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1. A [C ii ] 158 μ m Survey of Damped Lyα Absorber Galaxies at z  ∼ 4: Observations of Dense and Metal-enriched Neutral Gas within the Circumgalactic Medium of Star-forming Galaxies

   https://doi.org/10.3847/1538-4357/adbbd1  

   Neeleman, Marcel; Kanekar, Nissim; Prochaska, J Xavier; Rafelski, Marc A; Kahinga, Lordrick A (April 2025, The Astrophysical Journal)

   Abstract We present a survey undertaken with the Atacama Large Millimeter/submillimeter Array (ALMA) to study the galaxies associated with a representative sample of 16 damped Lyαabsorbers (DLAs) atz ≈ 4.1–4.5, using the [Cii] 158μm ([Cii]) line. We detect seven [Cii]-emitting galaxies in the fields of five DLAs, all of which have absorption metallicity [M/H] > −1.5. We find that the detectability of these Hi-selected galaxies with ALMA is a strong function of DLA metallicity, with a detection rate of $71_{-20}^{+11}$% for DLAs with [M/H] > −1.5 and 0⁺¹⁸% for DLAs with [M/H] < −1.5. The identified DLA galaxies have far-IR properties similar to those of typical star-forming galaxies atz ∼ 4, with estimated obscured star formation rates ranging from ≲6M_⊙yr⁻¹to 110M_⊙yr⁻¹. High-metallicity DLAs therefore provide an efficient way to identify and study samples of high-redshift, star-forming galaxies, without preselecting the galaxies by their emission properties. The agreement between the velocities of the metal absorption lines of the DLA and the [Cii] emission line of the DLA galaxy indicates that the metals within the DLA originated in the galaxy. With observed impact parameters between 14 and 59 kpc, this indicates that star-forming galaxies atz ∼ 4 have a substantial reservoir of dense, cold, neutral gas within their circumgalactic medium that has been enriched with metals from the galaxy. 

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2. KODIAQ-Z: Metals and Baryons in the Cool Intergalactic and Circumgalactic Gas at 2.2 ≲ z ≲ 3.6

   https://doi.org/10.3847/1538-4357/ac7400  

   Lehner, Nicolas; Kopenhafer, Claire; O’Meara, John M.; Howk, J. Christopher; Fumagalli, Michele; Prochaska, J. Xavier; Acharyya, Ayan; O’Shea, Brian W.; Peeples, Molly S.; Tumlinson, Jason; et al (September 2022, The Astrophysical Journal)

   Abstract We present the KODIAQ-Z survey aimed to characterize the cool, photoionized gas at 2.2 ≲z≲ 3.6 in 202 Hi-selected absorbers with 14.6 ≤ $\log N_{\mathrm{H}\mathrm{I}}$< 20 that probe the interface between galaxies and the intergalactic medium (IGM). We find that gas with $14.6\le \log N_{\mathrm{H}\mathrm{I}}<20$at 2.2 ≲z≲ 3.6 can be metal-rich (−1.6 ≲ [X/H] ≲ − 0.2) as seen in damped Lyαabsorbers (DLAs); it can also be very metal-poor ([X/H] < − 2.4) or even pristine ([X/H] < − 3.8), which is not observed in DLAs but is common in the IGM. For $16<\log N_{\mathrm{H}\mathrm{I}}<20$absorbers, the frequency of pristine absorbers is about 1%–10%, while for $14.6\le \log N_{\mathrm{H}\mathrm{I}}\le 16$absorbers it is 10%–20%, similar to the diffuse IGM. Supersolar gas is extremely rare (<1%) at these redshifts. The factor of several thousand spread from the lowest to highest metallicities and large metallicity variations (a factor of a few to >100) between absorbers separated by less than Δv< 500 km s⁻¹imply that the metals are poorly mixed in $14.6\le \log N_{\mathrm{H}\mathrm{I}}<20$gas. We show that these photoionized absorbers contribute to about 14% of the cosmic baryons and 45% of the cosmic metals at 2.2 ≲z≲ 3.6. We find that the mean metallicity increases withN_Hi, consistent with what is found inz< 1 gas. The metallicity of gas in this column density regime has increased by a factor ∼8 from 2.2 ≲z≲ 3.6 toz< 1, but the contribution of the $14.6\le \log N_{\mathrm{H}\mathrm{I}}<19$absorbers to the total metal budget of the universe atz< 1 is a quarter of that at 2.2 ≲z≲ 3.6. We show that FOGGIE cosmological zoom-in simulations have a similar evolution of [X/H] withN_Hi, which is not observed in lower-resolution simulations. In these simulations, very metal-poor absorbers with [X/H] < − 2.4 atz∼ 2–3 are tracers of inflows, while higher-metallicity absorbers are a mixture of inflows and outflows. 

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3. SILVERRUSH. XIV. Ly α Luminosity Functions and Angular Correlation Functions from 20,000 Ly α Emitters at z  ∼ 2.2–7.3 from up to 24 deg ² HSC-SSP and CHORUS Surveys: Linking the Postreionization Epoch to the Heart of Reionization

   https://doi.org/10.3847/1538-4365/adb1c0  

   Umeda, Hiroya; Ouchi, Masami; Kikuta, Satoshi; Harikane, Yuichi; Ono, Yoshiaki; Shibuya, Takatoshi; Inoue, Akio K; Shimasaku, Kazuhiro; Liang, Yongming; Matsumoto, Akinori; et al (March 2025, The Astrophysical Journal Supplement Series)

