy (rp), local and global background densities, and the host morphology. The spatial distributions of dwarf satellites of early-type galaxies are much different from those of dwarf satellites of late-type galaxies, suggesting the host morphology combined with rp plays a decisive role on the morphology of the dwarf satellite galaxies. The local and global background densities play no significant role on the morphology of dwarfs in the satellite systems hosted by early-type galaxies. However, in the satellite system hosted by late-type galaxies, the global background densities of dE and dSph satellites are significantly different from those of dEbc, dEblue, and dI satellites. The blue-cored dwarf satellites (dEbc) of early-type galaxies are likely to be located at rp > 0.3 Mpc to keep their cold gas from the ram pressure stripping by the hot corona of early-type galaxies. The spatial distribution of dEbc satellites of early-type galaxies and their global background densities suggest that their cold gas is intergalactic material accreted before they fall into the satellite systems.
We present the spatially resolved star formation history (SFH) of the Carina dwarf spheroidal galaxy, obtained from deep, wide-field g,r imaging and a metallicity distribution from the literature. Our photometry covers $\sim2$ deg$^2$, reaching up to $\sim10$ times the half-light radius of Carina with a completeness higher than $50\%$ at $g\sim24.5$, more than one magnitude fainter than the oldest turnoff. This is the first time a combination of depth and coverage of this quality has been used to derive the SFH of Carina, enabling us to trace its different populations with unprecedented accuracy. We find that Carina's SFH consists of two episodes well separated by a star formation temporal gap. These episodes occurred at old ($>10$ Gyr) and intermediate ($2$-$8$ Gyr) ages. Our measurements show that the old episode comprises the majority of the population, accounting for $54\pm5\%$ of the stellar mass within $1.3$ times the King tidal radius, while the total stellar mass derived for Carina is $1.60\pm0.09\times 10^{6} M_{\rm{\odot}}$, and the stellar mass-to-light ratio $1.8\pm0.2$. The SFH derived is consistent with no recent star formation which hints that the observed blue plume is due to blue stragglers. We conclude that the SFH of Carina evolved independently of the tidal field of the Milky Way, since the frequency and duration of its star formation events do not correlate with its orbital parameters. This result is supported by the age/metallicity relation observed in Carina, and the gradients calculated indicating that outer regions are older and more metal poor.
We perform a systematic Bayesian analysis of rotation vs. dispersion support
($v_{\rm rot} / \sigma$) in $40$ dwarf galaxies throughout the Local Volume
(LV) over a stellar mass range $10^{3.5} M_{\rm \odot} < M_{\star} < 10^8
M_{\rm \odot}$. We find that the stars in $\sim 90\%$ of the LV dwarf galaxies
studied -- both satellites and isolated systems -- are dispersion-supported. In
particular, we show that $7/10$ *isolated* dwarfs in our sample have stellar
populations with $v_{\rm rot} / \sigma < 0.6$. All have $v_{\rm rot} / \sigma
\lesssim 2$. These results challenge the traditional view that the stars in
gas-rich dwarf irregulars (dIrrs) are distributed in cold,
rotationally-supported stellar disks, while gas-poor dwarf spheroidals (dSphs)
are kinematically distinct in having dispersion-supported stars. We see no
clear trend between $v_{\rm rot} / \sigma$ and distance to the closest $\rm
L_{\star}$ galaxy, nor between $v_{\rm rot} / \sigma$ and $M_{\star}$ within
our mass range. We apply the same Bayesian analysis to four FIRE hydrodynamic
zoom-in simulations of isolated dwarf galaxies ($10^9 M_{\odot} < M_{\rm vir} <
10^{10} M_{\rm \odot}$) and show that the simulated *isolated* dIrr galaxies
have stellar ellipticities and stellar $v_{\rm rot} / \sigma$ ratios that are
consistent with the observed population of dIrrs *and* dSphs without the need
to subject these dwarfs to any external perturbations or tidal forces. We posit
that most dwarf galaxies form as puffy, dispersion-supported systems, rather
than cold, angular momentum-supported disks. If this is the case, then
transforming a dIrr into a dSph may require little more than removing its gas.
Dwarf spheroidal galaxies (dSphs) are mostly investigated in the Local Group.
DSphs are difficult targets for observations because of their small size and
very low surface brightness. Here we measure spectroscopic and photometric
parameters of three candidates for isolated dSphs, KKH65=BTS23, KK180, and
KK227, outside the Local Group. The galaxies are found to be of low metallicity
and low velocity dispersion. They are among the lowest surface brightness
objects in the Local Universe. According to the measured radial velocities,
metallicities, and structural and photometric parameters, KKH65 and KK227 are
representatives of the ultra-diffuse quenched galaxies. KKH65 and KK227 belong
to the outer parts of the groups NGC3414 and NGC5371, respectively. KK180 is
located in the Virgo cluster infall region.
We discuss how knowledge of the whole evolutionary history of dwarf galaxies,
including details on the early star formation events, can provide insight on
the origin of the different dwarf galaxy types. We suggest that these types may
be imprinted by the early conditions of formation rather than being only the
result of a recent morphological transformation driven by environmental
effects. We present precise star formation histories of a sample of Local Group
dwarf galaxies, derived from colour-magnitude diagrams reaching the oldest
main-sequence turnoffs. We argue that these galaxies can be assigned to two
basic types: fast dwarfs that started their evolution with a dominant and short
star formation event, and slow dwarfs that formed a small fraction of their
stars early and have continued forming stars until the present time (or
almost). These two different evolutionary paths do not map directly onto the
present-day morphology (dwarf spheroidal vs dwarf irregular). Slow and fast
dwarfs also differ in their inferred past location relative to the Milky Way
and/or M31, which hints that slow dwarfs were generally assembled in lower
density environments than fast dwarfs. We propose that the distinction between
a fast and slow dwarf galaxy reflects primarily the characteristic density of
the environment where they form. At a later stage, interaction with a large
host galaxy may play a role in the final gas removal and ultimate termination
of star formation.
We analyze the stellar kinematics of 39 dwarf early-type galaxies (dEs) in the Virgo Cluster. Based on the specific stellar angular momentum λRe and the ellipticity, we find 11 slow rotators and 28 fast rotators. The fast rotators in the outer parts of the Virgo Cluster rotate significantly faster than fast rotators in the inner parts of the cluster. Moreover, 10 out of the 11 slow rotators are located in the inner 3° (D < 1 Mpc) of the cluster. The fast rotators contain subtle disk-like structures that are visible in high-pass filtered optical images, while the slow rotators do not exhibit these structures. In addition, two of the dEs have kinematically decoupled cores and four more have emission partially filling in the Balmer absorption lines. These properties suggest that Virgo Cluster dEs may have originated from late-type star-forming galaxies that were transformed by the environment after their infall into the cluster. The correlation between λRe and the clustercentric distance can be explained by a scenario where low luminosity star-forming galaxies fall into the cluster, their gas is rapidly removed by ram-pressure stripping, although some of it can be retained in their core, their star formation is quenched but their stellar kinematics are preserved. After a long time in the cluster and several passes through its center, the galaxies are heated up and transformed into slow rotating dEs.
