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1. 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 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 CaHK for EMPs. This study demonstrates the power of deep HST CaHK imaging for measuring the MDFs of UFDs.

    

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2. 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, at 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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3. Rest-frame UV Colors for Faint Galaxies at z ∼ 9–16 with the JWST NGDEEP Survey

   https://doi.org/10.3847/2041-8213/ad2de4  

   Morales, Alexa M. ; Finkelstein, Steven L. ; Leung, Gene C. K. ; Bagley, Micaela B. ; Cleri, Nikko J. ; Dave, Romeel ; Dickinson, Mark ; Ferguson, Henry C. ; Hathi, Nimish P. ; Jones, Ewan ; et al ( March 2024 , The Astrophysical Journal Letters)

   We present measurements of the rest-frame UV spectral slope,β, for a sample of 36 faint star-forming galaxies atz∼ 9–16 discovered in one of the deepest JWST NIRCam surveys to date, the Next Generation Deep Extragalactic Exploratory Public Survey. We use robust photometric measurements for UV-faint galaxies (down toM_UV∼ −16), originally published in Leung et al., and measure values of the UV spectral slope via photometric power-law fitting to both the observed photometry and stellar population models obtained through spectral energy distribution (SED) fitting withBagpipes. We obtain a median and 68% confidence interval forβfrom photometric power-law fitting of${\beta }_{\mathrm{PL}}=-{2.7}_{-0.5}^{+0.5}$and from SED fitting,${\beta }_{\mathrm{SED}}=-{2.3}_{-0.1}^{+0.2}$for the full sample. We show that when only two to three photometric detections are available, SED fitting has a lower scatter and reduced biases than photometric power-law fitting. We quantify this bias and find that after correction the median${\beta }_{\mathrm{SED},\mathrm{corr}}=-{2.5}_{-0.2}^{+0.2}$. We measure physical properties for our galaxies withBagpipesand find that our faint ($M_{\mathrm{UV}}=-{18.1}_{-0.9}^{+0.7}$) sample is low in mass ($\log [M_{*}/M_{\odot }]={7.7}_{-0.5}^{+0.5}$), fairly dust-poor ($A_{\mathrm{v}}={0.1}_{-0.1}^{+0.2}$mag), and modestly young ($\log [\mathrm{age}]={7.8}_{-0.8}^{+0.2}$yr) with a median star formation rate of$\log (\mathrm{SFR})=-{0.3}_{-0.4}^{+0.4}{M}_{\odot }{\mathrm{yr}}^{-1}$. We find no strong evidence for ultrablue UV spectral slopes (β∼ −3) within our sample, as would be expected for exotically metal-poor (Z/Z_⊙< 10⁻³) stellar populations with very high Lyman continuum escape fractions. Our observations are consistent with model predictions that galaxies of these stellar masses atz∼ 9–16 should have only modestly low metallicities (Z/Z_⊙∼ 0.1–0.2).

    

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4. The Pan-STARRS1 z > 5.6 Quasar Survey. III. The z ≈ 6 Quasar Luminosity Function

   https://doi.org/10.3847/1538-4357/aca7ca  

   Schindler, Jan-Torge ; Bañados, Eduardo ; Connor, Thomas ; Decarli, Roberto ; Fan, Xiaohui ; Farina, Emanuele Paolo ; Mazzucchelli, Chiara ; Nanni, Riccardo ; Rix, Hans-Walter ; Stern, Daniel ; et al ( January 2023 , The Astrophysical Journal)

