Search

We present ∼300 stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI (M V = −7.5) and Andromeda XXVIII (M V = –8.8), derived using metallicity-sensitive calcium H and K narrowband Hubble Space Telescope imaging. These are the first individual stellar metallicities in And XVI (95 stars). Our And XXVIII sample (191 stars) is a factor of ∼15 increase over literature metallicities. For And XVI, we measure 〈 [Fe/H] 〉 = − 2.17 − 0.05 + 0.05 , σ [Fe/H] = 0.33 − 0.07 + 0.07 , and ∇[Fe/H] = −0.23 ± 0.15 dex R e − 1 . We find that And XVI is more metal-rich than Milky Way ultrafaint dwarf galaxies of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure 〈 [Fe/H] 〉 = − 1.95 − 0.04 + 0.04 , σ [Fe/H] = 0.34 − 0.05 + 0.05 , and ∇[Fe/H]= −0.46 ± 0.10 dex R e − 1 , placing it on the dwarf galaxy mass–metallicity relation. Neither galaxy has a metallicity distribution function (MDF) with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint (L ≲ 106 L ⊙) galaxies outside the Milky Way halo. Both galaxies' gradients are consistent with predictions from the FIRE simulations, where an age–gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal-poor ([Fe/H] < –3.0) star candidates, which make up 7% of And XVI's MDF and 6% of And XXVIII's. [ABSTRACT FROM AUTHOR]

The tip of the red giant branch (TRGB) based distance method in the I band is one of the most efficient and precise techniques for measuring distances to nearby galaxies (D ≲ 15 Mpc). The TRGB in the near-infrared (NIR) is 1–2 mag brighter relative to the I band, and has the potential to expand the range over which distance measurements to nearby galaxies are feasible. Using Hubble Space Telescope (HST) imaging of 12 fields in eight nearby galaxies, we determine color-based corrections and zero-points of the TRGB in the Wide Field Camera 3 IR (WFC3/IR) F110W and F160W filters. First, we measure TRGB distances in the I band equivalent Advanced Camera System (ACS) F814W filter from resolved stellar populations with the HST. The TRGB in the ACS F814W filter is used for our distance anchor and to place the WFC3/IR magnitudes on an absolute scale. We then determine the color dependence (a proxy for metallicity/age) and zero-point of the NIR TRGB from photometry of WFC3/IR fields that overlap with the ACS fields. The new calibration is accurate to ∼1% in distance relative to the F814W TRGB. Validating the accuracy of the calibrations, we find that the distance modulus for each field using the NIR TRGB calibration agrees with the distance modulus of the same field as determined from the F814W TRGB. This is a JWST preparatory program, and the work done here will directly inform our approach to calibrating the TRGB in JWST NIRCam and NIRISS photometric filters. [ABSTRACT FROM AUTHOR]

We present a detailed analysis of the structure of the Local Group flocculent spiral galaxy M33, as measured using the Panchromatic Hubble Andromeda Treasury Triangulum Extended Region (PHATTER) survey. Leveraging the multiwavelength coverage of PHATTER, we find that the oldest populations are dominated by a smooth exponential disk with two distinct spiral arms and a classical central bar—completely distinct from what is seen in broadband optical imaging, and the first-ever confirmation of a bar in M33. We estimate a bar extent of ∼1 kpc. The two spiral arms are asymmetric in orientation and strength, and likely represent the innermost impact of the recent tidal interaction responsible for M33's warp at larger scales. The flocculent multiarmed morphology for which M33 is known is only visible in the young upper main-sequence population, which closely tracks the morphology of the interstellar medium. We investigate the stability of M33's disk, finding Q ∼ 1 over the majority of the disk. We fit multiple components to the old stellar density distribution and find that, when considering recent stellar kinematics, M33's bulk structure favors the inclusion of an accreted halo component, modeled as a broken power law. The best-fit halo has an outer power-law index of −3 and accurately describes observational evidence of M33's stellar halo from both resolved stellar spectroscopy in the disk and its stellar populations at large radius. Integrating this profile yields a total halo stellar mass of ∼5 × 108 M ⊙, for a stellar halo mass fraction of 16%, most of which resides in the innermost 2.5 kpc. [ABSTRACT FROM AUTHOR]

We present the lifetime star formation histories (SFHs) for six ultrafaint dwarf (UFD; M V > − 7.0, 4.9 < log 10 (M * (z = 0) / M ⊙) < 5.5) satellite galaxies of M31 based on deep color–magnitude diagrams constructed from Hubble Space Telescope imaging. These are the first SFHs obtained from the oldest main-sequence turnoff of UFDs outside the halo of the Milky Way (MW). We find that five UFDs formed at least 50% of their stellar mass by z = 5 (12.6 Gyr ago), similar to known UFDs around the MW, but that 10%–40% of their stellar mass formed at later times. We uncover one remarkable UFD, And xiii, which formed only 10% of its stellar mass by z = 5, and 75% in a rapid burst at z ∼ 2–3, a result that is robust to choices of underlying stellar model and is consistent with its predominantly red horizontal branch. This "young" UFD is the first of its kind and indicates that not all UFDs are necessarily quenched by reionization, which is consistent with predictions from several cosmological simulations of faint dwarf galaxies. SFHs of the combined MW and M31 samples suggest reionization did not homogeneously quench UFDs. We find that the least-massive MW UFDs (M *(z = 5) ≲ 5 × 104 M ⊙) are likely quenched by reionization, whereas more-massive M31 UFDs (M *(z = 5) ≳ 105 M ⊙) may only have their star formation suppressed by reionization and quench at a later time. We discuss these findings in the context of the evolution and quenching of UFDs. [ABSTRACT FROM AUTHOR]

