Probing the Physical Conditions of Atomic Gas at High Redshift

TL;DR

This study introduces a novel method to determine the physical conditions of atomic gas in high-redshift DLAs, revealing the presence of dense, cold, and turbulent gas phases similar to local interstellar medium conditions.

Contribution

The paper develops a new spectroscopic approach using fine-structure level ratios to constrain the physical parameters of DLAs, providing insights into their density, temperature, and pressure.

Findings

At least 5% of DLAs contain dense, cold gas with densities around 100 cm-3 and temperatures below 500 K.

The typical pressure of DLAs is comparable to the local interstellar medium, log(P/k) = 3.4.

Eight systems show high-pressure signatures indicating a turbulent, star-forming ISM environment.

Abstract

A new method is used to measure the physical conditions of the gas in damped Lyman-alpha systems (DLAs). Using high resolution absorption spectra of a sample of 80 DLAs, we are able to measure the ratio of the upper to lower fine-structure levels of the ground state of C II and Si II. These ratios are determined solely by the physical conditions of the gas. We explore the allowed physical parameter space using a Monte Carlo Markov Chain method to constrain simultaneously the temperature, neutral hydrogen density, and electron density of each DLA. The results indicate that at least 5 % of all DLAs have the bulk of their gas in a dense, cold phase with typical densities of ~100 cm-3 and temperatures below 500 K. We further find that the typical pressure of DLAs in our sample is log(P/k) = 3.4 [K cm-3], which is comparable to the pressure of the local interstellar medium (ISM), and that the components containing the bulk of the neutral gas can be quite small with absorption sizes as small as a few parsec. We show that the majority of the systems are consistent with having densities significantly higher than expected from a purely canonical WNM, indicating that significant quantities of dense gas (i.e. n_H > 0.1 cm-3) are required to match observations. Finally, we identify 8 systems with positive detections of Si II*. These systems have pressures (P/k) in excess of 20000 K cm-3, which suggest that these systems tag a highly turbulent ISM in young, star-forming galaxies.