   Abstract We present luminosity functions (LFs) and angular correlation functions (ACFs) derived from 18,960 Lyαemitters (LAEs) atz = 2.2−7.3 over a wide survey area of ≲24 deg²that are identified in the narrowband data of the HSC-SSP and CHORUS surveys. Confirming the large sample with 241 spectroscopically identified LAEs, we determine LyαLFs and ACFs in the brighter luminosity range down to 0.5L_⋆, and confirm that our measurements are consistent with previous studies but offer significantly reduced statistical uncertainties. The improved precision of our ACFs allows us to clearly detect one-halo terms at some redshifts, and provides large-scale bias measurements that indicate host halo masses of ∼10¹¹M_⊙overz ≃ 2−7. By comparing our LyαLF (ACF) measurements with reionization models, we estimate the neutral hydrogen fractions in the intergalactic medium to bex_Hi < 0.05 (= ${0.06}_{-0.03}^{+0.12}$) atz= 5.7 andx_Hi= $0.15_{-0.08}^{+0.10}$( ${0.21}_{-0.14}^{+0.19}$), $0.18_{-0.12}^{+0.14}$, and $0.75_{-0.13}^{+0.09}$atz= 6.6, 7.0, and 7.3, respectively. Our findings suggest that the neutral hydrogen fraction remains relatively low,x_Hi ≲ 0.2, atz = 5−7, but increases sharply atz > 7, reachingx_Hi ∼ 0.9 byz ≃ 8−9, as indicated by recent JWST studies. The combination of our results from LAE observations with recent JWST observations suggests that the major epoch of reionization occurred atz ∼ 7−8, likely driven by the emergence of massive sources emitting significant ionizing photons. 

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4. Looking at the Distant Universe with the MeerKAT Array: The H i Mass Function in the Local Universe

   https://doi.org/10.3847/1538-4357/ad9f3f  

   Kazemi-Moridani, Amir; Baker, Andrew J; Verheijen, Marc; Gawiser, Eric; Blyth, Sarah-Louise; Obreschkow, Danail; Chemin, Laurent; Collier, Jordan D; Cook, Kyle W; Delhaize, Jacinta; et al (March 2025, The Astrophysical Journal)

   Abstract We present measurements of the neutral atomic hydrogen (Hi) mass function (HiMF) and cosmic Hidensity (Ω_{H I}) at 0 ≤z≤ 0.088 from the Looking at the Distant Universe with MeerKAT Array (LADUMA) survey. Using LADUMA Data Release 1 (DR1), we analyze the HiMF via a new “recovery matrix” method that we benchmark against a more traditional modified maximum likelihood (MML) method. Our analysis, which implements a forward modeling approach, corrects for survey incompleteness and uses extensive synthetic source injections to ensure robust estimates of the HiMF parameters and their associated uncertainties. This new method tracks the recovery of sources in mass bins different from those in which they were injected and incorporates a Poisson likelihood in the forward modeling process, allowing it to correctly handle uncertainties in bins with few or no detections. The application of our analysis to a high-purity subsample of the LADUMA DR1 spectral line catalog in turn mitigates any possible biases that could result from the inconsistent treatment of synthetic and real sources. For the surveyed redshift range, the recovered Schechter function normalization, low-mass slope, and “knee” mass are ${\varphi }_{*}=3.56_{-1.92}^{+0.97}\times 10^{-3}$Mpc⁻³dex⁻¹, $\alpha =-1.18_{-0.19}^{+0.08}$, and $\log (M_{*}/M_{\odot })=10.01_{-0.12}^{+0.31}$, respectively, which together imply a comoving cosmic Hidensity of ${\Omega }_{\mathrm{H}\mathrm{I}}=3.09_{-0.47}^{+0.65}\times 10^{-4}$. Our results show consistency between recovery matrix and MML methods and with previous low-redshift studies, giving confidence that the cosmic volume probed by LADUMA, even at low redshifts, is not an outlier in terms of its Hicontent. 

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5. H i Properties of Field Galaxies at z  ≈ 0.2–0.6: Insights into Declining Cosmic Star Formation

   https://doi.org/10.3847/2041-8213/ae0d8b  

   DePalma, David; Gupta, Neeraj; Chen, Hsiao-Wen; Simcoe, Robert A; Balashev, Sergei; Boettcher, Erin; Cantalupo, Sebastiano; Chen, Mandy C; Combes, Françoise; Faucher-Giguère, Claude-André; et al (October 2025, The Astrophysical Journal Letters)

   Abstract We report statistically significant detection of Hi21 cm emission from intermediate-redshift (z ≈ 0.2–0.6) galaxies. By leveraging multisightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey and deep radio observations from the MeerKAT Absorption Line Survey, we have established a sample of ≈6000 spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass, from $\log M_{\mathrm{star}}/M_{\odot }\approx 8$to $\log M_{\mathrm{star}}/M_{\odot }\approx 11$, with a median of ${〈\log M_{\mathrm{star}}/M_{\odot }〉}_{\mathrm{med}}\approx 10$and a wide range in redshift fromz ≈ 0.24 toz ≈ 0.63 with a median of 〈z〉_med = 0.44. While no individual galaxies show detectable Hiemission, the emission line signal is detected in the stacked spectra of all subsamples at greater than 4σsignificance. The observed total Hi21 cm line flux translates to a Himass,M_{H I}≈10¹⁰M_⊙. We find a high Hi-to-stellar-mass ratio ofM_HI/M_star ≈ 6 for low-mass galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 9.3$(>3.7σ). For galaxies with $〈\log M_{\mathrm{star}}/M_{\odot }〉\approx 10.6$, we findM_HI/M_star ≈ 0.3 (>4.7σ). In addition, the redshift evolution of Himass, 〈M_{H I}〉, in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21 cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of Higas in their extended surroundings. 

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