Low-metallicity Gas on the Outskirts of the Local Group: The Circumgalactic Medium of Sextans B*

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 (LG). Using Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy of two active galactic nucleus sight lines passing through the Sextans B CGM at small impact parameters of 4 and 8 kpc (≈0.04–0.08 R 200 ), 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 the 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 gas-phase silicon and carbon abundances in the Sextans B CGM of [Si/H] = −1.7 ± 0.2 and [C/H] = −2.1 ± 0.2, respectively, among the lowest gas-phase abundances anywhere in the LG. We calculate a cool CGM gas mass within 8 kpc of ≈4 × 10 7 M ⊙ , comparable to the H i mass and the stellar mass of Sextans B.