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Abstract Spectroscopic studies of elliptical galaxies show that their stellar population ages, mean metallicity, andαenhancement 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 atz∼ 0 andz∼ 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, 〈age〉, 〈[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]〉–〈age〉 and 〈[Mg/Fe]〉–〈age〉 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 (SFE) and outflow efficiency affect the early and late evolution of 〈[Mg/H]〉, respectively; explaining observed trends requires higher SFE and lower outflows in more massive galaxies. With core-collapse supernova yields calibrated to the plateau [Mg/Fe]_cc≈ 0.45 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]_cc= 0.55 improves the agreement, though the models remain 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]〉. 

Abstract Our understanding of the assembly timeline of the Milky Way has been transforming along with the dramatic increase in astrometric and spectroscopic data available over the past several years. Many substructures in chemo-dynamical space have been discovered and identified as the remnants of various galactic mergers. To investigate the timeline of these mergers, we select main-sequence turnoff and subgiant stars (MSTOs) from the H3 survey, finding members in seven metal-poor components of the halo: Gaia-Sausage/Enceladus (GSE), the Helmi Streams, Thamnos, Sequoia, Wukong/LMS-1, Arjuna, and I’itoi. We also select out a metal-poor in situ population to facilitate comparison to the evolution of the Milky Way itself at these early epochs. We fit individual isochrone ages to the MSTOs in each of these substructures and use the resulting age distributions to infer simple star formation histories (SFHs). For GSE, we resolve an extended SFH that truncates ≈10 Gyr ago, as well as a clear age–metallicity relation. From this age distribution and measured SFH we infer that GSE merged with the Milky Way at a time 9.5–10.2 Gyr ago, in agreement with previous estimates. We infer that the other mergers occurred at various times ranging from 9 to 13 Gyr ago, and that the metal-poor in situ Galaxy built up within only a few billion years. These results reinforce the emerging picture that both the disk and halo of the Milky Way experienced a rapid assembly. 

Abstract Eruptive mass loss in massive stars is known to occur, but the mechanism(s) are not yet well understood. One proposed physical explanation appeals to opacity-driven super-Eddington luminosities in stellar envelopes. Here, we present a 1D model for eruptive mass loss and implement this model in theMESAstellar evolution code. The model identifies regions in the star where the energy associated with a local super-Eddington luminosity exceeds the binding energy of the overlaying envelope. The material above such regions is ejected from the star. Stars with initial masses of 10−100M_⊙at solar and SMC metallicities are evolved through core helium burning, with and without this new eruptive mass-loss scheme. We find that eruptive mass loss of up to ∼10⁻²M_⊙yr⁻¹can be driven by this mechanism, and occurs in a vertical band on the H-R diagram between $3.5\lesssim \log \left(T_{\mathrm{eff}}/\mathrm{K}\right)\lesssim 4.0$. This predicted eruptive mass loss prevents stars of initial masses ≳20M_⊙from evolving to become red supergiants (RSGs), with the stars instead ending their lives as blue supergiants, and offers a possible explanation for the observed lack of RSGs in that mass regime. 

Abstract Stellar streams in the Milky Way are promising detectors of low-mass dark matter (DM) subhalos predicted by ΛCDM. Passing subhalos induce perturbations in streams that indicate the presence of the subhalos. Understanding how known DM-dominated satellites impact streams is a crucial step toward using stream perturbations to constrain the properties of dark perturbers. Here, we cross-match a Gaia Early Data Release 3 and SEGUE member catalog of the Cetus-Palca stream (CPS) with H3 for additional radial velocity measurements and fit the orbit of the CPS using this six-dimensional (6D) data. We demonstrate for the first time that the ultra-faint dwarf Segue 2 had a recent (77 ± 5 Myr ago) close flyby (within the stream's 2σwidth) with the CPS. This interaction enables constraints on Segue 2’s mass and density profile at larger radii ( $O\left(1\right)$kpc) than are probed by its stars ( $O\left(10\right)$pc). While Segue 2 is not expected to strongly affect the portion of the stream covered by our 6D data, we predict that if Segue 2’s mass within ∼ 6 kpc is 5 × 10⁹M_⊙, the CPS's velocity dispersion will be ∼ 40 km s⁻¹larger atϕ₁ > 20° than atϕ₁ < 0°. If no such heating is detected, Segue 2’s mass cannot exceed 10⁹M_⊙within ∼ 6 kpc. The proper motion distribution of the CPS near the impact site is mildly sensitive to the shape of Segue 2’s density profile. This study presents a critical test for frameworks designed to constrain properties of dark subhalos from stream perturbations. 