The Spitzer Survey of Stellar Structure in Galaxies (S4G) is the largest
available database of deep, homogeneous middle-infrared (mid-IR) images of
galaxies of all types. The survey, which includes 2352 nearby galaxies, reveals
galaxy morphology only minimally affected by interstellar extinction. This
paper presents an atlas and classifications of S4G galaxies in the
Comprehensive de Vaucouleurs revised Hubble-Sandage (CVRHS) system. The CVRHS
system follows the precepts of classical de Vaucouleurs (1959) morphology,
modified to include recognition of other features such as inner, outer, and
nuclear lenses, nuclear rings, bars, and disks, spheroidal galaxies, X patterns
and box/peanut structures, OLR subclass outer rings and pseudorings, bar ansae
and barlenses, parallel sequence late-types, thick disks, and embedded disks in
3D early-type systems. We show that our CVRHS classifications are internally
consistent, and that nearly half of the S4G sample consists of extreme
late-type systems (mostly bulgeless, pure disk galaxies) in the range Scd-Im.
The most common family classification for mid-IR types S0/a to Sc is SA while
that for types Scd to Sm is SB. The bars in these two type domains are very
different in mid-IR structure and morphology. This paper examines the bar,
ring, and type classification fractions in the sample, and also includes
several montages of images highlighting the various kinds of "stellar
structures" seen in mid-IR galaxy morphology.
We present an overview of a new integral field spectroscopic survey called
MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of three core
programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began
on 2014 July 1. MaNGA will investigate the internal kinematic structure and
composition of gas and stars in an unprecedented sample of 10,000 nearby
galaxies. We summarize essential characteristics of the instrument and survey
design in the context of MaNGA's key science goals and present prototype
observations to demonstrate MaNGA's scientific potential. MaNGA employs
dithered observations with 17 fiber-bundle integral field units that vary in
diameter from 12" (19 fibers) to 32" (127 fibers). Two dual-channel
spectrographs provide simultaneous wavelength coverage over 3600-10300 A at
R~2000. With a typical integration time of 3 hr, MaNGA reaches a target r-band
signal-to-noise ratio of 4-8 (per A, per 2" fiber) at 23 AB mag per sq. arcsec,
which is typical for the outskirts of MaNGA galaxies. Targets are selected with
stellar mass greater than 1e9 Msun using SDSS-I redshifts and i-band luminosity
to achieve uniform radial coverage in terms of the effective radius, an
approximately flat distribution in stellar mass, and a sample spanning a wide
range of environments. Analysis of our prototype observations demonstrates
MaNGA's ability to probe gas ionization, shed light on recent star formation
and quenching, enable dynamical modeling, decompose constituent components, and
map the composition of stellar populations. MaNGA's spatially resolved spectra
will enable an unprecedented study of the astrophysics of nearby galaxies in
the coming 6 yr.
We present a sample of dwarf galaxies that suffer ongoing disruption by the
tidal force of nearby massive galaxies. Analysing structural and stellar
population properties using the archival imaging and spectroscopic data from
the Sloan Digital Sky Survey (SDSS), we find that they are likely a `smoking
gun' example of the formation of early-type dwarf galaxies (dEs) in the galaxy
group environment through the tidal stirring. Inner cores of these galaxies are
fairly intact and the observed light profiles are well fitted with the Sersic
functions, while the tidally stretched stellar halos are prominent in the outer
parts. They are all located within the 50 kpc sky-projected distance from the
center of host galaxies and no dwarf galaxies have relative line-of-sight
velocity larger than 205 km/s to their hosts. We derive the Composite Stellar
Population (CSP) properties these galaxies by fitting the SDSS optical spectra
to a multiple-burst composite stellar population model. We find that these
galaxies accumulate a significant fraction of stellar mass within the last 1
Gyr, while they contain a majority stellar population of intermediate age of 2
to 4 Gyr. With these evidences, we argue that tidal stirring, particularly
through the galaxy-galaxy interaction, might have an important role in the
formation and evolution of dEs in the group environment, where the influence of
other gas stripping mechanism might be limited.
We present new constraints on the star formation histories of six ultra-faint
dwarf galaxies: Bootes I, Canes Venatici II, Coma Berenices, Hercules, Leo IV,
and Ursa Major I. Our analysis employs a combination of high-precision
photometry obtained with the Advanced Camera for Surveys on the Hubble Space
Telescope, medium-resolution spectroscopy obtained with the DEep Imaging
Multi-Object Spectrograph on the W.M. Keck Observatory, and updated
Victoria-Regina isochrones tailored to the abundance patterns appropriate for
these galaxies. The data for five of these Milky Way satellites are best fit by
a star formation history where at least 75% of the stars formed by z~10 (13.3
Gyr ago). All of the galaxies are consistent with 80% of the stars forming by
z~6 (12.8 Gyr ago) and 100% of the stars forming by z~3 (11.6 Gyr ago). The
similarly ancient populations of these galaxies support the hypothesis that
star formation in the smallest dark matter sub-halos was suppressed by a global
outside influence, such as the reionization of the universe.
We use a compilation of star formation histories (SFHs) and cosmological simulations to explore the impact of cosmic reionization
on nearby isolated dwarf galaxies. Nearby dwarfs show a wide diversity of SFHs; from ancient systems that completed their
star formation (SF) ∼10 Gyr ago to young dwarfs that formed the majority of their stars in the past ∼5 Gyr to ‘two-component’
systems characterized by the overlap of old and young stars. As an ensemble, SF in nearby dwarfs dips to lower-than-average
rates at intermediate times (4 < t/Gyr < 8), a feature caused in the simulation by cosmic reionization. Reionization heats the gas and drives it out of low-mass
haloes, affecting especially systems with virial temperatures of ∼2 × 104 K at zreion. SF begins before zreion in systems above this threshold; its associated feedback compounds the effects of reionization, emptying the haloes of gas
and leaving behind old stellar systems. In haloes below the threshold at zreion, reionization leads to a delay in the onset of SF that lasts until the halo grows massive enough to allow gas to cool and
form stars, leading to a system with a prominent young stellar component. ‘Two-component’ systems may be traced to late accretion
events that allow young stars to form in systems slightly above the threshold at zreion. The dearth of intermediate-age stars in nearby dwarfs might be the clearest signature of the imprint of cosmic reionization
on the SFHs of dwarf galaxies.
