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1. Stellar Metallicities and Gradients in the Isolated, Quenched Low-mass Galaxy Tucana

   https://doi.org/10.3847/1538-4357/ad25ed  

   Fu, Sal Wanying ; Weisz, Daniel R. ; Starkenburg, Else ; Martin, Nicolas ; Mercado, Francisco J. ; Savino, Alessandro ; Boylan-Kolchin, Michael ; Côté, Patrick ; Dolphin, Andrew E. ; Longeard, Nicolas ; et al ( April 2024 , The Astrophysical Journal)

   We measure the metallicities of 374 red giant branch (RGB) stars in the isolated, quenched dwarf galaxy Tucana using Hubble Space Telescope narrowband (F395N) calcium H and K imaging. Our sample is a factor of ∼7 larger than what is available from previous studies. Our main findings are as follows. (i) A global metallicity distribution function (MDF) with$〈\mathrm{[Fe/H]}〉=-{1.55}_{-0.04}^{+0.04}$and${\sigma }_{\mathrm{[Fe/H]}}={0.54}_{-0.03}^{+0.03}$. (ii) A metallicity gradient of −0.54 ± 0.07 dex$R_e^{-1}$(−2.1 ± 0.3 dex kpc⁻¹) over the extent of our imaging (∼2.5R_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$〈\mathrm{[Fe/H]}〉=-{1.78}_{-0.06}^{+0.06}$and${\sigma }_{\mathrm{[Fe/H]}}={0.44}_{-0.06}^{+0.07}$and a red RGB with$〈\mathrm{[Fe/H]}〉=-{1.08}_{-0.07}^{+0.07}$and${\sigma }_{\mathrm{[Fe/H]}}={0.42}_{-0.06}^{+0.06}$. (iv) At fixed stellar mass, Tucana is more metal-rich than Milky Way satellites by ∼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) About 2% of Tucana’s stars have [Fe/H] < −3% and 20% have [Fe/H] > −1. We provide a catalog for community spectroscopic follow-up.

    

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2. Spectroscopic Confirmation of a Protocluster at z = 3.37 with a High Fraction of Quiescent Galaxies

   https://doi.org/10.3847/1538-4357/ac2b9f  

   McConachie, Ian ; Wilson, Gillian ; Forrest, Ben ; Marsan, Z. Cemile ; Muzzin, Adam ; Cooper, M. C. ; Annunziatella, Marianna ; Marchesini, Danilo ; Chan, Jeffrey C. C. ; Gomez, Percy ; et al ( February 2022 , The Astrophysical Journal)

   We report the discovery of MAGAZ3NE J095924+022537, a spectroscopically confirmed protocluster at$z={3.3665}_{-0.0012}^{+0.0009}$around a spectroscopically confirmedUVJ-quiescent ultramassive galaxy (UMG;$M_{\star }={2.34}_{-0.34}^{+0.23}\times {10}^{11}{M}_{\odot }$) in the COSMOS UltraVISTA field. We present a total of 38 protocluster members (14 spectroscopic and 24 photometric), including the UMG. Notably, and in marked contrast to protoclusters previously reported at this epoch that have been found to contain predominantly star-forming members, we measure an elevated fraction of quiescent galaxies relative to the coeval field (${73.3}_{-16.9}^{+26.7}\mathrm{\%}$versus${11.6}_{-4.9}^{+7.1}\mathrm{\%}$for galaxies with stellar massM_⋆≥ 10¹¹M_⊙). This high quenched fraction provides a striking and important counterexample to the seeming ubiquitousness of star-forming galaxies in protoclusters atz> 2 and suggests, rather, that protoclusters exist in a diversity of evolutionary states in the early universe. We discuss the possibility that we might be observing either “early mass quenching” or nonclassical “environmental quenching.” We also present the discovery of MAGAZ3NE J100028+023349, a second spectroscopically confirmed protocluster, at a very similar redshift of$z={3.3801}_{-0.0281}^{+0.0213}$. We present a total of 20 protocluster members, 12 of which are photometric and eight spectroscopic including a poststarburst UMG ($M_{\star }={2.95}_{-0.20}^{+0.21}\times {10}^{11}{M}_{\odot }$). Protoclusters MAGAZ3NE J0959 and MAGAZ3NE J1000 are separated by 18′ on the sky (35 comoving Mpc), in good agreement with predictions from simulations for the size of “Coma”-type cluster progenitors at this epoch. It is highly likely that the two UMGs are the progenitors of Brightest Cluster Galaxies seen in massive virialized clusters at lower redshift.