We use deep narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST) to construct the metallicity distribution function (MDF) of Local Group ultra-faint dwarf galaxy Eridanus II (Eri II). When combined with archival F475W and F814W data, we measure metallicities for 60 resolved red giant branch stars as faint as m F475W ∼ 24 mag, a factor of ∼4Ă— more stars than current spectroscopic MDF determinations. We find that Eri II has a mean metallicity of [Fe/H] = âˆ'2.50 âˆ' 0.07 + 0.07 and a dispersion of σ [ Fe / H ] = 0.42 âˆ' 0.06 + 0.06 , which are consistent with spectroscopic MDFs, though more precisely constrained owing to a larger sample. We identify a handful of extremely metal-poor star candidates (EMP; [Fe/H] < âˆ'3) that are marginally bright enough for spectroscopic follow-up. The MDF of Eri II appears well described by a leaky box chemical evolution model. We also compute an updated orbital history for Eri II using Gaia eDR3 proper motions, and find that it is likely on first infall into the Milky Way. Our findings suggest that Eri II underwent an evolutionary history similar to that of an isolated galaxy. Compared to MDFs for select cosmological simulations of similar mass galaxies, we find that Eri II has a lower fraction of stars with [Fe/H] < âˆ'3, though such comparisons should currently be treated with caution due to a paucity of simulations, selection effects, and known limitations of CaHK for EMPs. This study demonstrates the power of deep HST CaHK imaging for measuring the MDFs of UFDs. [ABSTRACT FROM AUTHOR]

The Survey of H i in Extremely Low-mass Dwarfs (SHIELD) includes a volumetrically complete sample of 82 gas-rich dwarfs with selected from the ALFALFA survey. We are obtaining extensive follow-up observations of the SHIELD galaxies to study their gas, stellar, and chemical content, and to better understand galaxy evolution at the faint end of the H i mass function. Here, we investigate the properties of 30 SHIELD galaxies using Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis Radio Telescope observations of their neutral hydrogen. We measure tip of the red giant branch (TRGB) distances, star formation activity, and gas properties. The TRGB distances are up to 4× greater than estimates from flow models, highlighting the importance of velocity-independent distance indicators in the nearby universe. The SHIELD galaxies are in underdense regions, with 23% located in voids; one galaxy appears paired with a more massive dwarf. We quantify galaxy properties at low masses including stellar and H i masses, star formation rate (SFRs), specific SFRs, star formation efficiencies, birth-rate parameters, and gas fractions. The lowest-mass systems lie below the mass thresholds where stellar mass assembly is predicted to be impacted by reionization. Even so, we find the star formation properties follow the same trends as higher-mass gas-rich systems, albeit with a different normalization. The H i disks are small (), making it difficult to measure the H i rotation using standard techniques; we develop a new methodology and report the velocity extent, and its associated spatial extent, with robust uncertainties. [ABSTRACT FROM AUTHOR]

We constrained the progenitor masses for 169 supernova remnants (SNRs), eight historically observed supernovae (SNe), and the black hole formation candidate in NGC 6946, finding that they are consistent with originating from a standard initial mass function. Additionally, there were 16 remnants that showed no sign of nearby star formation consistent with a core-collapse SN, making them good Type Ia candidates. Using Hubble Space Telescope broadband imaging, we measured the stellar photometry of ACS/WFC fields in the F435W, F555W, F606W, and F814W filters, as well as WFC3/UVIS fields in F438W, F606W, and F814W. We then fitted this photometry with stellar evolutionary models to determine the ages of the young populations present at the positions of the SNRs and SNe. We then inferred a progenitor mass probability distribution from the fitted age distribution. For 37 SNRs, we tested how different filter combinations affected the inferred masses. We find that filters sensitive to Hα, [N ii ], and [S ii ] gas emission can bias mass estimates for remnants that rely on our technique. Using a Kolmogorov–Smirnov test analysis on our most reliable measurements, we find that the progenitor mass distribution is well matched by a power-law index of , which is consistent with a standard initial mass function. [ABSTRACT FROM AUTHOR]

The presence of asymptotic giant branch (AGB) stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g., massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (∼450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and 10 secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 108 yr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of at 108 yr down to at 1010 yr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalog is released together with finding charts to facilitate follow-up studies. [ABSTRACT FROM AUTHOR]

We present an analysis of the star formation history (SFH) of a field near the half-light radius in the Local Group dwarf irregular galaxy IC 1613 based on deep Hubble Space Telescope Advanced Camera for Surveys imaging. Our observations reach the oldest main sequence turn-off, allowing a time resolution at the oldest ages of ∼1 Gyr. Our analysis shows that the SFH of the observed field in IC 1613 is consistent with being constant over the entire lifetime of the galaxy. These observations rule out an early dominant episode of star formation in IC 1613. We compare the SFH of IC 1613 with expectations from cosmological models. Since most of the mass is in place at early times for low-mass halos, a naive expectation is that most of the star formation should have taken place at early times. Models in which star formation follows mass accretion result in too many stars formed early and gas mass fractions that are too low today (the “over-cooling problem”). The depth of the present photometry of IC 1613 shows that, at a resolution of ∼1 Gyr, the star formation rate is consistent with being constant, at even the earliest times, which is difficult to achieve in models where star formation follows mass assembly. [ABSTRACT FROM AUTHOR]

We measured the tip of the red giant branch distances to nine galaxies in the direction to the Virgo cluster using the Advanced Camera for Surveys on the Hubble Space Telescope. These distances put seven galaxies (GR 34, UGC 7512, NGC 4517, IC 3583, NGC 4600, VCC 2037, and KDG 215) in front of Virgo and two galaxies (IC 3023 and KDG 177) likely inside the cluster. Distances and radial velocities of the galaxies situated between us and the Virgo core clearly exhibit the infall phenomenon toward the cluster. In the case of spherically symmetric radial infall, we estimate the radius of the “zero-velocity surface” to be (7.2 ± 0.7) Mpc, which yields a total mass of the Virgo cluster of (8.0 ± 2.3) × 1014M☼, in good agreement with its virial mass estimates. We conclude that the Virgo outskirts do not contain significant amounts of dark matter beyond their virial radius. [ABSTRACT FROM AUTHOR]

We have mapped the distribution of young and old stars in the gaseous H I warp of NGC 4565. We find a clear correlation of young stars (<600 Myr) with the warp but no coincident old stars (>1 Gyr), which places an upper limit on the age of the structure. The formation rate of the young stars, which increased ∼300 Myr ago relative to the surrounding regions, is M☼ yr–1 kpc–2. This implies a ∼60 ± 20 Gyr depletion time of the H I warp, similar to the timescales calculated for the outer H I disks of nearby spiral galaxies. While some stars associated with the warp fall into the asymptotic giant branch (AGB) region of the color-magnitude diagram, where stars could be as old as 1 Gyr, further investigation suggests that they may be interlopers rather than real AGB stars. We discuss the implications of these age constraints for the formation of H I warps and the gas fueling of disk galaxies. [ABSTRACT FROM AUTHOR]

