The Bimodal Metallicity Distribution of the Cool Circumgalactic Medium at z<1

TL;DR

This study reveals a bimodal metallicity distribution in the cool circumgalactic medium at z<1, indicating the coexistence of metal-rich outflows and metal-poor accretion streams around galaxies.

Contribution

First to analyze the metallicity distribution of the low-redshift cool CGM using an H I-selected sample, revealing its bimodal nature and implications for galaxy gas flows.

Findings

Bimodal metallicity distribution with peaks at -1.6 and -0.3 in [X/H]

Approximately equal numbers of metal-rich and metal-poor absorbers

Presence of significant cold, metal-poor gas in the CGM at z<1

Abstract

We assess the metal content of the cool (10^4 K) circumgalactic medium (CGM) about galaxies at z<1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2<log N(H I)<18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N(H I) selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N(H I) in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H]=-1.6 and -0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z<1 galaxies.