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1. 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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2. The Diversity of Metal Enrichment and Abundance Patterns at High Redshift: A Magellan Survey of Gas-rich Galaxies Traced by Damped Ly α Absorbers at z  ∼ 5

   https://doi.org/10.3847/1538-4357/adf63e  

   Huyan, J; Kulkarni, V P; Poudel, S; Tejos, N; Péroux, C; Lopez, S (September 2025, The Astrophysical Journal)

   A powerful technique to trace the signatures of the first stars is through the metal enrichment in concentrated reservoirs of hydrogen, such as the damped Lyα absorbers (DLAs) in the early Universe. We conducted a survey aimed at discovering DLAs along sight lines to high-z quasars in order to measure element abundances at z > 4. Here we report our first results from this survey for 10 DLAs with redshifts of ≈4.2–5.0. We determine abundances of C, O, Si, S, and Fe, and thereby the metallicities and dust depletions. We find that DLA metallicities at z > 4.5 show a wide diversity spanning ∼3 orders of magnitude. The metallicities of DLAs at 3.7 < z < 5.3 show a larger dispersion compared to that at lower redshifts. Combining our sample with the literature, we find a relatively smooth evolution of metallicity with redshift out to z ∼ 5.3, with a tentative (∼2 sigma) indication of a slight rise in metallicity at 4.5 < z < 5.3. The relative abundances exhibit C enhancement for both metal-poor and metal-enriched DLAs. In addition, α-element enhancement is evident in some DLAs, including a DLA at z = 4.7 with a supersolar metallicity. Comparing [C/O] and [Si/O] with model predictions, four DLAs in our survey seem consistent with a nonzero Population III contribution (three with >~30% Population III contribution). Combining our sample and the literature, we find the dust depletion strength and dust-to-metal ratios to correlate positively with the total (gas+solid phase) metallicity, confirming the presence of metal-rich, dusty DLAs even at ∼1 billion years after the Big Bang. 

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3. Evidence of Pop III stars’ chemical signature in neutral gas at z ∼ 6: A study based on the E-XQR-30 spectroscopic sample

   https://doi.org/10.1051/0004-6361/202349062  

   Sodini, Alessio; D’Odorico, Valentina; Salvadori, Stefania; Vanni, Irene; Bischetti, Manuela; Cupani, Guido; Davies, Rebecca; Becker, George D; Bañados, Eduardo; Bosman, Sarah; et al (July 2024, Astronomy & Astrophysics)

   Aims.This study explores the metal enrichment signatures attributed to the first generation of stars (Pop III) in the Universe, focusing on the E-XQR-30 sample – a collection of 42 high signal-to-noise ratio spectra of quasi-stellar objects (QSOs) with emission redshifts ranging from 5.8 to 6.6. We aim to identify traces of Pop III metal enrichment by analyzing neutral gas in the interstellar medium of primordial galaxies and their satellite clumps, detected in absorption. Methods.To chase the chemical signature of Pop III stars, we studied metal absorption systems in the E-XQR-30 sample, selected through the detection of the neutral oxygen absorption line at 1302 Å. The O Iline is a reliable tracer of neutral hydrogen and allowed us to overcome the challenges posed by the Lyman-αforest’s increasing saturation at redshifts above ∼5 to identify damped Lyman-αsystems (DLAs). We detected and analyzed 29 O Isystems atz ≥ 5.4, differentiating between proximate DLAs (PDLAs) and intervening DLAs. Voigt function fits were applied to obtain ionic column densities, and relative chemical abundances were determined for 28 systems. These were then compared with the predictions of theoretical models. Results.Our findings expand the study of O Isystems atz ≥ 5.4 fourfold. No systematic differences were observed in the average chemical abundances between PDLAs and intervening DLAs. The chemical abundances in our sample align with literature systems atz >  4.5, suggesting a similar enrichment pattern for this class of absorption systems. A comparison between these DLA-analogs at 4.5 <  z <  6.5 with a sample of very metal-poor DLAs at 2 <  z <  4.5 shows in general similar average values for the relative abundances, with the exception of [C/O], [Si/Fe] and [Si/O] which are significantly larger for the high-zsample. Furthermore, the dispersion of the measurements significantly increases in the high-redshift bin. This increase is predicted by the theoretical models and indicates a potential retention of Pop III signatures in the probed gas. Conclusions.This work represents a significant advancement in the study of the chemical properties of highly neutral gas atz ≥ 5.4, shedding light on its potential association with the metal enrichment from Pop III stars. Future advancements in observational capabilities, specifically high-resolution spectrographs, are crucial for refining measurements and addressing current limitations in the study of these distant absorption systems. 