Based on Hubble Space Telescope observations from the Local Cosmology from Isolated Dwarfs project, we present the star formation histories, as a function of galactocentric radius, of four isolated Local Group dwarf galaxies: two dSph galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the uncertainties, coeval (∼13 Gyr) at all galactocentric radii. We find that there are no significative differences between the four galaxies in the fundamental properties (such as the normalized star formation rate or age-metallicity relation) of their outer regions (radii greater than four exponential scale lengths); at large radii, these galaxies consist exclusively of old (≳ 10.5 Gyr) metal-poor stars. The duration of star formation in the inner regions varies from galaxy to galaxy, and the extended central star formation in the dTrs produces the dichotomy between dSph and dTr galaxy types. The dTr galaxies show prominent radial stellar population gradients: The centers of these galaxies host young (≲ 1 Gyr) populations, while the age of the last formation event increases smoothly with increasing radius. This contrasts with the two dSph galaxies. Tucana shows a similar, but milder, gradient, but no gradient in age is detected Cetus. For the three galaxies with significant stellar population gradients, the exponential scale length decreases with time. These results are in agreement with outside-in scenarios of dwarf galaxy evolution, in which a quenching of the star formation toward the center occurs as the galaxy runs out of gas in the outskirts. [ABSTRACT FROM AUTHOR]

Turbulent neutral hydrogen (H I) line widths are often thought to be driven primarily by star formation (SF), but the timescale for converting SF energy to H I kinetic energy is unclear. As a complication, studies on the connection between H I line widths and SF in external galaxies often use broadband tracers for the SF rate, which must implicitly assume that SF histories (SFHs) have been constant over the timescale of the tracer. In this paper, we compare measures of Hi energy to time-resolved SFHs in a number of nearby dwarf galaxies. We find that H I energy surface density is strongly correlated only with SF that occurred 30-40 Myr ago. This timescale corresponds to the approximate lifetime of the lowest mass supernova progenitors (~8 M⊙). This analysis suggests that the coupling between SF and the neutral interstellar medium is strongest on this timescale, due either to an intrinsic delay between the release of the peak energy from SF or to the coherent effects of many supernova explosions during this interval. At ΣSFR > 10-3 M⊙ yr-1 kpc-2, we find a mean coupling efficiency between SF energy and H I energy of ∈ = 0.11 ± 0.04 using the 30-40 Myr timescale. However, unphysical efficiencies are required in lower ΣSFR systems, implying that SF is not the primary driver of Hi kinematics at ⊙SFR < 10-3 M⊙ yr-1 kpc-2. [ABSTRACT FROM AUTHOR]

We use empirical star formation histories (SFHs), measured from Hubble-Space-Telescope-based resolved star color-magnitude diagrams, as input into population synthesis codes to model the broadband spectral energy distributions (SEDs) of 50 nearby dwarf galaxies (6.5 < logM"/M☉ < 8.5, with metallicities ~10% solar). In the presence of realistic SFHs, we compare the modeled and observed SEDs from the ultraviolet (UV) through near-infrared and assess the reliability of widely used UV-based star formation rate (SFR) indicators. In the FUV through i bands, we find that the observed and modeled SEDs are in excellent agreement. In the Spitzer 3.6 μm and 4.5 μm bands, we find that modeled SEDs systematically overpredict observed luminosities by up to ~0.2 dex, depending on treatment of the TP-AGB stars in the synthesis models. We assess the reliability of UV luminosity as a SFR indicator, in light of independently constrained SFHs. We find that fluctuations in the SFHs alone can cause factor of ~2 variations in the UV luminosities relative to the assumption of a constant SFH over the past 100 Myr. These variations are not strongly correlated with UV-optical colors, implying that correcting UV-based SFRs for the effects of realistic SFHs is difficult using only the broadband SED. Additionally, for this diverse sample of galaxies, we find that stars older than 100 Myr can contribute from <5%-100% of the present day UV luminosity, highlighting the challenges in defining a characteristic star formation timescale associated with UV emission. We do find a relationship between UV emission timescale and broadband UV-optical color, though it is different than predictions based on exponentially declining SFH models. Our findings have significant implications for the comparison of UV-based SFRs across low-metallicity populations with diverse SFHs. [ABSTRACT FROM AUTHOR]

We have examined resolved stellar photometry from HST imaging surrounding 18 high-mass ×-ray binary (HM×B) candidates in NGC 300 and NGC 2403 as determined from combined Chandra/HST analysis. We have fit the color-magnitude distribution of the surrounding stars with stellar evolution models. All but one region in NGC 300 and two in NGC 2403 contain a population with an age between 20 and 70 Myr. One of the candidates is the ultraluminous ×-ray source in NGC 2403, which we associate with a 60 ± 5 Myr old population. These age distributions provide additional evidence that 16 of these 18 candidates are HM×Bs. Furthermore, our results suggest that the most common HM×B age in these galaxies is 40-55 Myr. This preferred age is similar to observations of HM×Bs in the SmallMagellanic Cloud, providing new evidence of this formation timescale, but in higher metallicity populations. We suggest that this preferred HM×B age is the result of the fortuitous combination of two physical effects. First, this is the age of a population when the greatest rate of core-collapse events should be occurring, maximizing neutron star production. Second, this is the age when B stars are most likely to be actively losing mass. We also discuss our results in the context of HM×B feedback in galaxies, confirming HM×Bs as a potentially important source of energy for the interstellar medium in low-mass galaxies. [ABSTRACT FROM AUTHOR]

