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1. A Glimpse of the Stellar Populations and Elemental Abundances of Gravitationally Lensed, Quiescent Galaxies at z ≳ 1 with Keck Deep Spectroscopy

   https://doi.org/10.3847/1538-4357/acc79b  

   Zhuang, Zhuyun ; Leethochawalit, Nicha ; Kirby, Evan N. ; Nightingale, J. W. ; Steidel, Charles C. ; Glazebrook, Karl ; Barone, Tania M. ; Skobe, Hannah ; Sweet, Sarah M. ; Nanayakkara, Themiya ; et al ( May 2023 , The Astrophysical Journal)

   Gravitational lenses can magnify distant galaxies, allowing us to discover and characterize the stellar populations of intrinsically faint, quiescent galaxies that are otherwise extremely difficult to directly observe at high redshift from ground-based telescopes. Here, we present the spectral analysis of two lensed, quiescent galaxies atz≳ 1 discovered by theASTRO 3D Galaxy Evolution with Lensessurvey:AGEL1323 (M_*∼ 10^11.1M_⊙,z= 1.016,μ∼ 14.6) andAGEL0014 (M_*∼ 10^11.5M_⊙,z= 1.374,μ∼ 4.3). We measured the age, [Fe/H], and [Mg/Fe] of the two lensed galaxies using deep, rest-frame-optical spectra (S/N ≳40 Å⁻¹) obtained on the Keck I telescope. The ages ofAGEL1323 andAGEL0014 are${5.6}_{-0.8}^{+0.8}$Gyr and${3.1}_{-0.3}^{+0.8}$Gyr, respectively, indicating that most of the stars in the galaxies were formed less than 2 Gyr after the Big Bang. Compared to nearby quiescent galaxies of similar masses, the lensed galaxies have lower [Fe/H] and [Mg/H]. Surprisingly, the two galaxies have comparable [Mg/Fe] to similar-mass galaxies at lower redshifts, despite their old ages. Using a simple analytic chemical evolution model connecting the instantaneously recycled element Mg with the mass-loading factors of outflows averaged over the entire star formation history, we found that the lensed galaxies may have experienced enhanced outflows during their star formation compared to lower-redshift galaxies,more »which may explain why they quenched early.

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2. Molecular Gas Reservoirs in Massive Quiescent Galaxies at z ∼ 0.7 Linked to Late-time Star Formation

   https://doi.org/10.3847/1538-4357/ac9af7  

   Woodrum, Charity ; Williams, Christina C. ; Rieke, Marcia ; Leja, Joel ; Johnson, Benjamin D. ; Bezanson, Rachel ; Kennicutt, Robert ; Spilker, Justin ; Tacchella, Sandro ( November 2022 , The Astrophysical Journal)

   We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies atz∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges fromf_burst≈ 0.3%–6% and ended betweent_end-burst≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr ($f_{M_{1\mathrm{Gyr}}}=2.6\%\pm 1.8\%$) compared to the CO-undetected galaxies ($f_{M_{1\mathrm{Gyr}}}=0.2\%\pm 0.1\%$). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven bymore »recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFR_UV+IRmeasurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

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3. Metallicity Distribution Function of the Eridanus II Ultra-faint Dwarf Galaxy from Hubble Space Telescope Narrowband Imaging

   https://doi.org/10.3847/1538-4357/ac3665  

   Fu, Sal Wanying ; Weisz, Daniel R. ; Starkenburg, Else ; Martin, Nicolas ; Ji, Alexander P. ; Patel, Ekta ; Boylan-Kolchin, Michael ; Côté, Patrick ; Dolphin, Andrew E. ; Longeard, Nicolas ; et al ( January 2022 , The Astrophysical Journal)

   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 EridanusII(EriII). When combined with archival F475W and F814W data, we measure metallicities for 60 resolved red giant branch stars as faint asm_F475W∼ 24 mag, a factor of ∼4× more stars than current spectroscopic MDF determinations. We find that EriIIhas a mean metallicity of [Fe/H] = −2.50${}_{-0.07}^{+0.07}$and a dispersion of${\sigma }_{[\mathrm{Fe}/\mathrm{H}]}={0.42}_{-0.06}^{+0.06}$, 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. The MDF of EriIIappears well described by a leaky box chemical evolution model. We also compute an updated orbital history for EriIIusing Gaia eDR3 proper motions, and find that it is likely on first infall into the Milky Way. Our findings suggest that EriIIunderwent an evolutionary history similar to that of an isolated galaxy. Compared to MDFs for select cosmological simulations of similar mass galaxies, we find that EriIIhas a lower fraction of stars withmore »[Fe/H] < −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.

