Kate H. R. Rubin

Red geysers are a population of massive ( log M / M ⊙ ∼ 10.5 ), quiescent galaxies with large-scale but weak, bisymmetric ionized gas outflows, interpreted as signatures of ongoing, low-level active galactic nucleus (AGN) feedback. We investigate the kinematics and prevalence of cool ( T ∼ 100–1000 K) neutral gas traced by Na i D absorption, and its connection to galaxy environment and AGN activity. Using 140 red geyser galaxies from the Sloan Digital Sky Survey-IV Mapping Nearby Galaxies at Apache Point Observatory (or MaNGA), we measure spatially resolved velocities and dispersions via double-Gaussian fits to the Na i D doublet. We find that ∼70% of the cool gas is inflowing, with a median velocity of ∼47 km s ⁻¹ (∼10% of the expected freefall velocity), and that it also exhibits kinematically ordered motions with σ _NaD / σ _* ∼ 0.4. Additionally, the Na i D absorption is more prevalent in red geysers than in a matched control sample, with a higher detection fraction (63% versus 40%) and reservoir areas ∼1.6 times larger. Acceleration (∼1 Myr) and accretion (∼20 Myr) timescales indicate that the Na I D-absorbing clouds are likely young and short-lived. Another intriguing result is that radio-detected red geysers (30% of the sample) show inflowing gas reservoirs ∼7 times larger than in non-radio systems. Similarly, galaxies subject to environmental effects host inflowing gas reservoirs ∼2.7 times larger than isolated red geysers. We take this as evidence that galaxy environments play a key role in replenishing the cool gas reservoirs of red geysers, fueling central AGN activity, sustaining radio emission, and regulating long-term quiescence. These findings reveal that quiescent systems are governed by cycles of inflow, feedback, and regulation.

High velocity clouds supply the Milky Way with gas that sustains star formation over cosmic timescales. Precise distance measurements are therefore essential to quantify their mass inflow rates and gauge their exact contribution to the Galaxy’s gas supply. We use a sample of 594 SDSS-V BOSS stellar spectra within 10° of the high-velocity Smith Cloud (SC) to trace Na I absorption and dust extinction as functions of distance. By fitting interstellar-medium-corrected MaStar templates to each spectrum, we isolate residual equivalent widths and extinction and then compare trends in the SC region to a same-latitude control field. Stars beyond 1 kpc toward the SC exhibit a significant Na I equivalent width excess (>0.2 Å, >3 σ ) relative to the control. Two-component linear fits of Na I equivalent width and A _V against both low and high-velocity H I column densities show that the low-velocity component is strongly correlated with both quantities, while the high-velocity term is marginally significant in extinction and Na I , consistent with a patchy, low dust-to-gas ratio. Given that the excess Na I begins at distances <2 kpc uniquely in the direction of the SC, and previous estimates of the SC place it at 12.4 ± 1.3 kpc, further investigation of its distance is warranted.

Barium stars are unusually enriched in barium ([Ba/Fe] ≥ 1.0 dex) and not predicted by current Galactic chemical evolution models. Previous observations of barium stars have found evidence that they form through mass transfer from a companion asymptotic giant branch (AGB) star or through radiative levitation. The chemical abundance and kinematic information of barium stars may help constrain AGB stellar nucleosynthesis, binary star evolution, and internal evolutionary processes that affect surface abundances. Using ∼450,000 stars from the Galactic Archaeology with Hermes (GALAH) survey, we identify nearly 3000 new barium-rich stars and separate them into hot ( T _eff > 6000 K) and cool ( T _eff < 6000 K) populations. Crossmatching with Gaia Data Release 3, we find that 47.7% of our barium stars within 1 kpc have an elevated renormalized unit weight error (RUWE; ≥1.4), compared to 16.3% of a comparable sample of the GALAH field, suggesting multiplicity plays an important role in the formation of both populations of barium stars. A subset of hot barium stars exhibit low RUWEs (RUWE < 1.2) and [ α /Fe] < −0.2, supporting radiative levitation as an origin as well. We determine Galactic memberships using both kinematics and chemistry and find that barium stars exist in the thin disk, thick disk, and halo though they are slightly more prevalent at lower metallicities. Overall, we show evidence for barium stars produced by mass transfer and for those produced by radiative levitation, with both formation mechanisms occurring ubiquitously across the Galaxy.

Barium stars are unusually enriched in barium ([Ba/Fe] >= 1.0 dex) and not predicted by current Galactic chemical evolution models. Previous observations of barium stars have found evidence that they form through mass transfer from a companion asymptotic giant branch (AGB) star or through radiative levitation. The chemical abundance and kinematic information of barium stars may help constrain AGB stellar nucleosynthesis, binary star evolution, and internal evolutionary processes that affect surface abundances. Using ~450,000 stars from the GALactic Archaeology with Hermes (GALAH) survey, we identify nearly 3000 new barium-rich stars and separate them into hot (Teff > 6000 K) and cool (Teff < 6000 K) populations. Cross-matching with Gaia DR3, we find that 47.7% of our barium stars within 1 kpc have elevated re-normalized unit weight error (RUWE >= 1.4), compared to 16.3% of a comparable sample of the GALAH field, suggesting multiplicity plays an important role in the formation of both populations of barium stars. A subset of hot barium stars exhibit low RUWE (RUWE < 1.2) and [alpha/Fe] < -0.2, supporting radiative levitation as an origin as well. We determine Galactic memberships using both kinematics and chemistry and find that barium stars exist in the thin disk, thick disk, and halo though they are slightly more prevalent at lower metallicities. Overall, we show evidence for barium stars produced by mass transfer and for those produced by radiative levitation, with both formation mechanisms occurring ubiquitously across the Galaxy.