We present detailed analysis of color-magnitude diagrams of NGC 2403, obtained from a deep (m ≲ 28) Hubble Space Telescope (HST)Wide Field Planetary Camera 2 observation of the outer disk of NGC 2403, supplemented by several shallow (m ≲ 26) HST Advanced Camera for Surveys fields.We derive the spatially resolved star formation history of NGC 2403 out to 11 disk scale lengths. In the inner portions of the galaxy, we compare the recent star formation rates (SFRs) we derive from the resolved stars with those measured using GALEX FUV + Spitzer 24μ fluxes, finding excellent agreement between the methods. Our measurements also show that the radial gradient in recent SFR mirrors the disk exponential profile to 11 scale lengths with no break, extending to SFR densities a factor of ~100 lower than those that can be measured with GALEX and Spitzer (~2×10-6 M☉ yr-1 kpc-2). Furthermore, we find that the cumulative stellar mass of the disk was formed at similar times at all radii. We compare these characteristics of NGC 2403 to those of its "morphological twins," NGC 300 and M 33, showing that the structure and age distributions of the NGC 2403 disk are more similar to those of the relatively isolated system NGC 300 than to those of the Local Group analog M 33. We also discuss the environments and Hi morphologies of these three nearby galaxies, comparing them to integrated light studies of larger samples of more distant galaxy disks. Taken together, the physical properties and evolutionary history of NGC 2403 suggest that the galaxy has had no close encounters with other M 81 group members and may be falling into the group for the first time. [ABSTRACT FROM AUTHOR]

We present the star formation history (SFH) of the faintest known star-forming galaxy, Leo T, based on deep imaging taken with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2). The HST/WFPC2 color-magnitude diagram (CMD) of Leo T is exquisitely deep, extending ~2 mag below the oldest main-sequence turnoff, permitting excellent constraints on star formation at all ages. We use a maximum likelihood CMD fitting technique to measure the SFH of Leo T assuming three different sets of stellar evolution models: Padova (solar-scaled metallicity) and BaSTI (both solar-scaled and α-enhanced metallicities). The resulting SFHs are remarkably consistent at all ages, indicating that our derived SFH is robust to the choice of stellar evolution model. From the lifetime SFH of Leo T, we find that 50% of the total stellar mass formed prior to z ~ 1 (7.6 Gyr ago). Subsequent to this epoch, the SFH of Leo T is roughly constant until the most recent ~25 Myr, where the SFH shows an abrupt drop. This decrease could be due to a cessation of star formation or stellar initial mass function sampling effects, but we are unable to distinguish between the two scenarios. Overall, our measured SFH is consistent with previously derived SFHs of Leo T. However, the HST-based solution provides improved age resolution and reduced uncertainties at all epochs. The SFH, baryonic gas fraction, and location of Leo T are unlike any of the other recently discovered faint dwarf galaxies in the Local Group, and instead bear strong resemblance to gas-rich dwarf galaxies (irregular or transition), suggesting that gas-rich dwarf galaxies may share common modes of star formation over a large range of stellar mass (~105-109 M⊙). [ABSTRACT FROM AUTHOR]

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color-magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended Hi structure in NGC 6822 is a warped disk inclined to the line of sight. [ABSTRACT FROM AUTHOR]

We compare the cumulative star formation histories (SFHs) of Local Group (LG) dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey Treasury (ANGST) sample (D ≲ 4 Mpc), in order to understand how typical the LG dwarf galaxies are relative to those in the nearby universe. The SFHs were derived in a uniform manner from high-quality optical color--magnitude diagrams constructed from Hubble Space Telescope imaging. We find that the mean cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of the ANGST sample for the three different morphological types (dwarf spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs: dTrans). We also discuss effects such as population gradients and systematic uncertainties in the stellar models that may influence the derived SFHs. Both the ANGST and LG dwarf galaxies show a consistent and strong morphology--density relationship, emphasizing the importance of environment in the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at lower densities and higher luminosities than dSphs, within this large sample. We also find that dTrans are located in similar environments to those occupied by dwarf irregular galaxies, but have systematically lower luminosities that are more comparable to those of dwarf spheroidals. The similarity of the SFHs and morphology-density relationships of the LG and ANGST dwarf galaxies suggests that the LG dwarfs are a good representation of dwarf galaxies in the local universe. [ABSTRACT FROM AUTHOR]

We derive the optical luminosity, colors, and ratios of the blue and red helium burning (HeB) stellar populations from archival Hubble Space Telescope observations of nineteen starburst dwarf galaxies and compare them with theoretical isochrones from Padova stellar evolution models across metallicities from Z = 0.001 to 0.009. We find that the observational data and the theoretical isochrones for both blue and red HeB populations overlap in optical luminosities and colors and the observed and predicted blue to red HeB ratios agree for stars older than 50 Myr over the time bins studied. These findings confirm the usefulness of applying isochrones to interpret observations of HeB populations. However, there are significant differences, especially for the red HeB population. Specifically, we find (1) offsets in color between the observations and theoretical isochrones of order 0.15 mag (0.5 mag) for the blue (red) HeB populations brighter than My ~ -4 mag, which cannot be solely due to differential extinction; (2) blue HeB stars fainter than My ~ -3 mag are bluer than predicted; (3) the slope of the red HeB sequence is shallower than predicted by a factor of ~3; and (4) the models overpredict the ratio of the most luminous blue to red HeB stars corresponding to ages ≲50 Myr. Additionally, we find that for the more metal-rich galaxies in our sample (Z ≳ 0.5 Z⊙), the red HeB stars overlap with the red giant branch stars in the color-magnitude diagrams, thus reducing their usefulness as indicators of star formation for ages ≳ 100 Myr. [ABSTRACT FROM AUTHOR]

