Search

Spectroscopic studies of elliptical galaxies show that their stellar population ages, mean metallicity, and $\alpha$-enhancement traced by [Mg/Fe] all increase with galaxy stellar mass or velocity dispersion. We use one-zone galactic chemical evolution (GCE) models with a flexible star formation history (SFH) to model the age, [Mg/H], and [Mg/Fe] inferred from simple stellar population (SSP) fits to observed ellipticals at $z \sim 0$ and $z \sim 0.7$. We show that an SSP fit to the spectrum computed from a full GCE model gives ages and abundances close to the light-weighted, logarithmically averaged values of the composite stellar population, , <[Mg/H]>, and <[Mg/Fe]>. With supernova Mg and Fe yields fixed to values motivated by Milky Way stellar populations, we find that predicted <[Mg/H]>- and <[Mg/Fe]>- relations are surprisingly insensitive to SFH parameters: older galaxies have higher <[Mg/Fe]>, but the detailed form of the SFH has limited impact. The star formation efficiency and outflow efficiency affect the early and late evolution of <[Mg/H]>, respectively; explaining observed trends requires higher star formation efficiency and lower outflows in more massive galaxies. With core collapse supernova yields calibrated to the plateau [Mg/Fe]$_{\rm cc} \approx0.45$ observed in many Milky Way studies, our models underpredict the observed <[Mg/Fe]> ratios of ellipticals by 0.05-0.1 dex. Increasing the core collapse yield ratio to [Mg/Fe]$_{\rm cc} = 0.55$ improves the agreement, though the models still lie below the data. We discuss potential resolutions of this discrepancy, including the possibility that many ellipticals terminate their star formation with a self-enriching, terminating burst that reduces the light-weighted age and boosts <[Mg/Fe]>.

We present $\sim300$ 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 & K narrow-band 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 $\sim15$ increase over literature metallicities. For And~XVI, we measure $\langle \mbox{[Fe/H]}\rangle = -2.17^{+0.05}_{-0.05}$, $\sigma_{\mbox{[Fe/H]}}=0.33^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.23\pm0.15$ dex $R_e^{-1}$. We find that And XVI is more metal-rich than MW UFDs of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure $\langle \mbox{[Fe/H]}\rangle = -1.95^{+0.04}_{-0.04}$, $\sigma_{\mbox{[Fe/H]}}=0.34^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.46 \pm 0.10$~dex~$R_e^{-1}$, placing it on the dwarf galaxy mass-metallicity relation. Neither galaxy has a metallicity distribution function with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint ($L \lesssim 10^6~L_{\odot}$) 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 ($\mbox{[Fe/H]} < -3.0$) star candidates, which make up 7% of And~XVI's MDF and 6% of And~XXVIII's., Comment: 17 pages, 5 figures, 4 tables, ApJ submitted; comments welcome!

The High-Resolution Multi-Object Spectrograph (HRMOS) is a facility instrument that we plan to propose for the Very Large Telescope (VLT) of the European Southern Observatory (ESO), following the initial presentation at the VLT 2030 workshop held at ESO in June 2019. HRMOS provides a combination of capabilities that are essential to carry out breakthrough science across a broad range of active research areas from stellar astrophysics and exoplanet studies to Galactic and Local Group archaeology. HRMOS fills a gap in capabilities amongst the landscape of future instrumentation planned for the next decade. The key characteristics of HRMOS will be high spectral resolution (R = 60000 - 80000) combined with multi-object (20-100) capabilities and long term stability that will provide excellent radial velocity precision and accuracy (10m/s). Initial designs predict that a SNR~100 will be achievable in about one hour for a star with mag(AB) = 15, while with the same exposure time a SNR~ 30 will be reached for a star with mag(AB) = 17. The combination of high resolution and multiplexing with wavelength coverage extending to relatively blue wavelengths (down to 380\,nm), makes HRMOS a spectrograph that will push the boundaries of our knowledge and that is envisioned as a workhorse instrument in the future. The science cases presented in this White Paper include topics and ideas developed by the Core Science Team with the contributions from the astronomical community, also through the wide participation in the first HRMOS Workshop (https://indico.ict.inaf.it/event/1547/) that took place in Firenze (Italy) in October 2021., Comment: 88 pages, 39 figures. Comments and expressions of interest are welcome by contacting members of the Core Science Team