A key unknown of the Milky Way (MW) satellites is their orbital history, and, in particular, the time they were accreted onto the MW system since it marks the point where they experience a multitude of environmental processes. We present a new methodology for determining infall times, namely using a neural network (NN) algorithm. The NN is trained on MW-analogues in the EAGLE hydrodynamical simulation to predict if a dwarf galaxy is at first infall or a backsplash galaxy and to infer its infall time. The resulting NN predicts with 85 per cent accuracy if a galaxy currently outside the virial radius is a backsplash satellite and determines the infall times with a typical 68 per cent confidence interval of 4.4 Gyrs. Applying the NN to MW dwarfs with Gaia EDR3 proper motions, we find that all of the dwarfs within 300 kpc had been inside the Galactic halo. The overall MW satellite accretion rate agrees well with the theoretical prediction except for late times when the MW shows a second peak at a lookback time of 1.5 Gyrs corresponding to the infall of the LMC and its satellites. We also find that the quenching times for ultrafaint dwarfs show no significant correlation with infall time and thus supporting the hypothesis that they were quenched during reionisation. In contrast, dwarfs with stellar masses above 105M⊙ are found to be consistent with environmental quenching inside the Galactic halo, with star-formation ceasing on average at $0.5^{+0.9}_{-1.2}$ Gyrs after infall.
We present the cumulative star-formation histories (SFHs) of >15000 dwarf galaxies (M* = 107 − 10 M⊙) simulated with the TNG50 run of the IllustrisTNG suite across a vast range of environments. The key factors that determine the dwarfs’ SFHs are their central/satellite status and stellar mass, with centrals and more massive dwarfs assembling their stellar mass at later times, on average, compared to satellites and lower mass dwarfs. Satellites (in hosts of mass M200c, host = 1012 − 14.3 M⊙) assembled 90 per cent of their stellar mass $\sim 7.0_{-5.5}^{+3.3}$ Gyr ago, on average and within the 10th–90th percentiles, while the centrals did so only $\sim 1.0_{-0.5}^{+4.0}$ Gyr ago. TNG50 predicts a large diversity in SFHs, so that individual dwarfs can have significantly different cumulative SFHs compared to the stacked median SFHs. Satellite dwarfs with the highest stellar mass to host cluster mass ratios have the latest stellar mass assembly. Conversely, satellites at fixed stellar and host halo mass found closer to the cluster centre or accreted at earlier times show significantly earlier stellar mass assembly. These trends and the shapes of the SFHs themselves are a manifestation of the varying proportions within a given subsample of quenched versus star-forming galaxies, which exhibit markedly distinct SFH shapes. Finally, satellite dwarfs in the most massive hosts have higher SFRs at early times, well before accretion into their z = 0 host, compared to a control sample of centrals mass-matched at the time of accretion. This is the result of the satellites being preprocessed in smaller hosts prior to accretion. Our findings are useful theoretical predictions for comparison to future resolved-stellar-population observations.
Recent progress in constraining the massive accretions (>1:10) experienced by the Milky Way (MW) and the Andromeda galaxy (M31) offers an opportunity to understand the dwarf galaxy population of the Local Group. Using zoom-in dark matter-only simulations of MW-mass haloes and concentrating on subhaloes that are thought to be capable of hosting dwarf galaxies, we demonstrate that the infall of a massive progenitor is accompanied with the accretion and destruction of a large number of subhaloes. Massive accreted progenitors do not increase the total number of infalling subhaloes onto a MW-mass host, but instead focus surrounding subhaloes onto the host causing a clustering in the infall time of subhaloes. This leads to a temporary elevation in the number of subhaloes as well as changes in their cumulative radial profile within the virial radius of the host. Surviving subhaloes associated with a massive progenitor have a large diversity in their orbits. We find that the star formation quenching times of Local Group dwarf spheroidal galaxies (105M⊙ ≲ M* ≲ 107M⊙) are clustered around the times of the most massive accretions suffered by the MW and M31. Our results imply that a) the quenching time of dwarf spheroidals is a good proxy of their infall time and b) the absence of recently quenched satellites around M31 suggests that M33 is not on its first infall and was accreted much earlier.
Eridanus II (Eri II) is an ultrafaint dwarf (UFD) galaxy (MV = −7.1) located at a distance close to the Milky Way virial radius. Early shallow color–magnitude diagrams (CMDs) indicated that it possibly hosted an intermediate-age or even young stellar population, which is unusual for a galaxy of this mass. In this paper, we present new Hubble Space Telescope/Advanced Camera for Surveys CMDs reaching the oldest main-sequence turnoff with excellent photometric precision and derive a precise star formation history (SFH) for this galaxy through CMD fitting. This SFH shows that the bulk of the stellar mass in Eri II formed in an extremely short star formation burst at the earliest possible time. The derived star formation rate profile has a width at half maximum of 500 Myr and reaches a value compatible with null star formation 13 Gyr ago. However, tests with mock stellar populations and with the CMD of the globular cluster M92 indicate that the star formation period could be shorter than 100 Myr. From the quantitative determination of the amount of mass turned into stars in this early star formation burst ( ∼2 × 105 M⊙) we infer the number of supernova (SN) events and the corresponding energy injected into the interstellar medium. For reasonable estimates of the Eri II virial mass and values of the coupling efficiency of the SN energy, we conclude that Eri II could be quenched by SN feedback alone, thus casting doubts on the need to invoke cosmic reionization as the preferred explanation for the early quenching of old UFD galaxies.
We present a new derivation of the star formation history (SFH) of the dSph galaxy Fornax in two central regions, characterised by unprecedented precision and age resolution. It reveals that star formation has proceeded in sharp bursts separated by periods of low-level or quiescent activity. The SFH was derived through colour-magnitude diagram (CMD) fitting of two extremely deep Hubble Space Telescope CMDs, sampling the centre and one core radius. The attained age resolution allowed us to single out a major star formation episode at early times, a second strong burst 4.6 ± 0.4 Gyr ago and recent intermittent episodes ∼2 − 0.2 Gyr ago. Detailed testing with mock stellar populations was used to estimate the duration of the main bursts and study the occurrence of low-level star formation between them. The SFHs in both regions show common features, with activity at the same epochs and similar age-metallicity relationship. However, clear indications of a spatial gradient were also found, with mean age increasing with radius and star formation episodes being more prolonged in the centre. While some galaxy evolution models predict bursty SFHs in dwarf galaxies and thus a secular origin of the observed SFH cannot be excluded in Fornax, other evidence points to possible mergers or interactions as the cause of its bursty SFH. In particular, we calculated the Fornax orbit relative to the closest dwarfs and the Milky Way and observed a correspondence between the main intermediate-age and young events and peri-passages of Fornax around the Milky Way, possibly indicating tidally-induced star formation.
