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1. ODIN: Clustering Analysis of 14,000 Ly α -emitting Galaxies at z = 2.4, 3.1, and 4.5

   https://doi.org/10.3847/2041-8213/adec82  

   Herrera, Danisbel; Gawiser, Eric; Benda, Barbara; Firestone, Nicole M; Ramakrishnan, Vandana; Moon, Byeongha; Lee, Kyoung-Soo; Park, Changbom; Valdes, Francisco; Yang, Yujin; et al (July 2025, The Astrophysical Journal Letters)

   Abstract Lyαemitters (LAEs) are star-forming galaxies that efficiently probe the spatial distribution of galaxies in the high-redshift Universe. The spatial clustering of LAEs reflects the properties of their individual host dark matter halos, allowing us to study the evolution of the galaxy–halo connection. We analyze the clustering of 5233, 5220, and 3706 LAEs atz= 2.4, 3.1, and 4.5, respectively, in the 9 deg²COSMOS field from the One-hundred-deg²DECam Imaging in Narrowbands survey. After correcting for redshift-space distortions, LAE contamination rates, and the integral constraint, the observed angular correlation functions imply linear galaxy bias factors ofb= $1.72_{-0.27}^{+0.26},2.01_{-0.29}^{+0.26},$and $2.95_{-0.46}^{+0.40}$forz= 2.4, 3.1, and 4.5, respectively. The median dark matter halo masses inferred from these measurements are $\log (M_h/M_{\odot })$= $11.44_{-0.28}^{+0.30},11.13_{-0.26}^{+0.26},\mathrm{and}10.85_{-0.24}^{+0.24}$for the three samples, respectively. The analysis also reveals that LAEs occupy roughly 3%–7% of the halos whose clustering strength matches that of the LAEs. 

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2. The Qz5 Survey. I. How the H i Mass Density of the Universe Evolves with Cosmic Time

   https://doi.org/10.3847/1538-4357/adb430  

   Oyarzún, Grecco A; Rafelski, Marc; Christensen, Lise; Ozyurt, Fiona; Jorgenson, Regina A; Neeleman, M; Fumagalli, Michele; Prochaska, J Xavier; Worseck, G; Wisz, M E; et al (April 2025, The Astrophysical Journal)

   Abstract We report that the neutral hydrogen (Hi) mass density of the Universe (ρ_Hi) increases with cosmic time sincez ∼ 5, peaks atz ∼ 3, and then decreases towardz ∼ 0. This is the first result of Qz5, our spectroscopic survey of 63 quasars atz ≳ 5 with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening Higas absorbers atz ∼ 5. The main feature of Qz5 is the high resolution (R ∼ 7000–9000) of the spectra, which allows us to (1) accurately detect high column density Higas absorbers in an increasingly neutral intergalactic medium atz ∼ 5 and (2) determine the reliability of previousρ_Himeasurements derived with lower resolution spectroscopy. We find five intervening damped Lyαabsorbers (DLAs) atz > 4.5, which corresponds to the lowest DLA incidence rate ( $0.034_{0.02}^{0.05}$) atz ≳ 2. We also measure the lowestρ_Hiatz ≳ 2 from our sample of DLAs and subDLAs, corresponding toρ_Hi $=0.56_{0.31}^{0.82}\times 10^8{M}_{\odot }$Mpc⁻³atz ∼ 5. Taking into account our measurements atz ∼ 5 and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude thatρ_Hidoubles fromz ∼ 5 toz ∼ 3. From these results emerges a qualitative agreement between how the cosmic densities of Higas mass, molecular gas mass, and star formation rate build up with cosmic time. 

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3. The Bimodal Absorption System Imaging Campaign (BASIC). I. A Dual Population of Low-metallicity Absorbers at z < 1

   https://doi.org/10.3847/1538-4357/acb047  

   Berg, Michelle_A; Lehner, Nicolas; Howk, J_Christopher; O’Meara, John_M; Schaye, Joop; Straka, Lorrie_A; Cooksey, Kathy_L; Tripp, Todd_M; Prochaska, J_Xavier; Oppenheimer, Benjamin_D; et al (February 2023, The Astrophysical Journal)

