MUSEQuBES: Physical conditions, origins, and multi-element abundances of the circumgalactic medium of an isolated, star-forming dwarf galaxy at z=0.57

Abstract

In dwarf galaxy models, outflows expel metal-enriched interstellar medium (ISM) into the circumgalactic medium (CGM) to reproduce their observed low metallicities, but measurements of dwarf CGM properties are scarce. We present a study of the CGM of an isolated dwarf at $z=0.5723$ with a stellar mass of $\approx5\times10^7\rm\,M_{\odot}$ and star-formation rate ($\approx0.05\,\rm M_\odot\,yr^{-1}$) and ISM metallicity ($\rm [O/H]\approx-0.9$) consistent with the star-forming main sequence and mass-metallicity relation. A background quasar sightline with archival UV spectra probes the dwarf's CGM at a projected distance of 28 kpc, corresponding to approximately half of the estimated virial radius. The dwarf's CGM is detected in \ion{H}{1}, intermediate metal ions of \ion{C}{3}, \ion{O}{3}, \ion{O}{4}, and \textcolor{black}{\ion{S}{5}}, and kinematically broader, highly-ionized \ion{O}{6}, but is undetected in \ion{N}{4} and \ion{Ne}{8}. Photoionization modeling of the intermediate ions indicates a modest volume-filling factor ($\sim 6\%$ along the sightline or $\sim 2\%$ globally), and a mass of $\sim2\times10^8 {\rm\,M_\odot}$, $\sim4\times$ higher than the dwarf's stellar mass, but $\sim10\times$ less than the highly ionized CGM. The \ion{O}{6} kinematics are comparable to the dwarf's estimated virial velocity, suggesting it is likely associated with cool, photoionized, and volume-filling CGM, with bulk motion or turbulence dominating over thermal pressure. The metallicity inferred for the intermediate ions is $\rm [O/H]=-0.6$, but with low relative abundances of $\rm [C/O]=-0.6$ and \textcolor{black}{$\rm [N/O]<-1.0$}. The [N/O] is below levels expected of the dwarf's ISM, but consistent with core-collapse supernova ejecta, suggesting that supernova-enriched gas escaped the dwarf without mixing significantly with ISM enriched in nitrogen from evolved, low-mass stars.