The effect of active galactic nuclei (AGN) feedback on the host galaxy, and its role in quenching or enhancing star-formation, is still uncertain due to the fact that usual star-formation rate (SFR) indicators – emission-line luminosities based on the assumption of photoionisation by young stars – cannot be used for active galaxies as the ionising source is the AGN. We thus investigate the use of SFR derived from the stellar population and its relation with that derived from the gas for a sample of 170 AGN hosts and a matched control sample of 291 galaxies. We compare the values of SFR densities obtained via the Hα emission line ($\rm \Sigma SFR_{Gas}$) for regions ionised by hot stars according to diagnostic diagrams with those obtained from stellar population synthesis ($\rm \Sigma SFR_\star$) over the last 1 to 100 Myr. We find that the $\rm \Sigma SFR_\star$over the last 20 Myrs closely reproduces the $\rm \Sigma SFR_{Gas}$, although a better match is obtained via the transformation: $\mbox{log($ \rm \Sigma SFR_\star $)} = (0.872\pm 0.004)\mbox{log($\rm \Sigma SFR_{Gas}$)} -(0.075\pm 0.006)$ (or $\mbox{log($\rm \Sigma SFR_{Gas}$)} = (1.147\pm 0.005)\mbox{log($ \rm \Sigma SFR_\star $)} +(0.086\pm 0.080)$), which is valid for both AGN hosts and non-active galaxies. We also compare the reddening obtained via the gas Hα/Hβ ratio with that derived via the full spectral fitting in the stellar population synthesis. We find that the ratio between the gas and stellar extinction is in the range 2.64 ≤AVg/AV⋆ ≤ 2.85, in approximate agreement with previous results from the literature, obtained for smaller samples. We interpret the difference as being due to the fact that the reddening of the stars is dominated by that affecting the less obscured underlying older population, while the reddening of the gas is larger as it is associated to a younger stellar population buried deeper in the dust.
We use the apostle and Auriga cosmological simulations to study the star formation histories (SFHs) of field and satellite dwarf galaxies. Despite sizeable galaxy-to-galaxy scatter, the SFHs of apostle and Auriga dwarfs exhibit robust average trends with galaxy stellar mass: faint field dwarfs (10⁵ < Mstar/M⊙ < 10⁶) have, on average, steadily declining SFHs, whereas brighter dwarfs (10⁷ < Mstar/M⊙ < 10⁹) show the opposite trend. Intermediate-mass dwarfs have roughly constant SFHs. Satellites exhibit similar average trends, but with substantially suppressed star formation in the most recent ∼5 Gyr, likely as a result of gas loss due to tidal and ram-pressure stripping after entering the haloes of their primaries. These simple mass and environmental trends are in good agreement with the derived SFHs of Local Group (LG) dwarfs whose photometry reaches the oldest main-sequence turn-off. SFHs of galaxies with less deep data show deviations from these trends, but this may be explained, at least in part, by the large galaxy-to-galaxy scatter, the limited sample size, and the large uncertainties of the inferred SFHs. Confirming the predicted mass and environmental trends will require deeper photometric data than currently available, especially for isolated dwarfs.
We present the star formation history (SFH) of the Sculptor dwarf spheroidal galaxy based on deep g, r photometry taken with Dark Energy Camera at the Blanco telescope, focusing our analysis on the central region of the galaxy extended up to ∼3 core radii. We have investigated how the SFH changes radially, subdividing the sampled area into four regions, and have detected a clear trend of star formation. All the SFHs show a single episode of star formation, with the innermost region presenting a longer period of star formation of ∼1.5 Gyr and for the outermost region the main period of star formation is confined to ∼0.5 Gyr. We observe a gradient in the mean age which is found to increase going towards the outer regions. These results suggest that Sculptor continued forming stars after the reionization epoch in its central part, while in the peripheral region, the majority of stars probably formed during the reionization epoch and soon after its end. From our analysis, Sculptor cannot be considered strictly as a fossil of the reionization epoch.
We measure the star formation histories (SFHs) of a sample of low-mass galaxies with M* < 109 M⊙ from the SDSS-IV MaNGA survey. The large number of IFU spectra for each galaxy are either combined to reach a high signal to noise ratio or used to investigate spatial variations. We use Bayesian inferences based on full spectrum fitting. Our analysis based on Bayesian evidence ratio indicates a strong preference for a model that allows the presence of an old stellar population, and that an improper model for the SFH can significantly underestimate the old population in these galaxies. The addition of near-infrared photometry to the constraining data can further distinguish between different SFH model families and significantly tighten the constraints on the mass fraction in the old population. On average more than half of the stellar mass in present-day low-mass galaxies formed at least 8 Gyr ago, while about 30 per cent within the past 4 Gyr. Satellite galaxies on average have formed their stellar mass earlier than central galaxies. The radial dependence of the SFH is quite weak. Our results suggest that most of the low-mass galaxies have an early episode of active star formation that produces a large fraction of their present stellar mass.
We derive the recent star formation histories (SFHs) of 23 active dwarf galaxies using Hubble Space Telescope observations from the Legacy Extragalactic UV Survey. We apply a color–magnitude diagram (CMD) fitting technique using two independent sets of stellar models, PARSEC-COLIBRI and MIST. Despite the nonnegligible recent activity, none of the 23 star-forming dwarfs show enhancements in the last 100 Myr larger than three times the 100 Myr average. The unweighted mean of the individual SFHs in the last 100 Myr is also consistent with a rather constant activity, irrespective of the atomic gas fraction. We confirm previous results that for dwarf galaxies, the CMD-based average star formation rates (SFRs) are generally higher than the FUV-based SFRs. For half of the sample, the 60 Myr average CMD-based SFR is more than two times the FUV SFR. In contrast, we find remarkable agreement between the 10 Myr average CMD-based SFR and the Hα-based SFR. Finally, using core helium-burning stars of intermediate mass, we study the pattern of star formation spatial progression over the past 60 Myr and speculate on the possible triggers and connections of the star formation activity with the environment in which these galaxies live. Approximately half of our galaxies show spatial progression of star formation in the last 60 Myr and/or very recent diffuse and off-center activity compared to RGB stars.
We study star formation histories (SFHs) of 500 dwarf galaxies (stellar mass M∗=10⁵−10⁹M⊙) from FIRE-2 cosmological zoom-in simulations. We compare dwarfs around individual Milky Way (MW)-mass galaxies, dwarfs in Local Group (LG)-like environments, and true field (i.e. isolated) dwarf galaxies. We reproduce observed trends wherein higher mass dwarfs quench later (if at all), regardless of environment. We also identify differences between the environments, both in terms of ‘satellite versus central’ and ‘LG versus individual MW versus isolated dwarf central.’ Around the individual MW-mass hosts, we recover the result expected from environmental quenching: central galaxies in the ‘near field’ have more extended SFHs than their satellite counterparts, with the former more closely resemble isolated (true field) dwarfs (though near-field centrals are still somewhat earlier forming). However, this difference is muted in the LG-like environments, where both near-field centrals and satellites have similar SFHs, which resemble satellites of single MW-mass hosts. This distinction is strongest for M* = 10⁶–10⁷M⊙ but exists at other masses. Our results suggest that the paired halo nature of the LG may regulate star formation in dwarf galaxies even beyond the virial radii of the MW and Andromeda. Caution is needed when comparing zoom-in simulations targeting isolated dwarf galaxies against observed dwarf galaxies in the LG.