    

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3. Resolved Measurements of the CO-to-H 2 Conversion Factor in 37 Nearby Galaxies

   https://doi.org/10.3847/1538-4357/ad23ed  

   Chiang 江, I-Da 宜達 ; Sandstrom, Karin M. ; Chastenet, Jérémy ; Bolatto, Alberto D. ; Koch, Eric W. ; Leroy, Adam K. ; Sun 孙, Jiayi 嘉懿 ; Teng, Yu-Hsuan ; Williams, Thomas G. ( March 2024 , The Astrophysical Journal)

   We measure the CO-to-H₂conversion factor (α_CO) in 37 galaxies at 2 kpc resolution, using the dust surface density inferred from far-infrared emission as a tracer of the gas surface density and assuming a constant dust-to-metal ratio. In total, we have ∼790 and ∼610 independent measurements ofα_COfor CO (2–1) and (1–0), respectively. The mean values forα_{CO (2–1)}andα_{CO (1–0)}are${9.3}_{-5.4}^{+4.6}$and${4.2}_{-2.0}^{+1.9}{M}_{\odot }{\mathrm{pc}}^{-2}{(\mathrm{K}\mathrm{km}{\mathrm{s}}^{-1})}^{-1}$, respectively. The CO-intensity-weighted mean is 5.69 forα_{CO (2–1)}and 3.33 forα_{CO (1–0)}. We examine howα_COscales with several physical quantities, e.g., the star formation rate (SFR), stellar mass, and dust-mass-weighted average interstellar radiation field strength ($\overline{U}$). Among them,$\overline{U}$, Σ_SFR, and the integrated CO intensity (W_CO) have the strongest anticorrelation with spatially resolvedα_CO. We provide linear regression results toα_COfor all quantities tested. At galaxy-integrated scales, we observe significant correlations betweenα_COandW_CO, metallicity,$\overline{U}$, and Σ_SFR. We also find thatα_COin each galaxy decreases with the stellar mass surface density (Σ_⋆) in high-surface-density regions (Σ_⋆≥ 100M_⊙pc⁻²), following the power-law relations${\alpha }_{\mathrm{CO}(2–1)}\propto {\mathrm{\Sigma }}_{\star }^{-0.5}$and${\alpha }_{\mathrm{CO}(1–0)}\propto {\mathrm{\Sigma }}_{\star }^{-0.2}$. The power-law index is insensitive to the assumed dust-to-metal ratio. We interpret the decrease inα_COwith increasing Σ_⋆as a result of higher velocity dispersion compared to isolated, self-gravitating clouds due to the additional gravitational force from stellar sources, which leads to the reduction inα_CO. The decrease inα_COat high Σ_⋆is important for accurately assessing molecular gas content and star formation efficiency in the centers of galaxies, which bridge “Milky Way–like” to “starburst-like” conversion factors.

    

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4. Short GRB Host Galaxies. II. A Legacy Sample of Redshifts, Stellar Population Properties, and Implications for Their Neutron Star Merger Origins

   https://doi.org/10.3847/1538-4357/ac91d1  

   Nugent, Anya E. ; Fong, Wen-Fai ; Dong 董, Yuxin 雨欣 ; Leja, Joel ; Berger, Edo ; Zevin, Michael ; Chornock, Ryan ; Cobb, Bethany E. ; Kelley, Luke Zoltan ; Kilpatrick, Charles D. ; et al ( November 2022 , The Astrophysical Journal)