We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope (HST) narrow-band (F395N) Calcium H & K (CaHK) imaging. Our sample is a factor of $\sim7$ larger than what is published. Our main findings are: (i) A global metallicity distribution function (MDF) with $\langle \mbox{[Fe/H]} \rangle = -1.55 \pm 0.04$ and $\sigma_{\mbox{[Fe/H]}}=0.54\pm0.03$; (ii) A metallicity gradient of $-0.54 \pm 0.07$ dex $R_e^{-1}$ ($-2.1 \pm 0.3$ dex kpc$^{-1}$) over the extent of our imaging ($\sim 2.5 R_e$), which is steeper than literature measurements. Our finding is consistent with predicted gradients from the publicly-available FIRE-2 simulations, in which bursty star formation creates stellar population gradients and dark matter cores; (iii) Tucana's bifurcated RGB has distinct metallicities: a blue RGB with $\langle \mbox{[Fe/H]} \rangle = -1.78 \pm 0.06$ and $\sigma_{\mbox{[Fe/H]}}=0.44^{+0.07}_{-0.06}$, and a red RGB with $\langle \mbox{[Fe/H]} \rangle = -1.08 \pm 0.07$ and $\sigma_{\mbox{[Fe/H]}}=0.42 \pm 0.06$; (iv) At fixed stellar mass, Tucana is more MR than MW satellites by $\sim 0.4$ dex, but its blue RGB is chemically comparable to the satellites. Tucana's MDF appears consistent with star-forming isolated dwarfs, though MDFs of the latter are not as well-populated; (v) $\sim2$% of Tucana's stars have $\mbox{[Fe/H]} < -3$ and 20% $\mbox{[Fe/H]} > -1$. We provide a catalog for community spectroscopic follow-up., Comment: Replaced with ApJ published version; 23 pages, 18 figures

We present uniformly measured stellar metallicities for 463 stars in 13 Milky Way (MW) ultra-faint dwarf galaxies (UFDs; $M_V = -7.1$ to $-0.8$) using narrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST). This represents the largest homogeneous set of stellar metallicities in UFDs, increasing the number of metallicities in these 13 galaxies by a factor of 5 and doubling the number of metallicities in all known MW UFDs. We provide the first well-populated MDFs for all galaxies in this sample, with [Fe/H] ranging from -3.0 dex to -2.0 dex, and $\sigma_{\mbox{[Fe/H]}}$ ranging from 0.3 dex to 0.7 dex. We find a nearly constant [Fe/H] ~ -2.6 over 3 decades in luminosity (~$10^2 - 10^5 L_{\odot}$), suggesting that the mass-metallicity relationship does not hold for such faint systems. We find a larger fraction (24%) of EMP ([Fe/H]<-3.0) stars across our sample compared to the literature (14%), but note that uncertainties in our most metal-poor measurements make this an upper limit. We find 19% of stars in our UFD sample to be metal-rich ([Fe/H]>-2.0), consistent with the sum of literature spectroscopic studies. MW UFDs are known to be predominantly >13 Gyr old, meaning that all stars in our sample are truly ancient, unlike metal-poor stars in the MW, which have a range of possible ages. Our UFD metallicities are not well-matched to known streams in the MW, providing further evidence that known MW substructures are not related to UFDs. We include a catalog of our stars to encourage community follow-up studies, including priority targets for ELT-era observations., Comment: 37 pages, 20 figures, ApJ accepted and published