We present the star formation histories (SFHs) of 20 faint M31 satellites (−12 ≲ M V ≲ −6) that were measured by modeling sub-horizontal branch depth color–magnitude diagrams constructed from Hubble Space Telescope ( HST ) imaging. Reinforcing previous results, we find that virtually all galaxies quenched between 3 and 9 Gyr ago, independent of luminosity, with a notable concentration 3–6 Gyr ago. This is in contrast to the Milky Way (MW) satellites, which are generally either faint with ancient quenching times or luminous with recent (<3 Gyr) quenching times. We suggest that systematic differences in the quenching times of M31 and MW satellites may be a reflection of the varying accretion histories of M31 and the MW. This result implies that the formation histories of low-mass satellites may not be broadly representative of low-mass galaxies in general. Among the M31 satellite population we identify two distinct groups based on their SFHs: one with exponentially declining SFHs ( τ ∼ 2 Gyr) and one with rising SFHs with abrupt quenching. We speculate how these two groups could be related to scenarios for a recent major merger involving M31. The Cycle 27 HST Treasury survey of M31 satellites will provide well-constrained ancient SFHs to go along with the quenching times we measure here. The discovery and characterization of M31 satellites with M V ≳ −6 would help quantify the relative contributions of reionization and environment to quenching of the lowest-mass satellites.
We report a new star formation history for the Tucana dwarf spheroidal galaxy, obtained from a new look at a deep HST/ACS colour-magnitude diagram. We combined information from the main sequence turn-off and the horizontal branch to resolve the ancient star formation rates on a finer temporal scale than previously possible. We show that Tucana experienced three major phases of star formation, two very close together at ancient times and the last one ending between 6 and 8 Gyr ago. We show that the three discrete clumps of stars on the horizontal branch are linked to the distinct episodes of star formation in Tucana. The spatial distribution of the clumps reveals that each generation of stars presents a higher concentration than the previous one. The simultaneous modelling of the horizontal branch and the main sequence turn-off also allows us to measure the amount of mass lost by red giant branch stars in Tucana with unprecedented precision, confirming dwarf spheroidals to be excellent laboratories to study the advanced evolution of low-mass stars.
Aims . To understand the stellar population content of early-type dwarf galaxies (dEs) and their environmental dependence, we compare the slopes and intrinsic scatter of color–magnitude relations (CMRs) for three nearby clusters, Fornax, Virgo, and Coma. Additionally, we present and compare internal color profiles of these galaxies to identify central blue regions with younger stars.
Methods . We use the imaging of the HST/ACS Fornax cluster in the magnitude range of −18.7 ≤ Mg ′ ≤ −16.0 to derive magnitudes, colors, and color profiles, which we compare with literature measurements from the HST/ACS Virgo and Coma Cluster Survey. We take advantage of HST accuracy to investigate and parameterize the ( g ′− z ′) color profiles of these dEs.
Results . Based on analysis of the color profiles, we report on a large number of dEs with young stellar populations at their center in all three clusters. While for Virgo and Coma the number of blue-cored dEs is found to be 85%±2% and 53%±3%, respectively, for Fornax, we find that all galaxies have a blue core. We show that bluer cores reside in fainter dEs, similar to the trend seen in nucleated dEs. We find no correlation between the luminosity of the galaxy and the size of its blue core. Moreover, a comparison of the CMRs of the three clusters shows that the scatter in the CMR of Virgo is considerably larger than in the Fornax and Coma clusters. Presenting adaptive smoothing we show that the galaxies on the blue side of the CMR often show evidence for dust extinction, which strengthens the interpretation that the bluer colors are due to young stellar populations. We also find that outliers on the red side of the CMR are more compact than what is expected for their luminosity. We find several of these red outliers in Virgo, often close to more massive galaxies. No red outlying compact early-types are found in Fornax and Coma in this magnitude range while we find three in the Virgo cluster. We show that the CMR of the Fornax and Virgo clusters are slightly bluer than that of Coma. We suggest that the large number of outliers and larger scatter found for Virgo CMR are a result of the alternative assembly history of this cluster.
In order to better understand the relationship between feedback and galactic chemical evolution, we have developed a new model for stellar feedback at grid resolutions of only a few parsecs in global disc simulations, using the adaptive mesh refinement hydrodynamics code ENZO. For the first time in galaxy-scale simulations, we simulate detailed stellar feedback from individual stars including asymptotic giant branch winds, photoelectric heating, Lyman- Werner radiation, ionizing radiation tracked through an adaptive ray-tracing radiative transfer method, and core-collapse and Type Ia supernovae. We furthermore follow the star-by-star chemical yields using tracer fields for 15 metal species: C, N, O, Na, Mg, Si, S, Ca, Mn, Fe, Ni, As, Sr, Y, and Ba. We include the yields ejected in massive stellar winds, but greatly reduce the winds' velocities due to computational constraints. We describe these methods in detail in this work and present the first results from 500 Myr of evolution of an isolated dwarf galaxy with properties similar to a Local Group, low-mass dwarf galaxy.We demonstrate that our physics and feedback model is capable of producing a dwarf galaxy whose evolution is consistent with observations in both theKennicutt-Schmidt relationship and extended Schmidt relationship. Effective feedback drives outflows with a greater metallicity than the interstellar medium (ISM), leading to low metal retention fractions consistent with observations. Finally, we demonstrate that these simulations yield valuable information on the variation in mixing behaviour of individual metal species within the multiphase ISM. © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.
Photoheating associated with reionization suppressed star formation in low-mass galaxies. Reionization was inhomogeneous, however, affecting different regions at different times. To establish the causal connection between reionization and suppression, we must take this local variation into account. We analyze the results of CoDa ('Cosmic Dawn') I, the first fully coupled radiation-hydrodynamical simulation of reionization and galaxy formation in the Local Universe, in a volume large enough to model reionization globally but with enough resolving power to follow all atomic-cooling galactic halos in that volume. For every halo identified at a given time, we find the redshift at which the surrounding IGM reionized, along with its instantaneous star formation rate ('SFR') and baryonic gas-to-dark matter ratio (Mgas/MDM). The average SFR per halo with M < 10⁹M⊙ was steady in regions not yet reionized, but declined sharply following local reionization. For M > 10¹⁰M⊙, this SFR continued through local reionization, increasing with time, instead. For 10⁹M⊙ < M < 10¹⁰M⊙, the SFR generally increased modestly through reionization, followed by a modest decline. In general, halo SFRs were higher for regions that reionized earlier. A similar pattern was found forMgas/MDM, which declined sharply following local reionization forM < 10⁹M⊙. Local reionization time correlates with local matter overdensity, which determines the local rates of structure formation and ionizing photon consumption. The earliest patches to develop structure and reionize ultimately produced more stars than they needed to finish and maintain their own reionization, exporting their 'surplus' starlight to help reionize regions that developed structure later.