Abstract Understanding the chemical enrichment of different elements is crucial to gaining a complete picture of galaxy chemical evolution. In this study, we present a new sample of 46 low-redshift, low-mass star-forming galaxies atM_*∼ 10⁸⁻¹⁰M_⊙along with two quiescent galaxies atM_*∼ 10^8.8M_⊙observed with the Keck Cosmic Web Imager, aiming to investigate the chemical evolution of galaxies in the transition zone between Local Group satellites and massive field galaxies. We develop a novel method to simultaneously determine stellar abundances of iron and magnesium in star-forming galaxies. With the gas-phase oxygen abundance (O/H)_gmeasured using the strong-line method, we are able to make the first-ever apples-to-apples comparison ofαelements in the stars and the interstellar medium. We find that the [Mg/H]_*–[O/H]_grelation is much tighter than the [Fe/H]_*–[O/H]_grelation, which can be explained by the similar production processes ofαelements. Most galaxies in our sample exhibit higher [O/H]_gthan [Fe/H]_*and [Mg/H]_*. In addition, we construct mass–metallicity relations (MZRs) measured as three different elements (Fe_*, Mg_*, O_g). Compared to the gas O-MZR, the stellar Fe- and Mg-MZRs show larger scatter driven by variations in specific star formation rates (sSFR), with star-forming galaxies exhibiting higher sSFR and lower stellar abundances at fixed mass. The excess of [O/H]_gcompared to stellar abundances as well as the anticorrelation between sSFR and stellar abundance suggests that galaxy quenching of intermediate-mass galaxies atM_*∼ 10⁸⁻¹⁰M_⊙is primarily driven by starvation. 

ABSTRACT We present the first detailed chemical-abundance analysis of stars from the dwarf-galaxy stellar stream Wukong/LMS-1 covering a wide metallicity range ($$-3.5 \lt \rm [Fe/H] \lesssim -1.3$$). We find abundance patterns that are effectively indistinguishable from the bulk of Indus and Jhelum, a pair of smaller stellar streams proposed to be dynamically associated with Wukong/LMS-1. We confirmed a carbon-enhanced metal-poor star ($$\rm [C/Fe] \gt +0.7$$ and $$\rm [Fe/H] \sim -2.9$$) in Wukong/LMS-1 with strong enhancements in Sr, Y, and Zr, which is peculiar given its solar-level [Ba/Fe]. Wukong/LMS-1 stars have high abundances of α elements up to $$\rm [Fe/H] \gtrsim -2$$, which is expected for relatively massive dwarfs. Towards the high-metallicity end, Wukong/LMS-1 becomes α-poor, revealing that it probably experienced fairly standard chemical evolution. We identified a pair of N- and Na-rich stars in Wukong/LMS-1, reminiscent of multiple stellar populations in globular clusters. This indicates that this dwarf galaxy contained at least one globular cluster that was completely disrupted in addition to two intact ones previously known to be associated with Wukong/LMS-1, which is possibly connected to similar evidence found in Indus. From these ≥3 globular clusters, we estimate the total mass of Wukong/LMS-1 to be $${\approx }10^{10} \, \mathrm{M}_\odot$$, representing ∼1 per cent of the present-day Milky Way. Finally, the [Eu/Mg] ratio in Wukong/LMS-1 continuously increases with metallicity, making this the first example of a dwarf galaxy where the production of r-process elements is clearly dominated by delayed sources, presumably neutron-star mergers. 