Large-scale surveys will provide spectroscopy for $\sim$50 million resolved stars in the Milky Way and Local Group. However, these data will have a high degree of heterogeneity and most will be low-resolution ($R<10000$), posing challenges to measuring consistent and reliable stellar labels. Here, we introduce a framework for identifying and remedying these issues. By simultaneously fitting the full spectrum and Gaia photometry with the Payne, we measure $\sim$40 abundances for 8 red giants in M15. From degraded quality Keck/HIRES spectra, we evaluate trends with resolution and S/N and find that (i) $\sim$20 abundances are recovered consistently within $\lesssim$0.1 dex agreement and with $\lesssim$0.05-0.15~dex systematic uncertainties from $10000\lesssim R\lesssim80000$; (ii) for 9 elements (C, Mg, Ca, Sc, Ti, Fe, Ni, Y, Nd), this systematic precision and accuracy extends down to $R\sim2500$; and (iii) while most elements do not exhibit strong S/N-dependent systematics, there are non-negligible biases for 4 elements (C, Mg, Ca, and Dy) below $\text{S/N}\sim10$ pixel$^{-1}$. We compare statistical uncertainties from MCMC sampling to the easier-to-compute Cram\'er-Rao bounds and find that they agree for $\sim$75% of elements, indicating the latter to be a reliable and faster way to estimate uncertainties. Our analysis illustrates the great promise of low-resolution spectroscopy for stellar chemical abundance work, and ongoing improvements to stellar models (e.g., 3D-NLTE physics) will only further extend its viability to more elements and to higher precision and accuracy., Comment: 46 pages, 26 figures, submitted to ApJS. Comments welcome!

We present elemental abundance patterns (C, N, Mg, Si, Ca, Ti, V, Cr, Fe, Co, and Ni) for a population of 135 massive quiescent galaxies at $z\sim0.7$ with ultra-deep rest-frame optical spectroscopy drawn from the LEGA-C survey. We derive average ages and elemental abundances in four bins of stellar velocity dispersion ($\sigma_v$) ranging from 150$~$km$\,$s$^{-1}$ to 250$~$km$\,$s$^{-1}$ using a full-spectrum hierarchical Bayesian model. The resulting elemental abundance measurements are precise to 0.05$\,$dex. The majority of elements, as well as the total metallicity and stellar age, show a positive correlation with $\sigma_v$. Thus, the highest dispersion galaxies formed the earliest and are the most metal-rich. We find only mild or non-significant trends between [X/Fe] and $\sigma_v$, suggesting that the average star-formation timescale does not strongly depend on velocity dispersion. To first order, the abundance patterns of the $z\sim0.7$ quiescent galaxies are strikingly similar to those at $z\sim0$. However, at the lowest velocity dispersions the $z\sim0.7$ galaxies have slightly enhanced N, Mg, Ti, and Ni abundance ratios and earlier formation redshifts than their $z\sim0$ counterparts. Thus, while the higher-mass quiescent galaxy population shows little evolution, the low-mass quiescent galaxies population has grown significantly over the past six billion years. Finally, the abundance patterns of both $z\sim0$ and $z\sim0.7$ quiescent galaxies differ considerably from theoretical prediction based on a chemical evolution model, indicating that our understanding of the enrichment histories of these galaxies is still very limited., Comment: 24 pages, 12 figures, accepted for publication in ApJ

We characterize massive stars (M>8 M_sun) in the nearby (D~0.8 Mpc) extremely metal-poor (Z~5% Z_sun) galaxy Leo A using Hubble Space Telescope ultra-violet (UV), optical, and near-infrared (NIR) imaging along with Keck/LRIS and MMT/Binospec optical spectroscopy for 18 main sequence OB stars. We find that: (a) 12 of our 18 stars show emission lines, despite not being associated with an H II region, suggestive of stellar activity (e.g., mass loss, accretion, binary star interaction), which is consistent with previous predictions of enhanced activity at low metallicity; (b) 6 are Be stars, which are the first to be spectroscopically studied at such low metallicity -- these Be stars have unusual panchromatic SEDs; (c) for stars well-fit by the TLUSTY non-local thermodynamic equilibrium (non-LTE) models, the photometric and spectroscopic values of T_eff and log(g) agree to within ~0.01 dex and ~0.18 dex, respectively, indicating that NUV/optical/NIR imaging can be used to reliably characterize massive (M ~ 8-30 M_sun) main sequence star properties relative to optical spectroscopy; (d) the properties of the most massive stars in H II regions are consistent with constraints from previous nebular emission line studies; and (e) 13 stars with M>8 M_sun are >40 pc from a known star cluster or H II region. Our sample comprises ~50% of all known massive stars at Z < 10% Z_sun with derived stellar parameters, high-quality optical spectra, and panchromatic photometry., Comment: 35 pages, 18 figures