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4. Magellan/IMACS Spectroscopy of Grus I: A Low Metallicity Ultra-faint Dwarf Galaxy*

   https://doi.org/10.3847/1538-4357/ac96ed  

   Chiti, Anirudh ; Simon, Joshua D. ; Frebel, Anna ; Pace, Andrew B. ; Ji, Alexander P. ; Li, Ting S. ( November 2022 , The Astrophysical Journal)

   We present a chemodynamical study of the Grus I ultra-faint dwarf galaxy (UFD) from medium-resolution (R∼ 11,000) Magellan/IMACS spectra of its individual member stars. We identify eight confirmed members of Grus I, based on their low metallicities and coherent radial velocities, and four candidate members for which only velocities are derived. In contrast to previous work, we find that Grus I has a very low mean metallicity of 〈[Fe/H]〉 = −2.62 ± 0.11 dex, making it one of the most metal-poor UFDs. Grus I has a systemic radial velocity of −143.5 ± 1.2 km s⁻¹and a velocity dispersion of${\sigma }_{\mathrm{rv}}={2.5}_{-0.8}^{+1.3}$km s⁻¹, which results in a dynamical mass of$M_{1/2}(r_h)=8_{-4}^{+12}\times {10}^5$M_⊙and a mass-to-light ratio ofM/L_V=${440}_{-250}^{+650}$M_⊙/L_⊙. Under the assumption of dynamical equilibrium, our analysis confirms that Grus I is a dark-matter-dominated UFD (M/L> 80M_⊙/L_⊙). However, we do not resolve a metallicity dispersion (σ_[Fe/H]< 0.44 dex). Our results indicate that Grus I is a fairly typical UFD with parameters that agree with mass–metallicity and metallicity-luminosity trends for faint galaxies. This agreement suggests that Grus I has not lost an especially significant amount of mass from tidal encounters with the Milky Way, in linemore »with its orbital parameters. Intriguingly, Grus I has among the lowest central densities (${\rho }_{1/2}\sim {3.5}_{-2.1}^{+5.7}\times {10}^7$M_⊙kpc⁻³) of the UFDs that are not known to be tidally disrupting. Models of the formation and evolution of UFDs will need to explain the diversity of these central densities, in addition to any diversity in the outer regions of these relic galaxies.

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5. The Direct-method Oxygen Abundance of Typical Dwarf Galaxies at Cosmic High Noon∗

   https://doi.org/10.3847/1538-4357/acb153  

   Gburek, Timothy ; Siana, Brian ; Alavi, Anahita ; Emami, Najmeh ; Richard, Johan ; Freeman, William R. ; Stark, Daniel P. ; Snapp-Kolas, Christopher ( May 2023 , The Astrophysical Journal)

   We present a Keck/MOSFIRE rest-optical composite spectrum of 16 typical gravitationally lensed star-forming dwarf galaxies at 1.7 ≲z≲ 2.6 (z_mean= 2.30), all chosen independent of emission-line strength. These galaxies have a median stellar mass of$\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.29}_{-0.43}^{+0.51}$and a median star formation rate of${\mathrm{S}\mathrm{F}\mathrm{R}}_{\mathrm{H}\alpha }^{\mathrm{m}\mathrm{e}\mathrm{d}}={2.25}_{-1.26}^{+2.15}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. We measure the faint electron-temperature-sensitive [Oiii]λ4363 emission line at 2.5σ(4.1σ) significance when considering a bootstrapped (statistical-only) uncertainty spectrum. This yields a direct-method oxygen abundance of$12+\log {(\mathrm{O}/\mathrm{H})}_{\mathrm{direct}}={7.88}_{-0.22}^{+0.25}$(${0.15}_{-0.06}^{+0.12}{Z}_{\odot }$). We investigate the applicability at highzof locally calibrated oxygen-based strong-line metallicity relations, finding that the local reference calibrations of Bian et al. best reproduce (≲0.12 dex) our composite metallicity at fixed strong-line ratio. At fixedM_*, our composite is well represented by thez∼ 2.3 direct-method stellar mass—gas-phase metallicity relation (MZR) of Sanders et al. When comparing to predicted MZRs from the IllustrisTNG and FIRE simulations, having recalculated our stellar masses with more realistic nonparametric star formation histories$(\log {(M_{*}/M_{\odot })}_{\mathrm{med}}={8.92}_{-0.22}^{+0.31})$, we find excellent agreement with the FIRE MZR. Our composite is consistent with no metallicity evolution, atmore »fixedM_*and SFR, of the locally defined fundamental metallicity relation. We measure the doublet ratio [Oii]λ3729/[Oii]λ3726 = 1.56 ± 0.32 (1.51 ± 0.12) and a corresponding electron density of$n_e=1_{-0}^{+215}{\mathrm{cm}}^{-3}$($n_e=1_{-0}^{+74}{\mathrm{cm}}^{-3}$) when considering the bootstrapped (statistical-only) error spectrum. This result suggests that lower-mass galaxies have lower densities than higher-mass galaxies atz∼ 2.

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