Origin of Cosmic Chemical Abundances

TL;DR

This study uses cosmological simulations with detailed chemistry and star formation to explore the origins of chemical abundance patterns in early galaxies, matching observations of metal-poor absorbers and predicting potential Population III sites.

Contribution

It introduces comprehensive hydrodynamic simulations including atomic/molecular chemistry and star formation to analyze chemical evolution and metal/molecular content in galaxy formation.

Findings

Simulations reproduce observed elemental ratios at high redshift.

Metal-poor cold clumps with high optical depth may host Population III stars.

Results align with observed dust content in early galaxies.

Abstract

Cosmological N-body hydrodynamic computations following atomic and molecular chemistry (e$^-$, H, H$^+$, H$^-$, He, He$^+$, He$^{++}$, D, D$^+$, H$_2$, H$_2^+$, HD, HeH$^+$), gas cooling, star formation and production of heavy elements (C, N, O, Ne, Mg, Si, S, Ca, Fe, etc.) from stars covering a range of mass and metallicity are used to explore the origin of several chemical abundance patterns and to study both the metal and molecular content during simulated galaxy assembly. The resulting trends show a remarkable similarity to up-to-date observations of the most metal-poor damped Lyman-$\alpha$ absorbers at redshift $z\gtrsim 2$. These exhibit a transient nature and represent collapsing gaseous structures captured while cooling is becoming effective in lowering the temperature below $\sim 10^4\,\rm K$, before they are disrupted by episodes of star formation or tidal effects. Our theoretical results agree with the available data for typical elemental ratios, such as [C/O], [Si/Fe], [O/Fe], [Si/O], [Fe/H], [O/H] at redshifts $z\sim 2-7$. Correlations between HI and H$_2$ abundances show temporal and local variations and large spreads as a result of the increasing cosmic star formation activity from $z\sim 6$ to $z\sim 3$. The scatter we find in the abundance ratios is compatible with the observational data and is explained by simultaneous enrichment by sources from different stellar phases or belonging to different stellar populations. Simulated synthetic spectra support the existence of metal-poor cold clumps with large optical depth at $z\sim 6$ that could be potential population~III sites at low or intermediate redshift. The expected dust content is in line with recent determinations.