We present novel constraints on the underlying galaxy formation physics (e.g., mass loading factor, star formation history, metal retention) at $z\gtrsim7$ for the low-mass ($M_*\sim10^5$ M$_\odot$) Local Group ultra-faint dwarf galaxy (UFD) Eridanus {\sc II} (Eri II). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri II's CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri II is well-characterized by a short, exponentially declining star-formation history ($\tau_\text{SFH}=0.39\pm_{0.13}^{0.18}$ Gyr), a low star-formation efficiency ($\tau_\text{SFE}=27.56\pm_{12.92}^{25.14}$ Gyr), and a large mass-loading factor ($\eta=194.53\pm_{42.67}^{33.37}$). Our results are consistent with Eri II forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri II and is in good agreement with theoretical predictions for the mass-scaling of galactic winds. It also results in the ejection of $>$90\% of the metals produced in Eri II. We make predictions for the distribution of [Mg/Fe]-[Fe/H] in Eri II as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri II ($\text{[Fe/H]} > -2$) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era., Comment: 20 pages; 12 figures, submitted to MNRAS

We use deep narrowband Ca H&K ($F395N$) imaging taken with the Hubble Space Telescope (HST) to construct the metallicity distribution function (MDF) of Local Group (LG) ultra-faint dwarf (UFD) 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 $m_{F475W}\sim24$ mag, a factor of $\sim4$x more stars than current spectroscopic MDF determinations. We find that Eri II has a mean metallicity of [Fe/H]$=$-2.50$^{+0.07}_{-0.07}$ and a dispersion of $\sigma_{\mbox{[Fe/H]}}=0.42^{+0.06}_{-0.06}$, 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] $< -3$) that are marginally bright enough for spectroscopic follow up. Eri II's MDF 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] $< -3$, though such comparisons should currently be treated with caution due to a paucity of simulations, selection effects, and known limitations of Ca H&K for EMPs. This study demonstrates the power of deep HST CaHK imaging for measuring the MDFs of UFDs., Comment: Resubmitted to ApJ with revisions following a positive referee report of the initial draft; 29 pages, 19 figures, and 3 tables, including appendix. Comments welcome!

Increasingly powerful and multiplexed spectroscopic facilities promise detailed chemical abundance patterns for millions of resolved stars in galaxies beyond the Milky Way (MW). Here, we employ the Cram\'er-Rao Lower Bound (CRLB) to forecast the precision to which stellar abundances for metal-poor, low-mass stars outside the MW can be measured for 41 current (e.g., Keck, MMT, VLT, DESI) and planned (e.g., MSE, JWST, ELTs) spectrograph configurations. We show that moderate resolution ($R\lesssim5000$) spectroscopy at blue-optical wavelengths ($\lambda\lesssim4500$ \AA) (i) enables the recovery of 2-4 times as many elements as red-optical spectroscopy ($5000\lesssim\lambda\lesssim10000$ \AA) at similar or higher resolutions ($R\sim 10000$) and (ii) can constrain the abundances of several neutron capture elements to $\lesssim$0.3 dex. We further show that high-resolution ($R\gtrsim 20000$), low S/N ($\sim$10 pixel$^{-1}$) spectra contain rich abundance information when modeled with full spectral fitting techniques. We demonstrate that JWST/NIRSpec and ELTs can recover (i) $\sim$10 and 30 elements, respectively, for metal-poor red giants throughout the Local Group and (ii) [Fe/H] and [$\alpha$/Fe] for resolved stars in galaxies out to several Mpc with modest integration times. We show that select literature abundances are within a factor of $\sim$2 (or better) of our CRLBs. We suggest that, like ETCs, CRLBs should be used when planning stellar spectroscopic observations. We include an open source python package, \texttt{Chem-I-Calc}, that allows users to compute CRLBs for spectrographs of their choosing., Comment: 60 pages, 24 figures, accepted for publication in ApJS