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4. Empirical Constraints on Core-collapse Supernova Yields Using Very Metal-poor Damped Lyα Absorbers

   https://doi.org/10.3847/1538-4357/ac470e  

   Nuñez, Evan H.; Kirby, Evan N.; Steidel, Charles C. (March 2022, The Astrophysical Journal)

   Abstract We place empirical constraints on the yields from zero- and low-metallicity core-collapse supernovae (CCSNe) using abundances measured in very metal-poor (VMP; [Fe/H] ≤ −2) damped Ly α absorbers (DLAs). For some abundance ratios ([N,Al,S/Fe]), VMP DLAs constrain the metal yields of the first SNe more reliably than VMP stars. We compile a large sample of high-S/N VMP DLAs from over 30 yr of literature, most with high-resolution spectral measurements. We infer the initial-mass-function-averaged CCSNe yield from the median values from the DLA abundance ratios of C, N, O, Al, Si, S, and Fe (over Fe and O). We assume that the DLAs are metal-poor enough that they represent galaxies in their earliest stages of evolution, when CCSNe are the only nucleosynthetic sources of the metals we analyze. We compare five sets of zero- and low-metallicity theoretical yields to the empirical yields derived in this work. We find that the five models agree with the DLA yields for ratios containing Si and S. Only one model (Heger & Woosley 2010, hereafter HW10) reproduced the DLA values for N, and one other model (Limongi & Chieffi 2018, hereafter LC18) reproduced [N/O]. We found little change in the theoretical yields with the adoption of an SN explosion landscape (where certain progenitor masses collapse into black holes, contributing no yields) onto HW10, but fixing explosion energy to progenitor mass results in wide disagreements between the predictions and DLA abundances. We investigate the adoption of a simple, observationally motivated initial distribution of rotational velocities for LC18 and find a slight improvement. 

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5. Metals and a search for molecules in the distant Universe: Magellan MIKE observations of sub-DLAs at 2 < z < 3

   https://doi.org/10.1093/mnras/stab926  

   Poudel, S; Kulkarni, V P; Som, D; Péroux, C (April 2021, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present abundance measurements of the elements Zn, S, O, C, Si, and Fe for four sub-DLAs at redshifts ranging from z = 2.173 to 2.635 using observations from the MIKE spectrograph on the Magellan telescope to constrain the chemical enrichment and star formation of gas-rich galaxies. Using weakly depleted elements O, S, and or Zn, we find the metallicities after the photoionization corrections to be [S/H] = −0.50 ± 0.11, [O/H] > −0.84, [O/H] = −1.27 ± 0.12, and [Zn/H]  = +0.40 ± 0.12 for the absorbers at z  = 2.173, 2.236, 2.539, and 2.635, respectively. Moreover, we are able to put constraints on the electron densities using the fine structure lines of C ii⋆ and Si ii⋆ for two of the sub-DLAs. We find that these values are much higher than the median values found in DLAs in the literature. Furthermore, we estimate the cooling rate lc = 1.20 × 10−26 erg s−1 per H atom for an absorber at z = 2.173, suggesting higher star formation rate density in this sub-DLA than the typical star formation rate density for DLAs at similar redshifts. We also study the metallicity versus velocity dispersion relation for our absorbers. Most of the absorbers follow the trend one can expect from the mass versus metallicity relation for sub-DLAs in the literature. Finally, we are able to put limits on the molecular column density from the non-detections of various strong lines of CO molecules. We estimate 3σ upper limits of logN(CO, J = 0) < 13.87, logN(CO, J = 0) < 13.17, and logN(CO, J = 0) < 13.08, respectively, from the non-detections of absorption from the J = 0 level in the CO AX 0–0, 1–0, and 2–0 bands near 1544, 1510, and 1478 Å. 

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