   We present the stellar population properties of 69 short gamma-ray burst (GRB) host galaxies, representing the largest uniformly modeled sample to date. Using theProspectorstellar population inference code, we jointly fit photometry and/or spectroscopy of each host galaxy. We find a population median redshift of$z={0.64}_{-0.32}^{+0.83}$(68% confidence), including nine photometric redshifts atz≳ 1. We further find a median mass-weighted age oft_m=${0.8}_{-0.53}^{+2.71}$Gyr, stellar mass of log(M_*/M_⊙) =${9.69}_{-0.65}^{+0.75}$, star formation rate of SFR =${1.44}_{-1.35}^{+9.37}$M_⊙yr⁻¹, stellar metallicity of log(Z_*/Z_⊙) =$-{0.38}_{-0.42}^{+0.44}$, and dust attenuation of$A_V={0.43}_{-0.36}^{+0.85}$mag (68% confidence). Overall, the majority of short GRB hosts are star-forming (≈84%), with small fractions that are either transitioning (≈6%) or quiescent (≈10%); however, we observe a much larger fraction (≈40%) of quiescent and transitioning hosts atz≲ 0.25, commensurate with galaxy evolution. We find that short GRB hosts populate the star-forming main sequence of normal field galaxies, but do not include as many high-mass galaxies as the general galaxy population, implying that their binary neutron star (BNS) merger progenitors are dependent on a combination of host star formation and stellar mass. The distribution of ages and redshifts implies a broad delay-time distribution, with a fast-merging channel atz> 1 and a decreased neutron star binary formation efficiency from high to low redshifts. If short GRB hosts are representative of BNS merger hosts within the horizon of current gravitational wave detectors, these results can inform future searches for electromagnetic counterparts. All of the data and modeling products are available on the Broadband Repository for Investigating Gamma-ray burst Host Traits website.

    

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5. A Luminous Red Supergiant and Dusty Long-period Variable Progenitor for SN 2023ixf

   https://doi.org/10.3847/2041-8213/ace618  

   Jencson, Jacob E. ; Pearson, Jeniveve ; Beasor, Emma R. ; Lau, Ryan M. ; Andrews, Jennifer E. ; Bostroem, K. Azalee ; Dong 董, Yize 一泽 ; Engesser, Michael ; Gomez, Sebastian ; Guolo, Muryel ; et al ( July 2023 , The Astrophysical Journal Letters)

   We analyze pre-explosion near- and mid-infrared (IR) imaging of the site of SN 2023ixf in the nearby spiral galaxy M101 and characterize the candidate progenitor star. The star displays compelling evidence of variability with a possible period of ≈1000 days and an amplitude of Δm≈ 0.6 mag in extensive monitoring with the Spitzer Space Telescope since 2004, likely indicative of radial pulsations. Variability consistent with this period is also seen in the near-IRJandK_sbands between 2010 and 2023, up to just 10 days before the explosion. Beyond the periodic variability, we do not find evidence for any IR-bright pre-supernova outbursts in this time period. The IR brightness ($M_{K_s}=-10.7$mag) and color (J−K_s= 1.6 mag) of the star suggest a luminous and dusty red supergiant. Modeling of the phase-averaged spectral energy distribution (SED) yields constraints on the stellar temperature ($T_{\mathrm{eff}}={3500}_{-1400}^{+800}$K) and luminosity ($\log L/L_{\odot }=5.1\pm 0.2$). This places the candidate among the most luminous Type II supernova progenitors with direct imaging constraints, with the caveat that many of these rely only on optical measurements. Comparison with stellar evolution models gives an initial mass ofM_init= 17 ± 4M_⊙. We estimate the pre-supernova mass-loss rate of the star between 3 and 19 yr before explosion from the SED modeling at$\dot{M}\approx 3\times {10}^{-5}$to 3 × 10⁻⁴M_⊙yr⁻¹for an assumed wind velocity ofv_w= 10 km s⁻¹, perhaps pointing to enhanced mass loss in a pulsation-driven wind.

    

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