The Epoch of Reionization (EoR) provides critical insights into the role of early galaxies in shaping the ionization state of the Universe. However, because of the opacity of the intergalactic medium, it is often not possible to make direct measurements of the ionizing photon escape fraction ( f esc LyC ) of high-redshift ( z ≳ 4) galaxies. To explore the agreement and systematics of common indirect approaches, we applied six empirically calibrated diagnostics to predict f esc LyC for the 45 nearby star-forming galaxies from the COS Legacy Spectroscopic SurveY. These methods—based on ultraviolet (UV) absorption lines, the UV continuum slope, Ly α kinematics, a multivariate model, radiation-hydrodynamic simulations, and nebular emission-line ratios—enable us to explore systematic differences between predictions and assess how galactic properties influence inferred LyC escape. Despite significant variations in method predictions, there is broad consistency in the resulting weak and strong LyC leaker classifications, with approximately half exhibiting predicted escape fractions >1%. We find evidence for two different pathways of LyC escape in nearby star-forming galaxies: (1) an early escape model driven by very young stellar populations, and (2) a delayed escape model that is consistent with supernova-driven outflows and time-dependent interstellar medium clearing. The early escape model is favored among galaxies with a single, intense burst of recent star formation. In contrast, the delayed escape model is common among galaxies with more extended starburst histories. To interpret ionizing photon escape during the EoR, it will be necessary to recognize and understand this diversity in LyC escape mechanisms.

For half a century now, the galaxy formation and evolution community has recognized the critical role of gas accretion to sustain star formation (e.g., Refs. 1, 2). Gas is thought to transition into galaxies through accretion processes and, in turn, get expelled from galaxies through feedback processes, an interplay that regulates galactic gas content. What is now broadly called “feedback” includes the ejection of both matter and energy from stellar winds, radiation, and supernovae as well as active galactic nuclei (AGN), and both theory and observation have implicated feedback as central to regulating, perhaps even halting (or quenching), star formation. The discovery of a ubiquitous circumgalactic medium (CGM; Fig. 1) from quasar absorption line studies supports this paradigm: (1) the dark matter halos in which galaxies reside contain a substantial baryonic component well beyond any stellar extent of a galaxy (a potential source of fuel for star formation)⁴; (2) the gaseous halos permeating their dark matter counterparts are correlated with their host galaxy star formation properties (connecting the CGM to central galaxy activity)^5,6; (3) galaxy environment impacts the gas reservoirs of galaxies living in overdense environments such as groups and clusters (where quenched galaxies reside in great numbers)⁷; and (4) more than half of the metals in the universe reside outside galaxies, which implicates ejection from sites of star formation (galaxies) into the CGM.⁸

Cool, dusty interstellar material plays an important role in the chemical evolution of galaxies. We present an analysis of this material across galaxy type through a spatially resolved spectral stacking analysis of galaxies from the MaNGA survey. With stellar population synthesis, we isolate neutral gas signals from resonance lines, comparing outcomes across model types, galactic geometry, and host stellar mass and age. We find that both synthetic and empirical models fail to capture the range of galactic chemical abundances. There is also notable Na i contamination from the Galaxy’s interstellar medium (ISM) in the MILES empirical stellar library. We are unable to reliably determine the column density of the gas due to the accuracy of absorption measurements, but differential analysis across radius and inclination reveals consistent and significant path-length dependent absorption in the equivalent width of Na i. We note similar but lesser trends in a narrow Ca ii index. We find no trends in Ca i or in a broad Ca ii index, indicating its ISM insensitivity and providing evidence in favor of its utility in determining the age and chemical content of stellar populations. Our data shows there is a cool ISM component in most external galaxies with D _n (4000) < 1.7 that can be traced by Na i. Lastly, we caution that the characterization of gas kinematics traced by Na i in such low-resolution spectra is subject to systematic effects due to the chosen approach to stellar population modeling.

We assess the impact of Ca ii λ λ3934, 3969 and Na i λ λ5891, 5897 absorption arising in the interstellar medium (ISM) on the Sloan Digital Sky Survey-IV MaNGA Stellar Library (MaStar) and produce corrected spectroscopy for 80% of the 24,162-star catalog. We model the absorption strength of these transitions as a function of the stellar distance, Galactic latitude, and dust reddening based on high-spectral resolution studies. With this model, we identify 6342 MaStar stars that have negligible ISM absorption (W ^ISM(Ca ii K) < 0.07 Å and W ^ISM(Na i 5891) < 0.05 Å). For 12,110 of the remaining stars, we replace their Na i D profile (and their Ca ii profile for effective temperatures T _eff > 9000 K) with a coadded spectrum of low-ISM stars with similar T _eff, surface gravity, and metallicity. For 738 additional stars with T _eff > 9000 K, we replace these spectral regions with a matching ATLAS9-based BOSZ model. This results in a mean reduction in W(Ca ii K) (W(Na i D)) of 0.4–0.7 Å (0.6–1.1 Å) for hot stars (T _eff > 7610 K), and a mean reduction in W(Na i D) of 0.1–0.2 Å for cooler stars. We show that interstellar absorption in the simple stellar population (SSP) model spectra constructed from the original library artificially enhances W(Ca ii K) by ≳20% at young ages (<400 Myr); dramatically enhances the strength of stellar Na i D in starbursting systems (by ≳50%); and enhances stellar Na i D in older stellar populations (≳10 Gyr) by ≳10%. We provide SSP spectra constructed from the cleaned library and discuss the implications of these effects for stellar population synthesis analyses constraining the stellar age, [Na/Fe] abundance, and initial mass function.