   We present thez≈ 6 type-1 quasar luminosity function (QLF), based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars atz≈ 5.7–6.2, with −28 ≲M₁₄₅₀≲ −25. With the addition of 48 fainter quasars from the SHELLQs survey, we evaluate thez≈ 6 QLF over −28 ≲M₁₄₅₀≲ −22. Adopting a double power law with an exponential evolution of the quasar density (Φ(z) ∝ 10^k(z−6);k= −0.7), we use a maximum likelihood method to model our data. We find a break magnitude of$M^{*}=-{26.38}_{-0.60}^{+0.79}\mathrm{mag}$, a faint-end slope of$\alpha =-{1.70}_{-0.19}^{+0.29}$, and a steep bright-end slope of$\beta =-{3.84}_{-1.21}^{+0.63}$. Based on our new QLF model, we determine the quasar comoving spatial density atz≈ 6 to be$n(M_{1450}<-26)={1.16}_{-0.12}^{+0.13}{\mathrm{cGpc}}^{-3}$. In comparison with the literature, we find the quasar density to evolve with a constant value ofk≈ −0.7, fromz≈ 7 toz≈ 4. Additionally, we derive an ionizing emissivity of${ϵ}_{912}(z=6)={7.23}_{-1.02}^{+1.65}\times {10}^{22}\mathrm{erg}{\mathrm{s}}^{-1}{\mathrm{Hz}}^{-1}{\mathrm{cMpc}}^{-3}$, based on the QLF measurement. Given standard assumptions, and the recent measurement of the mean free path by Becker et al. atz≈ 6, we calculate an Hiphotoionizing rate of Γ_{H I}(z= 6) ≈ 6 × 10⁻¹⁶s⁻¹, strongly disfavoring a dominant role of quasars in hydrogen reionization.

    

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5. Reading between the (Spectral) Lines: Magellan/IMACS Spectroscopy of the Ultrafaint Dwarf Galaxies Eridanus IV and Centaurus I

   https://doi.org/10.3847/1538-4357/ad0cf7  

   Heiger, M. E. ; Li, T. S. ; Pace, A. B. ; Simon, J. D. ; Ji, A. P. ; Chiti, A. ; Bom, C. R. ; Carballo-Bello, J. A. ; Carlin, J. L. ; Cerny, W. ; et al ( January 2024 , The Astrophysical Journal)

   We present a spectroscopic analysis of Eridanus IV (Eri IV) and Centaurus I (Cen I), two ultrafaint dwarf galaxies of the Milky Way. Using IMACS/Magellan spectroscopy, we identify 28 member stars of Eri IV and 34 member stars of Cen I. For Eri IV, we measure a systemic velocity of$v_{\mathrm{sys}}=-{31.5}_{-1.2}^{+1.3}\mathrm{km}{\mathrm{s}}^{-1}$, and velocity dispersion${\sigma }_v={6.1}_{-0.9}^{+1.2}\mathrm{km}{\mathrm{s}}^{-1}$. Additionally, we measure the metallicities of 16 member stars of Eri IV. We find a metallicity of$[\mathrm{Fe}/\mathrm{H}]=-{2.87}_{-0.07}^{+0.08}$, and resolve a dispersion ofσ_[Fe/H]=0.20 ± 0.09. The mean metallicity is marginally lower than all other known ultrafaint dwarf galaxies, making it one of the most metal-poor galaxies discovered thus far. Eri IV also has a somewhat unusual right-skewed metallicity distribution. For Cen I, we find a velocityv_sys= 44.9 ± 0.8 km s⁻¹, and velocity dispersion${\sigma }_v={4.2}_{-0.5}^{+0.6}\mathrm{km}{\mathrm{s}}^{-1}$. We measure the metallicities of 27 member stars of Cen I, and find a mean metallicity [Fe/H] = −2.57 ± 0.08, and metallicity dispersion${\sigma }_{[\mathrm{Fe}/\mathrm{H}]}={0.38}_{-0.05}^{+0.07}$. We calculate the systemic proper motion, orbit, and the astrophysical J-factor for each system, the latter of which indicates that Eri IV is a good target for indirect dark matter detection. We also find no strong evidence for tidal stripping of Cen I or Eri IV. Overall, our measurements confirm that Eri IV and Cen I are dark-matter-dominated galaxies with properties largely consistent with other known ultrafaint dwarf galaxies. The low metallicity, right-skewed metallicity distribution, and high J-factor make Eri IV an especially interesting candidate for further follow-up.

    

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