The Physical Nature of the Most Metal-Poor Damped Lyman Alpha Systems

TL;DR

This study uses high-resolution cosmological simulations to explore the physical properties and distribution of extremely metal-poor damped Lyman alpha systems at redshift 3, revealing their origins, characteristics, and evolution within galaxy environments.

Contribution

It provides a detailed physical model of mpDLAs, including their typical locations, properties, and how they evolve and interact with host galaxies, which was not previously well understood.

Findings

mpDLAs mainly reside in infalling cold gas streams at >20 kpc from galaxy centers.

They have low densities, low star formation rates, and are mostly diffuse and metal-poor.

Simulated observables match observations except for gas density, which is lower in simulations.

Abstract

Utilizing the high-resolution, large-scale LAOZI cosmological simulations we investigate the nature of the metal-poor (${\rm [Z/H]<-2}$) damped Lyman alpha systems (mpDLA) at $z=3$. The following physical picture of mpDLAs emerges. The majority of mpDLAs inhabit regions $\ge 20$~kpc from the host galaxy center on infalling cold gas streams originating from the intergalactic medium, with infall velocity of $\sim 100$ km/s and temperature of $\sim 10^{4}$ K. For each host galaxy, on average, about $1\%$ of the area within a radius $150$~kpc is covered by mpDLAs. The mpDLAs are relatively diffuse ($n_{\rm{gas}} \sim 10^{-2}$ cm$^{-3}$), Jeans quasi-stable, and have very low star formation rate ($\dot{\Sigma} \le 10^{-4} M_{\odot} \rm{\ yr}^{-1} \rm{\ kpc}^{-2}$). As mpDLAs migrate inward to the galaxy center, they mix with high metallicity gas and stellar outflows in the process, removing themselves from the metal-poor category and rendering the central ($\le 5$ kpc) regions of galaxies devoid of mpDLAs. Thus, the central regions of the host galaxies are populated by mostly metal-rich DLAs instead of mpDLAs. All observables of the simulated mpDLAs are in excellent agreement with observations, except the gas density, which is about a factor of ten lower than the value inferred observationally. However, the observationally inferred value is based on simplified assumptions that are not borne out in the simulations.