The Diversity of Metal-Enrichment and Abundance Patterns at High Redshift: A Magellan Survey of Gas-rich Galaxies Traced by Damped Lyman-alpha Absorbers at z~5

TL;DR

The study probes first-star nucleosynthesis by measuring detailed element abundances in 10 DLAs at $z\simeq 4.2$–$5.0$ using high-resolution Magellan MIKE spectroscopy. It derives H I and metal-column densities via Voigt-profile fitting, applies ionization corrections with Cloudy, and corrects for dust depletion with Jenkins sequences to obtain intrinsic metallicities and dust-to-metal ratios. The results reveal a wide, non-monotonic metallicity distribution at $z>4.5$, a generally smooth metallicity evolution to $z\sim5.3$, and evidence for Pop III enrichment signatures in several DLAs, alongside persistent $\alpha$-element enhancements even after depletion corrections. The findings indicate metal-rich, dusty DLAs exist within the first billion years, suggesting inner-galaxy regions with accelerated chemical enrichment and informing models of early star formation and IMF variations.

Abstract

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 Lyman-alpha 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, alpha-element enhancement is evident in some DLAs, including a DLA at z=4.7 with a super-solar metallicity. Comparing [C/O] and [Si/O] with model predictions, 4 DLAs in our survey seem consistent with a non-zero Pop III contribution (3 with >=30% Pop-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.

Figures (19)

• Figure 1: Voigt profiles of Lyman-$\alpha$ lines for 10 DLAs in our survey. The fitted profiles are in solid red, normalized quasar spectra are in black, and the 1 $\sigma$ uncertainties in the normalized flux are in green. The dashed black curves show profiles corresponding to $\pm 0.1$ conservative uncertainties in ${\rm log}N_{\rm H I}$ values. The DLAs in the sightlines toward J0007-5701 and J0838-0440, the $z=4.555$ DLA toward J1135-1526, and the $z=4.889$ DLA toward J2328+0217 are located in the quasar proximity zones, with velocity separations $\Delta v \le 5000 \ {\rm km/s}$ relative to the quasar emission redshifts.
• Figure 2: Metallicity-redshift relationship for DLAs from our survey as well as the literature. The small grey points show individual DLAs from the literature at $z < 4.5$, and the blue circles show $N_{\rm H I}$-weighted mean metallicities for the DLAs in various redshift bins. The purple diamonds and green diamonds show the DLAs at $z > 4.5$ from this work and the literature. The magenta triangle shows a super-solar metallicity DLA toward J0025-0145 reported in 2023ApJ...954L..19H. The orange square shows the binned $N_{\rm H I}$-weighted mean metallicity for the DLAs at $z > 4.5$, including all the measurements in this work and the literature except the supersolar metallicity DLA toward J0025-0145. DLAs at $z > 4.5$ show a wider diversity in metallicities compared to those at $z < 4.5$. The dashed blue line shows the best linear regression fit for the binned relation for DLAs at $z < 4.5$ given by $\mathrm{[M/H]} = -0.190\times z_{abs}-0.655$, and the shaded region in blue shows the $\pm 1 \sigma$ uncertainty in this best fit for the $z < 4.5$ DLAs. All of the metallicity measurements shown are determined from undepleted elements (Zn, S, or O). The solid black, dot-dashed brown, and dotted purple curves show predicted metallicity-redshift relations from various models. Our findings suggest a relatively smooth DLA metallicity-redshift relation.
• Figure 3: The scatter in DLA metallicities as a function of redshift. Blue circles denote 1 standard deviations in the metallicities ($\sigma_{\mathrm{[X/H]}}$) of individual DLAs in the various redshift bins, and red diamonds denote the range of metallicities ($\mathrm{[X/H]_{\rm max} - \mathrm [X/H]_{\rm min}}$) for DLAs in various redshift bins. Both the metallicity range and the standard deviation show a increasing trend for the bins at redshift $3.7<z<4.5$ and $4.5<z<5.3$ compared to those at $z < 3.7$.
• Figure 4: Comparison of the dust-depletion corrected abundances [C/O] and [Si/O] for DLAs from our study and the literature with the the maximum extent of [C/O] and [Si/O] in simulations from 2023MNRAS.526.2620V for different levels of enrichment from Pop III stars. A selected sample of very metal-poor DLAs from 2011MNRAS.412.1047C2011MNRAS.417.1534C and references therein are shown as filled yellow circles. Proximate DLAs from 2024AA...687A.314S shown as filled blue circles. Red diamonds show DLAs from our survey, and diamonds enclosed in circles with different colors are the DLAs that are consistent with a non-zero contribution from Pop III stars (J1129: orange; J1135 at $z_{DLA}=4.346$: magenta; J2202: cyan; J2328 at $z_{DLA}=4.889$: green).
• Figure 5: The chemical abundance patterns are overplotted in different colors for the different DLAs from our survey presented in this work and 2023ApJ...954L..19H. The chemical enrichment [X/H] is plotted in the order of X$=$C, O, Mg, S, Si and Fe on the y-axis. Comparison of different elements reveals enhanced observed [$\alpha$/Fe] ratios for O, Mg, Si and S for several DLAs.