We present uniformly measured star formation histories (SFHs) of 60 nearby (D ≲ 4 Mpc) dwarf galaxies based on color-magnitude diagrams of resolved stellar populations from images taken with the Hubble Space Telescope and analyzed as part of the ACS Nearby Galaxy Survey Treasury program (ANGST). This volume-limited sample contains 12 dwarf spheroidal (dSph)/dwarf elliptical (dE), 5 dwarf spiral, 28 dwarf irregular (dI), 12 dSph/dI (transition), and 3 tidal dwarf galaxies. The sample spans a range of ~10 mag in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best-fit SFHs, we find three significant results for dwarf galaxies in the ANGST volume: (1) the majority of dwarf galaxies formed the bulk of their mass prior to z ~ 1, regardless of current morphological type; (2) the mean SFHs of dis, transition dwarf galaxies (dTrans), and dSphs are similar over most of cosmic time, and only begin to diverge a few Gyr ago, with the clearest differences between the three appearing during the most recent 1 Gyr; and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single bursts, constant star formation rates (SFRs), or smooth, exponentially declining SFRs. The mean SFHs show clear divergence from the cosmic SFH at z ≲ 0.7, which could be evidence that low-mass systems have experienced delayed star formation relative to more massive galaxies. The sample shows a strong density-morphology relationship, such that the dSphs in the sample are less isolated than the dis. We find that the transition from a gas-rich to gas-poor galaxy cannot be solely due to internal mechanisms such as stellar feedback, and instead is likely the result of external mechanisms, e.g., ram pressure and tidal stripping and tidal forces. In terms of their environments, SFHs, and gas fractions, the majority of the dTrans appear to be low-mass dis that simply lack Hα emission, similar to Local Group (LG) dTrans DDO 210. However, a handful of dTrans have remarkably low gas fractions, suggesting that they have nearly exhausted their gas supply, analogous to LG dTrans such as Phoenix. Finally, we have also included extensive exploration of uncertainties in the SFH recovery method, including the optimization of time resolution, the effects of photometric depth, and impact of systematic uncertainties due to the limitations in current stellar evolution models. [ABSTRACT FROM AUTHOR]

We present our investigation of pre-main-sequence (PMS) stellar populations in the Large Magellanic Cloud (LMC) from imaging with Hubble Space Telescope Wide-Field Planetary Camera 2. Our targets of interest are four star-forming regions located at the periphery of the super-giant shell LMC 4 (Shapley Constellation III). The PMS stellar content of the regions is revealed through the differential Hess diagrams and the observed color-magnitude diagrams (CMDs). Further statistical analysis of stellar distributions along cross sections of the faint part of the CMDs allowed the quantitative assessment of the PMS stars census, and the isolation of faint PMS stars as the true low-mass stellar members of the regions. These distributions are found to be well represented by a double-Gaussian function, the first component of which represents the main-sequence field stars and the second the native PMS stars of each region. Based on this result, a cluster membership probability was assigned to each PMS star according to its CMD position. The higher extinction in the region LH 88 did not allow the unambiguous identification of its native stellar population. The CMD distributions of the PMS stars with the highest membership probability in the regions LH 60, LH 63, and LH 72 exhibit an extraordinary similarity among the regions, suggesting that these stars share common characteristics, as well as common recent star formation history. Considering that the regions are located at different areas of the edge of LMC 4, this finding suggests that star formation along the super-giant shell may have occurred almost simultaneously. [ABSTRACT FROM AUTHOR]

The origin of kpc-scale holes in the atomic hydrogen (H I) distributions of some nearby dwarf irregular galaxies presents an intriguing problem. Star formation histories (SFHs) derived from resolved stars give us the unique opportunity to study past star-forming events that may have helped shape the currently visible H I distribution. Our sample of five nearby dwarf irregular galaxies spans over an order of magnitude in both total H I mass and absolute B-band magnitude and is at the low-mass end of previously studied systems. We use Very Large Array H I line data to estimate the energy required to create the centrally dominant hole in each galaxy. We compare this energy estimate to the past energy released by the underlying stellar populations computed from SFHs derived from data taken with the Hubble Space Telescope. The inferred integrated stellar energy released within the characteristic ages exceeds our energy estimates for creating the holes in all cases, assuming expected efficiencies. Therefore, it appears that stellar feedback provides sufficient energy to produce the observed holes. However, we rind no obvious signature of single star-forming events responsible for the observed structures when comparing the global SFHs of each galaxy in our sample to each other or to those of dwarf irregular galaxies reported in the literature. We also fail to find evidence of a central star cluster in FUV or Ha imaging. We conclude that large H I holes are likely formed from multiple generations of star formation and only under suitable interstellar medium conditions. [ABSTRACT FROM AUTHOR]

We compare the stellar populations and complex neutral gas dynamics of the M81 group dIrr galaxy DDO 165 using data from the Hubble Space Telescope and the Very Large Array. Cannon et al. in Paper I of this series identified two kinematically distinct H I components, multiple localized high velocity gas features, and eight H I holes and shells (the largest of which spans ~2.2 × 1.1 kpc). Using the spatial and temporal information from the stellar populations in DDO 165, we compare the patterns of star formation (SF) over the past 500 Myr with the H I dynamics. We extract localized star formation histories within 6 of the 8 H I holes identified in Paper I, as well as 23 other regions that sample a range of stellar densities and neutral gas properties. From population synthesis modeling, we derive the energy outputs (from stellar winds and supernovae) of the stellar populations within these regions over the last 100 Myr, and compare with refined estimates of the energies required to create the H I holes. In all cases, we find that "feedback" is energetically capable of creating the observed structures in the interstellar medium (ISM). Numerous regions with significant energy inputs from feedback lack coherent H I structures but show prominent localized high velocity gas features; this feedback signature is a natural product of temporally and spatially distributed SF. In DDO 165, the extended period of heightened SF activity (lasting more than 1 Gyr) is energetically capable of creating the observed holes and high velocity gas features in the neutral ISM. [ABSTRACT FROM AUTHOR]

We present new multi-configuration Very Large Array H I spectral line observations of the M81 group dwarf irregular post-starburst galaxy DDO 165. The Hi morphology is complex, with multiple column density peaks surrounding a large region of very low H I surface density that is offset from the center of the stellar distribution. The bulk of the neutral gas is associated with the southern section of the galaxy; a secondary peak in the north contains ~15% of the total Hi mass. These components appear to be kinematically distinct, suggesting that either tidal processes or large-scale blowout have recently shaped the interstellar medium (ISM) of DDO 165. Using spatially resolved position-velocity maps, we find multiple localized high-velocity gas features. Cross-correlating with radius-velocity analyses, we identify eight shell/hole structures in the ISM with a range of sizes (~400-900 pc) and expansion velocities (~7-11 km s-1). These structures are compared with narrow- and broadband imaging from the Kitt Peak National Observatory and the Hubble Space Telescope (HST). Using the latter data, recent works have shown that DDO 165's previous "burst" phase was extended temporally (≳1 Gyr). We thus interpret the high-velocity gas features, H I holes, and kinematically distinct components of the galaxy in the context of the immediate effects of "feedback" from recent star formation (SF). In addition to creating H I holes and shells, extended SF events are capable of creating localized high-velocity motion of the surrounding interstellar material. A companion paper connects the energetics from the H I and HST data. [ABSTRACT FROM AUTHOR]