A recent paper by Ge et al. performs a series of experiments with two full spectral fitting codes, PPXF and STARLIGHT, finding that the two yield consistent results when the input spectrum is not heavily reddened. For E(B - V) ≳ 0.2, however, they claim STARLIGHT leads to severe biases in the derived properties. Counterintuitively, and at odds with previous simulations, they find that this behaviour worsens significantly as the signal-to-noise ratio of the input spectrum increases. This communication shows that this is entirely due to an AV < 1 mag condition imposed while initializing the Markov chains in the code. This choice is normally irrelevant in real-life galaxy work but can become critical in artificial experiments. Alleviating this usually harmless initialization constraint changes the Ge et al. results completely, as was explained to the authors before their publication. We replicate their spectral fitting experiments, finding much smaller biases. Furthermore, both bias and scatter in the derived properties converge as signal to noise increases, as one would expect. We also show how the very output of the code provides ways of diagnosing anomalies in the fits.
The removal of gas by ram pressure stripping of galaxies is treated by a purely kinematic description. The solution has two asymptotic limits: if the duration of the ram pressure pulse exceeds the period of vertical oscillations perpendicular to the galactic plane, the commonly used quasi-static criterion of Gunn and Gott is obtained that uses the maximum ram pressure that the galaxy has experienced along its orbit. For shorter pulses, the outcome depends on the time-integrated ram pressure. This parameter pair fully describes the gas mass fraction that is stripped from a given galaxy. This approach closely reproduces results from smoothed particle hydrodynamics (SPH) simulations. We show that typical galaxies follow a very tight relation in this parameter space corresponding to a pressure pulse length of about 300 Myr. Thus, the Gunn and Gott criterion provides a good description for galaxies in larger clusters. Applying the analytic description to a sample of 232 Virgo galaxies from the GoldMine data base, we show that the intracluster medium provides indeed the ram pressures needed to explain the deficiencies. We also can distinguish current and past strippers, including objects whose stripping state was unknown. © 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
The Local Group hosts a number of star-forming dwarf galaxies that show evidence of periods of little to no star formation. We use a suite of cosmological simulations to study how star formation is reignited in such galaxies. We focus on isolated galaxies at $z=0$ with halo masses between 9.2$\times$10$^8$ M$_\odot$ and 8.4$\times$10$^9$ M$_\odot$, where star formation is typically shut off by reionization or by supernova feedback. Nearly 20% of these simulated galaxies later restart star formation, due to interactions with streams of gas in the intergalactic medium, indicating that this mechanism is relatively common in this mass range and that many isolated dwarfs at $z=0$ may not have been isolated throughout their histories. The source of this gas is not necessarily cosmic filaments. Rather, the dwarfs interact with gas thrown off by nearby galaxy mergers or streams extending from neighboring galaxies. While high ram pressure interactions of this nature lead to stripping, the encounters that reignite star formation are low density and/or low velocity and thus low ram pressure, resulting in compression of the hot gas in the halos of our dwarfs. The gas mass bound up in hot halos can be substantial -- at least an order of magnitude greater than the mass contained in HI. Consequently, we find that dwarfs that have experienced reignition tend to be more HI-rich and have a higher M$_{HI}$/M$_{*}$ ratio at $z=0$ than galaxies with continuous star formation. Using this fact, we identify galaxies in the Local Volume that might have "gappy" star formation histories, and can be studied by the Hubble Space Telescope or the James Webb Space Telescope.
We present the star formation history (SFH) of the Sextans dwarf spheroidal galaxy based on deep archive $B$,$I$ photometry taken with Suprime-Cam at Subaru telescope focusing our analysis on the inner region of the galaxy, fully located within the core radius. Within the errors of our SFH we have not detected any metallicity gradient along the considered radial distance interval. As a main result of this work we can state that the Sextans dwarf spheroidal stopped forming stars less than $\sim1.3$ Gyr after Big Bang in correspondance to the end of the reionization epoch. We have been able to constrain the duration of the main burst of star formation to $\sim0.6$ Gyr. From the calculation of the mechanical luminosity released from supernovae (SNe) during the brief episode of star formation, there are strong indications that SNe could have played an important role in the fate of Sextans, by removing almost completely the gas component, so preventing a prolonged star formation.
The faint-end slope of the galaxy mass function is usually reproduced in LCDM galaxy formation models by assuming that the fraction of baryons that turns into stars drops steeply with decreasing halo mass and essentially vanishes in haloes with maximum circular velocities V_{max}<20-30 km/s. Dark matter-dominated dwarfs should therefore have characteristic velocities of about that value, unless they are small enough to probe only the rising part of the halo circular velocity curve (i.e., half-mass radii, r_{1/2} << 1 kpc). Many dwarfs have properties in disagreement with this prediction: they are large enough to probe their halo $V_{\rm max}$ but their characteristic velocities are well below 20 km/s. These cold faint giants (an extreme example is the recently discovered Crater~2 Milky Way satellite) can only be reconciled with our $\Lambda$CDM models if they are the remnants of once massive objects heavily affected by tidal stripping. We examine this possibility using the APOSTLE cosmological hydrodynamical simulations of the Local Group. Assuming that low velocity dispersion satellites have been affected by stripping, we infer their progenitor masses, radii, and velocity dispersions, and find them in remarkable agreement with those of isolated dwarfs. Tidal stripping also explains the large scatter in the mass discrepancy-acceleration relation in the dwarf galaxy regime: tides remove preferentially dark matter from satellite galaxies, lowering their accelerations below the $a_{min} ~ 10^{-11} m/s^2 minimum expected for isolated dwarfs. In many cases, the resulting velocity dispersions are inconsistent with the predictions from Modified Newtonian Dynamics, a result that poses a possibly insurmountable challenge to that scenario.
We compare a suite of four simulated dwarf galaxies formed in 10$^{10} M_{\odot}$ haloes of collisionless Cold Dark Matter (CDM) with galaxies simulated in the same haloes with an identical galaxy formation model but a non-zero cross-section for dark matter self-interactions. These cosmological zoom-in simulations are part of the Feedback In Realistic Environments (FIRE) project and utilize the FIRE-2 model for hydrodynamics and galaxy formation physics. We find the stellar masses of the galaxies formed in Self-Interacting Dark Matter (SIDM) with $\sigma/m= 1\, cm^2/g$ are very similar to those in CDM (spanning $M_{\star} \approx 10^{5.7 - 7.0} M_{\odot}$) and all runs lie on a similar stellar mass -- size relation. The logarithmic dark matter density slope ($\alpha=d\log \rho / d\log r$) in the central $250-500$ pc remains steeper than $\alpha= -0.8$ for the CDM-Hydro simulations with stellar mass $M_{\star} \sim 10^{6.6} M_{\odot}$ and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with $\alpha >-0.4$. Moreover, the central density profiles predicted in SIDM runs without baryons are similar to the SIDM runs that include FIRE-2 baryonic physics. Thus, SIDM appears to be much more robust to the inclusion of (potentially uncertain) baryonic physics than CDM on this mass scale, suggesting SIDM will be easier to falsify than CDM using low-mass galaxies. Our FIRE simulations predict that galaxies less massive than $M_{\star} < 3 \times 10^6 M_{\odot}$ provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.
