Low Metallicity Gas on the Outskirts of the Local Group: the Circumgalactic Medium of Sextans B

TL;DR

This study analyzes the low-metallicity circumgalactic medium of Sextans B using UV spectroscopy, revealing low metal abundances and a significant cool gas mass, providing insights into dwarf galaxy halos in the Local Group.

Contribution

First detailed UV absorption-line analysis of Sextans B's CGM, measuring low metallicities and gas mass, enhancing understanding of dwarf galaxy halos.

Findings

Detected low-metallicity CGM in Sextans B.

Measured low silicon and carbon abundances.

Estimated substantial cool gas mass in the CGM.

Abstract

We present a UV absorption-line analysis of the circumgalactic medium (CGM) of Sextans B, a dwarf irregular galaxy at 1.3 Mpc distance on the outer frontier of the Local Group. Using HST/COS spectroscopy of two AGN sightlines passing through the Sextans B CGM at small impact parameters of 4 kpc and 8 kpc (0.04 and 0.08 r_vir), we detect the CGM in Si II, Si III, Si IV, and C II absorption. All four ions show a column-density profile that declines with radius. The profiles fall below the average CGM profiles of other nearby dwarfs (by 0.3-0.6 dex, depending on ion), likely due to the low halo mass and low metallicity of Sextans B. Using Cloudy photoionization models and interferometric measurements of the H I column density, we find low gas-phase silicon and carbon abundances in the Sextans B CGM, [Si/H]=-1.7+/-0.2 and [C/H]=-2.1+/-0.2, among the lowest gas-phase abundances anywhere in the Local Group. We calculate a cool CGM gas mass within 8 kpc of $\sim4\times10^7 M_\odot$, comparable to the H I mass and the stellar mass of Sextans B.