We present deep Hubble Space Telescope WFPC2 optical observations obtained as part of the ACS Nearby Galaxy Survey Treasury as well as early release Wide Field Camera 3 ultraviolet and infrared observations of the nearby dwarf starbursting galaxy NGC 4214. Our data provide a detailed example of how covering such a broad range in wavelength provides a powerful tool for constraining the physical properties of stellar populations. The deepest data reach the ancient red clump at MF814W ~ -0.2. All of the optical data reach the main-sequence turnoff for stars younger than ~300 Myr and the blue He-burning sequence for stars younger than 500 Myr. The full color-magnitude diagram (CMD) fitting analysis shows that all three fields in our data set are consistent with ~75% of the stellar mass being older than 8 Gyr, in spite of showing a wide range in star formation rates at present. Thus, our results suggest that the scale length of NGC 4214 has remained relatively constant for many gigayears. As previously noted by others, we also find the galaxy has recently ramped up production consistent with its bright UV luminosity and its population of UV-bright massive stars. In the central field we find UV point sources with F336W magnitudes as bright as -9.9. These are as bright as stars with masses of at least 52-56 M... and ages near 4 Myr in stellar evolution models. Assuming a standard initial mass function, our CMD is well fitted by an increase in star formation rate beginning 100 Myr ago. The stellar populations of this late-type dwarf are compared with those of NGC 404, an early-type dwarf that is also the most massive galaxy in its local environment. The late-type dwarf appears to have a similar high fraction of ancient stars, suggesting that these dominant galaxies may form at early epochs even if they have low total mass and very different present-day morphologies. [ABSTRACT FROM AUTHOR]

Using Hubble Space Telescope photometry to measure star formation histories, we age-date the stellar populations surrounding supernova remnants (SNRs) in M31 and M33. We then apply stellar evolution models to the ages to infer the corresponding masses for their supernova progenitor stars. We analyze 33 M33 SNR progenitors and 29 M31 SNR progenitors in this work. We then combine these measurements with 53 previously published M31 SNR progenitor measurements to bring our total number of progenitor mass estimates to 115. To quantify the mass distributions, we fit power laws of the form dN/dM∝M–α. Our new larger sample of M31 progenitors follows a distribution with , and the M33 sample follows a distribution with . Thus both samples are consistent within the uncertainties, and the full sample across both galaxies gives . Both the individual and full distributions display a paucity of massive stars when compared to a Salpeter initial mass function, which we would expect to observe if all massive stars exploded as SN that leave behind observable SNR. If we instead fix α = 2.35 and treat the maximum mass as a free parameter, we find Mmax ∼ 35-45 M☼, indicative of a potential maximum cutoff mass for SN production. Our results suggest that either SNR surveys are biased against finding objects in the youngest (<10 Myr old) regions, or the highest mass stars do not produce SNe. [ABSTRACT FROM AUTHOR]

We have obtained deep images of the highly isolated (d = 1 Mpc) Aquarius dwarf irregular galaxy (DDO 210) with the Hubble Space Telescope Advanced Camera for Surveys. The resulting color-magnitude diagram (CMD) reaches more than a magnitude below the oldest main-sequence turnoff, allowing us to derive the star formation history (SFH) over the entire lifetime of the galaxy with a timing precision of ≈10% of the lookback time. Using a maximum likelihood fit to the CMD we find that only ≈10% of all star formation in Aquarius took place more than 10 Gyr ago (lookback time equivalent to redshift z ≈ 2). The star formation rate increased dramatically ≈6-8 Gyr ago (z ≈ 0.7-1.1) and then declined until the present time. The only known galaxy with a more extreme confirmed delay in star formation is Leo A, a galaxy of similar MH I /M⋆, dynamical mass, mean metallicity, and degree of isolation. The delayed stellar mass growth in these galaxies does not track the mean dark matter accretion rate from CDM simulations. The similarities between Leo A and Aquarius suggest that if gas is not removed from dwarf galaxies by interactions or feedback, it can linger for several gigayears without cooling in sufficient quantity to form stars efficiently. We discuss possible causes for the delay in star formation including suppression by reionization and late-time mergers. We find reasonable agreement between our measured SFHs and select cosmological simulations of isolated dwarfs. Because star formation and merger processes are both stochastic in nature, delayed star formation in various degrees is predicted to be a characteristic (but not a universal) feature of isolated small galaxies. [ABSTRACT FROM AUTHOR]

Using resolved stellar photometry measured from archival Hubble Space Telescope imaging, we generate color-magnitude diagrams of the stars within 50 pc of the locations of historic core-collapse supernovae (SNe) that took place in galaxies within 8 Mpc. We fit these color-magnitude distributions with stellar evolution models to determine the best-fit age distribution of the young population. We then translate these age distributions into probability distributions for the progenitor mass of each SN. The measurements are anchored by the main-sequence stars surrounding the event, making them less sensitive to assumptions about binarity, post-main-sequence evolution, or circumstellar dust. We demonstrate that, in cases where the literature contains masses that have been measured from direct imaging, our measurements are consistent with (but less precise than) these measurements. Using this technique, we constrain the progenitor masses of 17 historic SNe, 11 of which have no previous estimates from direct imaging. Our measurements still allow the possibility that all SN progenitor masses are <20 M☼. However, the large uncertainties for the highest-mass progenitors also allow the possibility of no upper-mass cutoff. [ABSTRACT FROM AUTHOR]

