Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy

TL;DR

This study discovers metal-enriched, small-scale gas clumps in the circumgalactic medium of a faint galaxy at redshift 2.5, revealing insights into galactic outflows and the limitations of current cosmological simulations.

Contribution

It provides detailed measurements of metal-enriched, tiny gas clouds in the CGM and discusses their implications for galaxy outflows and simulation resolution requirements.

Findings

Gas clouds are metal-enriched with 0.1-0.6 Z_sun.

Cloud sizes are extremely small, less than 100-500 parsecs.

Outflow rate estimated at approximately 5 solar masses per year.

Abstract

We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$ cm$^{-2}$) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalise over uncertainties in the shape and normalisation of the ionizing radiation spectrum, we derive the total hydrogen column density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all the absorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_\odot$. These metallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of $\sim5 M_\odot\,$yr$^{-1}$. Our photoionization model yields extremely small sizes ($<$100-500 pc) for the absorbing clouds, which we argue are typical of high column density absorbers in the circumgalactic medium (CGM). Given these small sizes and extreme kinematics, it is unclear how the clumps survive in the CGM without being destroyed by hydrodynamic instabilities. The small cloud sizes imply that even state-of-the-art cosmological simulations require more than a $1000$-fold improvement in mass resolution to resolve the hydrodynamics relevant for cool gas in the CGM.