   Abstract The bimodal absorption system imaging campaign (BASIC) aims to characterize the galaxy environments of a sample of 36 Hi-selected partial Lyman limit systems (pLLSs) and Lyman limit systems (LLSs) in 23 QSO fields atz≲ 1. These pLLSs/LLSs provide a unique sample of absorbers with unbiased and well-constrained metallicities, allowing us to explore the origins of metal-rich and low-metallicity circumgalactic medium (CGM) atz< 1. Here we present Keck/KCWI and Very Large Telescope/MUSE observations of 11 of these QSO fields (19 pLLSs) that we combine with Hubble Space Telescope/Advanced Camera for Surveys imaging to identify and characterize the absorber-associated galaxies at 0.16 ≲z≲ 0.84. We find 23 unique absorber-associated galaxies, with an average of one associated galaxy per absorber. For seven absorbers, all with <10% solar metallicities, we find no associated galaxies with $\log M_{\star }\gtrsim 9.0$withinρ/R_virand ∣Δv∣/v_esc≤ 1.5 with respect to the absorber. We do not find any strong correlations between the metallicities or Hicolumn densities of the gas and most of the galaxy properties, except for the stellar mass of the galaxies: the low-metallicity ([X/H] ≤ −1.4) systems have a probability of ${0.39}_{-0.15}^{+0.16}$for having a host galaxy with $\log M_{\star }\ge 9.0$withinρ/R_vir≤ 1.5, while the higher metallicity absorbers have a probability of ${0.78}_{-0.13}^{+0.10}$. This implies metal-enriched pLLSs/LLSs atz< 1 are typically associated with the CGM of galaxies with $\log M_{\star }>9.0$, whereas low-metallicity pLLSs/LLSs are found in more diverse locations, with one population arising in the CGM of galaxies and another more broadly distributed in overdense regions of the universe. Using absorbers not associated with galaxies, we estimate the unweighted geometric mean metallicity of the intergalactic medium to be [X/H] ≲ −2.1 atz< 1, which is lower than previously estimated. 

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4. (Nearly) Model-independent Constraints on the Neutral Hydrogen Fraction in the Intergalactic Medium at z ∼ 5–7 Using Dark Pixel Fractions in Lyα and Lyβ Forests

   https://doi.org/10.3847/1538-4357/aca678  

   Jin, Xiangyu; Yang, Jinyi; Fan, Xiaohui; Wang, Feige; Bañados, Eduardo; Bian, Fuyan; Davies, Frederick B.; Eilers, Anna-Christina; Farina, Emanuele Paolo; Hennawi, Joseph F.; et al (January 2023, The Astrophysical Journal)

   Abstract Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral atz> 7 and largely ionized byz∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volume-averaged neutral fraction of the IGM is either relatively low ( ${\overline{x}}_{\mathrm{H}\mathrm{I}}\lesssim {10}^{-3}$) or close to unity ( ${\overline{x}}_{\mathrm{H}\mathrm{I}}\sim 1$). In particular, the neutral fraction evolution of the IGM at the critical redshift range ofz= 6–7 is poorly constrained. We present new constraints on ${\overline{x}}_{\mathrm{H}\mathrm{I}}$atz∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 <z< 7.09. We derive model-independent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyαand Lyβforests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first model-independent constraints on the IGM neutral hydrogen fraction atz∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.3)<0.79\pm 0.04$(1σ), ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.5)<0.87\pm 0.03$(1σ), and ${\overline{x}}_{\mathrm{H}\mathrm{I}}(z=6.7)<{0.94}_{-0.09}^{+0.06}$(1σ). The dark pixel fractions atz> 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018. 

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5. Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang

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

   Huyan 呼延, Jianghao 江豪; Kulkarni, Varsha P.; Poudel, Suraj; Tejos, Nicolas; Péroux, Celine; Lopez, Sebastian (August 2023, The Astrophysical Journal Letters)

   Abstract Abundances of chemical elements in the interstellar and circumgalactic media of high-redshift galaxies offer important constraints on the nucleosynthesis by early generations of stars. Damped Lyαabsorbers (DLAs) in spectra of high-redshift background quasars are excellent sites for obtaining robust measurements of element abundances in distant galaxies. Past studies of DLAs at redshiftsz> 4 have measured abundances of ≲0.01 solar. Here we report the discovery of a DLA atz= 4.7372 with an exceptionally high degree of chemical enrichment. We estimate the Hicolumn density in this absorber to be log (N_{H I}/cm⁻²) = 20.48 ± 0.15. Our analysis shows unusually high abundances of carbon and oxygen ([C/H] = 0.88 ± 0.17, [O/H] = 0.71 ± 0.16). Such a high level of enrichment a mere 1.2 Gyr after the Big Bang is surprising because of insufficient time for the required amount of star formation. To our knowledge, this is the first supersolar absorber found atz> 4.5. We find the abundances of Si and Mg to be [Si/H] = $-{0.56}_{-0.35}^{+0.40}$and [Mg/H] = ${0.59}_{-0.50}^{+0.27}$, confirming the metal-rich nature of this absorber. By contrast, Fe shows a much lower abundance ([Fe/H] = $-{1.53}_{-0.15}^{+0.15}$). We discuss implications of our results for galactic chemical evolution models. The metallicity of this absorber is higher than that of any other known DLA and is >2 orders of magnitude above predictions of chemical evolution models and theN_{H I}-weighted mean metallicity from previous studies atz> 4.5. The relative abundances (e.g., [O/Fe] = 2.29 ± 0.05, [C/Fe] = 2.46 ± 0.08) are also highly unusual compared to predictions for enrichment by early stars. 

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