The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. In this work, we analyze the numbers and luminosity functions of TP-AGB stars in six quiescent, low metallicity ([Fe/H] ≲ –0.86) galaxies taken from the ACS Nearby Galaxy Survey Treasury sample, using Hubble Space Telescope (HST) photometry in both optical and near-infrared filters. The galaxies contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed TP-AGB luminosity functions and relative numbers of TP-AGB and red giant branch (RGB) stars, NTP-AGB/NRGB, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories derived from the HST deep optical observations. We test various mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). These comparisons confirm that pre-dust mass loss is important, since models that neglect pre-dust mass loss fail to explain the observed NTP-AGB/NRGB ratio or the luminosity functions. In contrast, models with more efficient pre-dust mass loss produce results consistent with observations. We find that for [Fe/H] ≲ –0.86, lower mass TP-AGB stars (M ≲ 1 M☼) must have lifetimes of ∼0.5 Myr and higher masses (M ≲ 3 M☼) must have lifetimes ≲ 1.2 Myr. In addition, assuming our best-fitting mass-loss prescription, we show that the third dredge-up has no significant effect on TP-AGB lifetimes in this mass and metallicity range. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

We present uniformly measured star formation histories (SFHs) of 40 Local Group (LG) dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble Space Telescope imaging. We demonstrate that accurate SFHs can be recovered from CMDs that do not reach the oldest main sequence turn-off (MSTO), but emphasize that the oldest MSTO is critical for precisely constraining the earliest epochs of star formation. We find that: (1) the average lifetime SFHs of dwarf spheroidals (dSphs) can be approximated by an exponentially declining SFH with τ ∼ 5 Gyr; (2) lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dwarf irregulars (dIrrs), transition dwarfs, and dwarf ellipticals can be approximated by the combination of an exponentially declining SFH (τ ∼ 3-4 Gyr) for lookback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z = 2 ranges considerably (80% for galaxies with M < 105M☼ to 30% for galaxies with M > 107M☼) and is largely explained by environment; (5) the distinction between “ultra-faint” and “classical” dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z = 2 than the Sloan Digital Sky Survey galaxies from Leitner and the SFHs from the abundance matching models of Behroozi et al. This may indicate higher than expected star formation efficiencies at early times in low mass galaxies. Finally, we provide all the SFHs in tabulated electronic format for use by the community. [ABSTRACT FROM AUTHOR]

We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys on board the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (And II; MV = –12.0; log(M⋆/M☼) ∼ 6.7) and Andromeda XVI (And XVI; MV = –7.5; log(M⋆/M☼) ∼ 4.9) yielding color-magnitude diagrams that extend at least 1 mag below the oldest main-sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50%-70% of their total stellar mass between 12.5 and 5 Gyr ago (z ∼ 5-0.5) and both were abruptly quenched ∼5 Gyr ago (z ∼ 0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context. [ABSTRACT FROM AUTHOR]

We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury to measure the recent (<500 Myr) star formation histories (SFHs) of 33 far-UV (FUV)-bright regions in M31. The region areas ranged from ∼104 to 106 pc2, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on subkiloparsec scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100 Myr mean SFRs of the SFHs to within the 1σ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than 105 pc2 or a few hundred parsecs. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3-4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux. [ABSTRACT FROM AUTHOR]

The Survey of H I in Extremely Low-mass Dwarf (SHIELD) galaxies is an ongoing multi-wavelength program to characterize the gas, star formation, and evolution in gas-rich, very low-mass galaxies. The galaxies were selected from the first ∼10% of the H I Arecibo Legacy Fast ALFA (ALFALFA) survey based on their inferred low H I mass and low baryonic mass, and all systems have recent star formation. Thus, the SHIELD sample probes the faint end of the galaxy luminosity function for star-forming galaxies. Here, we measure the distances to the 12 SHIELD galaxies to be between 5 and 12 Mpc by applying the tip of the red giant method to the resolved stellar populations imaged by the Hubble Space Telescope. Based on these distances, the H I masses in the sample range from 4 × 106 to 6 × 107M☼, with a median H I mass of 1 × 107M☼. The tip of the red giant branch distances are up to 73% farther than flow-model estimates in the ALFALFA catalog. Because of the relatively large uncertainties of flow-model distances, we are biased toward selecting galaxies from the ALFALFA catalog where the flow model underestimates the true distances. The measured distances allow for an assessment of the native environments around the sample members. Five of the galaxies are part of the NGC 672 and NGC 784 groups, which together constitute a single structure. One galaxy is part of a larger linear ensemble of nine systems that stretches 1.6 Mpc from end to end. Three galaxies reside in regions with 1-9 neighbors, and four galaxies are truly isolated with no known system identified within a radius of 1 Mpc. [ABSTRACT FROM AUTHOR]

Using Hubble Space Telescope photometry, we age-date 59 supernova remnants (SNRs) in the spiral galaxy M31 and use these ages to estimate zero-age main-sequence masses (MZAMS) for their progenitors. To accomplish this, we create color-magnitude diagrams (CMDs) and employ CMD fitting to measure the recent star formation history of the regions surrounding cataloged SNR sites. We identify any young coeval population that likely produced the progenitor star, then assign an age and uncertainty to that population. Application of stellar evolution models allows us to infer the MZAMS from this age. Because our technique is not contingent on identification or precise location of the progenitor star, it can be applied to the location of any known SNRs. We identify significant young star formation around 53 of the 59 SNRs and assign progenitor masses to these, representing a factor of ∼2 increase over currently measured progenitor masses. We consider the remaining six SNRs as either probable Type Ia candidates or the result of core-collapse progenitors that have escaped their birth sites. In general, the distribution of recovered progenitor masses is bottom-heavy, showing a paucity of the most massive stars. If we assume a single power-law distribution, dN/dM∝Mα, then we find a distribution that is steeper than a Salpeter initial mass function (IMF) (α = –2.35). In particular, we find values of α outside the range –2.7 ⩾ α ⩾ –4.4 to be inconsistent with our measured distribution at 95% confidence. If instead we assume a distribution that follows a Salpeter IMF up to some maximum mass, then we find that values of MMax > 26 are inconsistent with the measured distribution at 95% confidence. In either scenario, the data suggest that some fraction of massive stars may not explode. The result is preliminary and requires more SNRs and further analysis. In addition, we use our distribution to estimate a minimum mass for core collapse between 7.0 and 7.8 M☼. [ABSTRACT FROM AUTHOR]