Abstract We presentAugustus, a catalog of distance, extinction, and stellar parameter estimates for 170 million stars from 14 mag <r< 20 mag and with ∣b∣ > 10° drawing on a combination of optical to near-infrared photometry from Pan-STARRS, 2MASS, UKIDSS, and unWISE along with parallax measurements from Gaia DR2 and 3D dust extinction maps. After applying quality cuts, we find 125 million objects have “high-quality” posteriors with statistical distance uncertainties of ≲10% for objects with well-constrained stellar types. This is a substantial improvement over the distance estimates derived from Gaia parallaxes alone and in line with the recent results from Anders et al. We find the fits are able to reproduce the dereddened Gaia color–magnitude diagram accurately, which serves as a useful consistency check of our results. We show that we are able to detect large, kinematically coherent substructures in our data clearly relative to the input priors, including the Monoceros Ring and the Sagittarius Stream, attesting to the quality of the catalog. Our results are publicly available at doi:10.7910/DVN/WYMSXV. An accompanying interactive visualization can be found athttp://allsky.s3-website.us-east-2.amazonaws.com. 

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 ∼ 0.7 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 (σv) ranging from 150–250 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 σv. Thus, the highest dispersion galaxies formed the earliest and are the most metal-rich. We find only mild or nonsignificant trends between [X/Fe] and σv, suggesting that the average star formation timescale does not strongly depend on velocity dispersion. To first order, the abundance patterns of the z ∼ 0.7 quiescent galaxies are strikingly similar to those at z ∼ 0. However, at the lowest-velocity dispersions, the z ∼ 0.7 galaxies have slightly enhanced N, Mg, Ti, and Ni abundance ratios and earlier formation redshifts than their z ∼ 0 counterparts. Thus, while the higher-mass quiescent galaxy population shows little evolution, the low-mass quiescent galaxies population has grown significantly over the past 6 Gyr. Finally, the abundance patterns of both z ∼ 0 and z ∼ 0.7 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. 

Abstract We present the elemental abundances and ages of 19 massive quiescent galaxies atz∼ 1.4 andz∼ 2.1 from the Keck Heavy Metal Survey. The ultradeep LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between [Fe/H] = −0.5 and −0.1 dex, with typical values of −0.2 [−0.3] atz∼ 1.4 [z∼ 2.1]. We also find a tentative $\log {\sigma }_v$–[Fe/H] relation atz∼ 1.4. The magnesium-to-iron ratios span [Mg/Fe] = 0.1–0.6 dex, with typical values of 0.3 [0.5] dex atz∼ 1.4 [z∼ 2.1]. The ages imply formation redshifts ofz_form= 2–8. Compared to quiescent galaxies at lower redshifts, we find that [Fe/H] was ∼0.2 dex lower atz= 1.4–2.1. We find no evolution in [Mg/Fe] out toz∼ 1.4, though thez∼ 2.1 galaxies are 0.2 dex enhanced compared toz= 0–0.7. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star formation timescales, with thez∼ 2.1 galaxies forming the bulk of their stars over 150 Myr atz_form∼ 4. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies atz∼ 0.0–0.7. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase [Fe/H], particularly in galaxy centers. Consequently, the buildup of massive quiescent galaxies sincez∼ 2.1 may require further mechanisms, such as major mergers and/or central star formation. 

Abstract Elongated bar-like features are ubiquitous in galaxies, occurring at the centers of approximately two-thirds of spiral disks in the nearby Universe. Due to gravitational interactions between the bar and the other components of galaxies, it is expected that angular momentum and matter will redistribute over long (Gyr) timescales in barred galaxies. Previous work ignoring the gas phase of galaxies has conclusively demonstrated that bars should slow their rotation over time due to their interaction with dark matter halos. We have performed a simulation of a Milky Way–like galactic disk hosting a strong bar, including a state-of-the-art model of the interstellar medium and a live dark matter halo. In this simulation, the bar pattern does not slow down over time, and instead it remains at a stable, constant rate of rotation. This behavior has been observed in previous simulations using more simplified models for the interstellar gas, but the apparent lack of secular evolution has remained unexplained. We find that the presence of the gas phase arrests the process by which the dark matter halo slows down a bar, a phenomenon we term bar locking. This locking is responsible for stabilizing the bar pattern speed. We find that, in a Milky Way–like disk, a gas fraction of only about 5% is necessary for this mechanism to operate. Our result naturally explains why nearly all observed bars rotate rapidly and is especially relevant for our understanding of how the Milky Way arrived at its present state.