We investigate the star formation history and chemical evolution of isolated analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass range of $10^3$ solar mass $<M_{\ast}<10^6$ solar mass), from the era of the first generation of stars down to $z=0$, by performing a suite of cosmological hydrodynamic zoom-in simulations. We confirm that reionization, combined with supernova feedback, is primarily responsible for the truncated star formation in dwarf galaxies. Our work further demonstrates the importance of Population~III stars and the associated external metal-enrichment in producing low-metallicity stars ($\rm [Fe/H]\lesssim-4$), and for the origin of carbon-enhanced metal-poor (CEMP) stars. We find that dwarf galaxies are composite systems, assembled from multiple progenitor haloes, some of which hosted only Population~II stars formed in environments externally enriched by SNe in neighboring haloes. We show that such external enrichment process naturally produces extremely low-metallicity Pop~II stars. Our simulations can also reproduce the population of CEMP stars by including Pop~III SNe with their intrinsically high $\rm [C/Fe]$ yields. We show that $\alpha$-elements are enhanced at all metallicities, in particular for haloes with $\rm M_{vir}\lesssim2\times10^9$ solar mass, implying that the gas in such small dwarfs has preferentially been contaminated by Type~II SNe from Pop~III and massive Pop~II stars. We illustrate how the simulated chemical enrichment can be used to constrain the star formation histories (SFHs) of true observed dwarf galaxies. Our results indicate that observed UFDs lacking stars with $\rm [Fe/H]>-1.04$ are likely to have SFHs truncated prior to $z=3$. Finally, we predict the existence of extremely low-metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Pop~III nucleosynthesis.
A conclusive model for the formation of dwarf spheroidal (dSph) galaxies still remains elusive. Owing to their proximity to the massive spirals Milky Way (MW) and M31, various environmental processes have been invoked to explain their origin. In this context, the tidal stirring model postulates that interactions with MW-sized hosts can transform rotationally supported dwarfs, resembling present-day dwarf irregular (dIrr) galaxies, into systems with the kinematic and structural properties of dSphs. Using N-body+SPH simulations, we investigate the dependence of this transformation mechanism on the gas fraction, fgas, in the disk of the progenitor dwarf. Our numerical experiments incorporate for the first time the combined effects of radiative cooling, ram-pressure stripping, star formation, supernova (SN) winds, and a cosmic UV background. For a given orbit inside the primary galaxy, rotationally supported dwarfs with gas fractions akin to those of observed dIrrs (fgas >= 0.5), demonstrate a substantially enhanced likelihood and efficiency of transformation into dSphs relative to their collisionless (fgas = 0) counterparts. We argue that the combination of ram-pressure stripping and SN winds causes the gas-rich dwarfs to respond more impulsively to tides, augmenting their transformation. When fgas >= 0.5, disky dwarfs on previously unfavorable low-eccentricity or large-pericenter orbits are still able to transform. On the widest orbits, the transformation is incomplete; the dwarfs retain significant rotational support, a relatively flat shape, and some gas, naturally resembling transition-type systems. We conclude that tidal stirring constitutes a prevalent evolutionary mechanism for shaping the structure of dwarf galaxies within the currently favored CDM cosmological paradigm.
We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33+-0.08 Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones. An ancient burst of star formation occurred 12-14 Gyr ago. There is no sign of recent/ongoing star formation in the last 1.5 Gyr. The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent, respectively, and the total stellar mass is 4.2x10^6 Msun. It is probable that the galaxy has not experienced an interaction with M31 in the past. We also discuss star formation processes of dSphs KKR 25, KKs 03, as well as dTr KK 258. Their star formation histories were uniformly measured by us from HST/ACS observations. All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated. Evidently, the evolution of these objects has proceeded without influence of neighbours.
Using kinematic maps from the Sloan Digital Sky Survey (SDSS) Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, we reveal that the majority of low-mass quenched galaxies exhibit coherent rotation in their stellar kinematics. Our sample includes all 39 quenched low-mass galaxies observed in the first year of MaNGA. The galaxies are selected with Mr > −19.1, stellar masses 109 M⊙ < M⋆ < 5 × 109 M⊙, EWHα < 2 Å, and all have red colours (u − r) > 1.9. They lie on the size-magnitude and σ-luminosity relations for previously studied dwarf galaxies. Just six (15 ± 5.7 per cent) are found to have rotation speeds ve, rot < 15 km s−1 at ∼1 Re, and may be dominated by pressure support at all radii. Two galaxies in our sample have kinematically distinct cores in their stellar component, likely the result of accretion. Six contain ionised gas despite not hosting ongoing star formation, and this gas is typically kinematically misaligned from their stellar component. This is the first large-scale Integral Field Unit (IFU) study of low mass galaxies selected without bias against low-density environments. Nevertheless, we find the majority of these galaxies are within ∼1.5 Mpc of a bright neighbour (MK < −23; or M⋆ > 5 × 1010 M⊙), supporting the hypothesis that galaxy-galaxy or galaxy-group interactions quench star formation in low-mass galaxies. The local bright galaxy density for our sample is ρproj = 8.2 ± 2.0 Mpc−2, compared to ρproj = 2.1 ± 0.4 Mpc−2 for a star forming comparison sample, confirming that the quenched low mass galaxies are preferentially found in higher density environments.
The Initial Star formation and Lifetimes of Andromeda Satellites (ISLAndS) project uses Hubble Space Telescope imaging to study a representative sample of six Andromeda dSph satellite companion galaxies. The main goal of the program is to determine whether the star formation histories (SFHs) of the Andromeda dSph satellites demonstrate significant statistical differences from those of the Milky Way, which may be attributable to the different properties of their local environments. Our observations reach the oldest main sequence turn-offs, allowing a time resolution at the oldest ages of ~ 1 Gyr, which is comparable to the best achievable resolution in the MW satellites. We find that the six dSphs present a variety of SFHs that are not strictly correlated with luminosity or present distance from M31. Specifically, we find a significant range in quenching times (lookback times from 9 to 6 Gyr), but with all quenching times more than ~ 6 Gyr ago. In agreement with observations of Milky Way companions of similar mass, there is no evidence of complete quenching of star formation by the cosmic UV background responsible for reionization, but the possibility of a degree of quenching at reionization cannot be ruled out. We do not find significant differences between the SFHs of the three members of the vast, thin plane of satellites and the three off-plane dSphs. The primary difference between the SFHs of the ISLAndS dSphs and Milky Way dSph companions of similar luminosities and host distances is the absence of very late quenching (< 5 Gyr ago) dSphs in the ISLAndS sample. Thus, models that can reproduce satellite populations with and without late quenching satellites will be of extreme interest.