The apparent age and mass of a stellar cluster can be strongly affected by stochastic sampling of the stellar initial mass function (IMF), when inferred from the integrated color of low-mass clusters (≲104M☼). We use simulated star clusters to show that these effects are minimized when the brightest, rapidly evolving stars in a cluster can be resolved, and the light of the fainter, more numerous unresolved stars can be analyzed separately. When comparing the light from the less luminous cluster members to models of unresolved light, more accurate age estimates can be obtained than when analyzing the integrated light from the entire cluster under the assumption that the IMF is fully populated. We show the success of this technique first using simulated clusters, and then with a stellar cluster in M31. This method represents one way of accounting for the discrete, stochastic sampling of the stellar IMF in less massive clusters and can be leveraged in studies of clusters throughout the Local Group and other nearby galaxies. [ABSTRACT FROM AUTHOR]

We map the spatial distribution of recent star formation over a few × 100 Myr timescales in 15 starburst dwarf galaxies using the location of young blue helium burning stars identified from optically resolved stellar populations in archival Hubble Space Telescope observations. By comparing the star formation histories from both the high surface brightness central regions and the diffuse outer regions, we measure the degree to which the star formation has been centrally concentrated during the galaxies' starbursts, using three different metrics for the spatial concentration. We find that the galaxies span a full range in spatial concentration, from highly centralized to broadly distributed star formation. Since most starbursts have historically been identified by relatively short timescale star formation tracers (e.g., Hα emission), there could be a strong bias toward classifying only those galaxies with recent, centralized star formation as starbursts, while missing starbursts that are spatially distributed. [ABSTRACT FROM AUTHOR]

As part of the Panchromatic Hubble Andromeda Treasury multi-cycle program, we observed a 12′ × 6.′5 area of the bulge of M31 with the WFC3/UVIS filters F275W and F336W. From these data we have assembled a sample of ∼4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars, and AGB-manqué stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manqué (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or α abundances when the mass loss on the red giant branch is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch (EHB) stars and their progeny. We construct the first radial profiles of these stellar populations and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UV excess. We calculate that only a few percent of main-sequence stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population. [ABSTRACT FROM AUTHOR]

We derive distance, density, and metallicity distribution of the stellar Monoceros Overdensity (MO) in the outer Milky Way, based on deep imaging with the Subaru Telescope. We applied color-magnitude diagram fitting techniques in three stripes at galactic longitudes, l ∼ 130°, 150°, 170°, and galactic latitudes, +15° ⩽ b ⩽ +25°. The MO appears as a wall of stars at a heliocentric distance of ∼10.1 ± 0.5 kpc across the observed longitude range with no distance change. The MO stars are more metal-rich ([Fe/H] ∼ –1.0) than the nearby stars at the same latitude. These data are used to test three different models for the origin of the MO: a perturbed disk model, which predicts a significant drop in density adjacent to the MO that is not seen; a basic flared disk model, which can give a good match to the density profile but the MO metallicity implies the disk is too metal-rich to source the MO stars; and a tidal stream model, which, from the literature, brackets the distances and densities we derive for the MO, suggesting that a model can be found that would fully fit the MO data. Further data and modeling will be required to confirm or rule out the MO feature as a stream or as a flaring of the disk. [ABSTRACT FROM AUTHOR]

We analyzed the radial surface brightness profile of the spiral galaxy NGC 7793 using HST/ACS images from the GHOSTS survey and a new HST/WFC3 image across the disk break. We used the photometry of resolved stars to select distinct populations covering a wide range of stellar ages. We found breaks in the radial profiles of all stellar populations at 280″ (∼5.1 kpc). Beyond this disk break, the profiles become steeper for younger populations. This same trend is seen in numerical simulations where the outer disk is formed almost entirely by radial migration. We also found that the older stars of NGC 7793 extend significantly farther than the underlying H I disk. They are thus unlikely to have formed entirely at their current radii, unless the gas disk was substantially larger in the past. These observations thus provide evidence for substantial stellar radial migration in late-type disks. [ABSTRACT FROM AUTHOR]

The Panchromatic Hubble Andromeda Treasury (PHAT) survey is an ongoing Hubble Space Telescope (HST) multi-cycle program to obtain high spatial resolution imaging of one-third of the M31 disk at ultraviolet through near-infrared wavelengths. In this paper, we present the first installment of the PHAT stellar cluster catalog. When completed, the PHAT cluster catalog will be among the largest and most comprehensive surveys of resolved star clusters in any galaxy. The exquisite spatial resolution achieved with HST has allowed us to identify hundreds of new clusters that were previously inaccessible with existing ground-based surveys. We identify 601 clusters in the Year 1 sample, representing more than a factor of four increase over previous catalogs within the current survey area (390 arcmin2). This work presents results derived from the first ∼25% of the survey data; we estimate that the final sample will include ∼2500 clusters. For the Year 1 objects, we present a catalog with positions, radii, and six-band integrated photometry. Along with a general characterization of the cluster luminosities and colors, we discuss the cluster luminosity function, the cluster size distributions, and highlight a number of individually interesting clusters found in the Year 1 search. [ABSTRACT FROM AUTHOR]

Using star formation histories derived from optically resolved stellar populations in 19 nearby starburst dwarf galaxies observed with the Hubble Space Telescope, we measure the stellar mass surface densities of stars newly formed in the bursts. By assuming a star formation efficiency (SFE), we then calculate the inferred gas surface densities present at the onset of the starbursts. Assuming an SFE of 1%, as is often assumed in normal star-forming galaxies, and assuming that the gas was purely atomic, translates to very high H I surface densities (∼102-103M☼ pc–2), which are much higher than have been observed in dwarf galaxies. This implies either higher values of SFE in these dwarf starburst galaxies or the presence of significant amounts of H2 in dwarfs (or both). Raising the assumed SFEs to 10% or greater (in line with observations of more massive starbursts associated with merging galaxies), still results in H I surface densities higher than observed in 10 galaxies. Thus, these observations appear to require that a significant fraction of the gas in these dwarf starbursts galaxies was in the molecular form at the onset of the bursts. Our results imply molecular gas column densities in the range 1019-1021 cm–2 for the sample. In the galaxies where CO observations have been made, these densities correspond to values of the CO–H2 conversion factor (XCO) in the range >(3-80) × 1020 cm–2 (K km s–1)–1, or up to 40 × greater than Galactic XCO values. [ABSTRACT FROM AUTHOR]