A Comprehensive Investigation of Metals in the Circumgalactic Medium of Nearby Dwarf Galaxies

Abstract

Dwarf galaxies are found to have lost most their metals via feedback processes; however, there still lacks consistent assessment on the retention rate of metals in their circumgalactic medium (CGM). Here we investigate the metal content in the CGM of 49 isolated dwarf galaxies with $M_*=10^{6.5-9.5}~M_\odot$ ($M_{\rm 200m}=10^{10.0-11.5}~M_\odot$) using HST/COS spectroscopy. While HI (Ly$\alpha$) is ubiquitously detected ($89\%$) within the CGM, we find low detection rates ($\approx5-21\%$) in CII, CIV, SiII, SiIII, and SiIV, largely consistent with literature values. Assuming these ions form in the cool ($T\approx10^4$ K) CGM with photoionization equilibrium, the observed HI and metal column density profiles can be best explained by an empirical model with low gas density and high volume filling factor. For a typical galaxy with $M_{\rm 200m}=10^{10.9}~M_\odot$ (median of the sample), our model predicts a cool gas mass of $M_{\rm CGM,cool}\sim10^{8.4}~M_\odot$, corresponding to $\sim2\%$ of the galaxy's baryonic budget. Assuming a metallicity of $0.3Z_\odot$, we estimate that the dwarf galaxy's cool CGM only harbors $\sim10\%$ of the metals ever produced, with the rest either in warmer phases yet to be detected, or transported to the intergalactic medium. We further examine the EAGLE simulation and show that HI and low ions may arise from a dense cool medium, while CIV from a diffuse warmer medium. Our work provides the community a uniform dataset on dwarf galaxies' CGM that combines our recent observations, additional archival data and literature compilation, which can be used to test various theoretical models of dwarf galaxies.