We derive stellar metallicities, light-weighted ages and stellar masses for a magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of these parameters using a large library of model spectra at medium-high resolution, covering a comprehensive range of star formation histories. The constraints we derive are set by the simultaneous fit of five spectral absorption features, which are well reproduced by our population synthesis models. By design, these constraints depend only weakly on the alpha/Fe element abundance ratio. Our sample includes galaxies of all types spanning the full range in star formation activity, from dormant early-type to actively star-forming galaxies. We show that, in the mean, galaxies follow a sequence of increasing stellar metallicity, age and stellar mass at increasing 4000AA-break strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption at fixed D4000 is associated with higher metallicity and younger age. We investigate how stellar metallicity and age depend on total galaxy stellar mass. Low-mass galaxies are typically young and metal-poor, massive galaxies old and metal-rich, with a rapid transition between these regimes over the stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration galaxies follow these relations, but there is a large dispersion in stellar metallicity at fixed stellar mass, especially for low-concentration galaxies of intermediate mass. Despite the large scatter, the relation between stellar metallicity and stellar mass is similar to the correlation between gas-phase oxygen abundance and stellar mass for star-forming galaxies. [abriged]
We present a detailed analysis of the Horizontal Branch of the Carina Dwarf
Spheroidal Galaxy by means of synthetic modelling techniques, taking
consistently into account the star formation history and metallicity evolution
as determined from main sequence and red giant branch spectroscopic
observations. We found that a range of integrated red giant branch mass loss
values of 0.1-0.14 M, increasing with metallicity, is able to reproduce the
colour extension of the old Horizontal Branch. However, leaving the mass loss
as the only free parameter is not enough to match the detailed morphology of
Carina Horizontal Branch. We explored the role played by the star formation
history on the discrepancies between synthetic and observed Horizontal
Branches. We derived a toy bursty star formation history that reproduces the
horizontal branch star counts, and also matches qualitatively the red giant and
the turn off regions. This star formation history is made of a subset of age
and [M/H] components of the star formation history based on turn off and red
giants only, and entails four separate bursts of star formation of different
strenghts, centred at 2, 5, 8.6 and 11.5 Gyr, with mean [M/H] decreasing from
\sim -1.7 to \sim -2.2 for increasing ages, and a Gaussian spread of 0.1 dex.
The comparison between the metallicity distribution function of our star
formation history and the one measured from the infrared CaT feature using a
CaT-[Fe/H] calibration shows a qualitative agreement, once taken into account
the range of [Ca/Fe] abundances measured in a sample of Carina stars, that
biases the derived [Fe/H] distribution toward too low values. In conclusion, we
have shown how the information contained within the horizontal branch of Carina
(and dwarf galaxies in general) can be extracted and interpreted to refine the
star formation history derived from red giants and turn off stars only.
Abridged
Segue 1 is the current best candidate for a "first galaxy", a system which
experienced only a single short burst of star formation and has since remained
unchanged. Here we present possible star formation scenarios which can explain
its unique metallicity distribution. While the majority of stars in all other
ultra-faint dwarfs (UFDs) are within 0.5 dex of the mean [Fe/H] for the galaxy,
5 of the 7 stars in Segue 1 have a spread of $\Delta$[Fe/H] $>0.8$ dex. We show
that this distribution of metallicities canot be explained by a gradual
build-up of stars, but instead requires clustered star formation. Chemical
tagging allows the separate unresolved delta functions in abundance space to be
associated with discrete events in space and time. This provides an opportunity
to put the enrichment events into a time sequence and unravel the history of
the system. We investigate two possible scenarios for the star formation
history of Segue 1 using Fyris Alpha simulations of gas in a $10^7$ M$_\odot$
dark matter halo. The lack of stars with intermediate metallicities $-3<$
[Fe/H] $<-2$ can be explained either by a pause in star formation caused by
supernova feedback, or by the spread of metallicities resulting from one or two
supernovae in a low-mass dark matter halo. Either possibility can reproduce the
metallicity distribution function (MDF), as well as the other observed
elemental abundances. The unusual MDF and the low luminosity of Segue 1 can be
explained by it being a first galaxy that originated with
$M_{\rm{vir}}\sim10^7$~M$_\odot$ at $z\sim10$.
The morphological types of 5836 galaxies were classified by a visual inspection of color images using the Sloan Digital Sky Survey Data Release 7 to produce a morphology catalog of a representative sample of local galaxies with . The sample galaxies are almost complete for galaxies brighter than . Our classification system is basically the same as that of the Third Reference Catalog of Bright Galaxies with some simplifications for giant galaxies. On the other hand, we distinguish the fine features of dwarf elliptical (dE)-like galaxies to classify five subtypes: dE, blue-cored dwarf ellipticals, dwarf spheroidals (dSph), blue dwarf ellipticals (dEblue), and dwarf lenticulars (dS0). In addition, we note the presence of nucleation in dE, dSph, and dS0. Elliptical galaxies and lenticular galaxies contribute only and of local galaxies, respectively, whereas spirals and irregulars contribute and , respectively. The dEblue galaxies, which are a recently discovered population of galaxies, contribute a significant fraction of dwarf galaxies. There seem to be structural differences between dSph and dE galaxies. The dSph galaxies are fainter and bluer with a shallower surface brightness gradient than dE galaxies. They also have a lower fraction of galaxies with small axis ratios () than dE galaxies. The mean projected distance to the nearest neighbor galaxy is ~260 kpc. About 1% of local galaxies have no neighbors with comparable luminosity within a projected distance of 2 Mpc.
We explore the quenching of low-mass galaxies (10^4 < Mstar < 10^8 Msun) as a
function of lookback time using the star formation histories (SFHs) of 38 Local
Group dwarf galaxies. The SFHs were derived from analyzing color-magnitude
diagrams of resolved stellar populations in archival Hubble Space
Telescope/Wide Field Planetary Camera 2 imaging. We find: (1) Lower mass
galaxies quench earlier than higher mass galaxies; (2) Inside of virial radius
there is no correlation between a satellite's current proximity to a massive
host and its quenching epoch; (3) There are hints of systematic differences in
quenching times of M31 and Milky Way (MW) satellites, although the sample
sample size and uncertainties in the SFHs of M31 dwarfs prohibit definitive
conclusions. Combined with literature results, we qualitatively consider the
redshift evolution (z=0-1) of the quenched galaxy fraction over ~7 dex in
stellar mass (10^4 < Mstar < 10^11.5 Msun). The quenched fraction of all
galaxies generally increases toward the present, with both the lowest and
highest mass systems exhibiting the largest quenched fractions at all
redshifts. In contrast, galaxies between Mstar ~ 10^8-10^10 Msun have the
lowest quenched fractions. We suggest that such intermediate-mass galaxies are
the least efficient at quenching. Finally, we compare our quenching times with
predictions for infall times of low-mass galaxies associated with the MW. We
find that some of the lowest-mass satellites (e.g., CVn II, Leo IV) may have
been quenched before infall while higher mass satellites (e.g., Leo I, Fornax)
typically quench ~1-4 Gyr after infall.
We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (104 < M
< 109M
☉). The SFHs are derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, and Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted "true fossils" identified by Bovill & Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., KKR 25).