Upper Limits on the Mass of Cool Gas in the Circumgalactic Medium of Dwarf Galaxies

We use HI absorption measurements to constrain the amount of cool ($\approx {10}^4$ K), photoionized gas in the circumgalactic medium (CGM) of dwarf galaxies with $M_{\ast }={10}^{6.5-9.5}{M}_{\odot }$ in the nearby Universe ($z<0.3$). We show analytically that volume-filling gas gives an upper limit on the gas mass needed to reproduce a given HI column density profile. We introduce a power-law density profile for the gas distribution and fit our model to archival HI observations to infer the cool CGM gas mass, $M_{\mathrm{c}\mathrm{C}\mathrm{G}\mathrm{M}}$, as a function of halo mass. For volume-filling ($f_V=1$) models, we find $M_{\mathrm{c}\mathrm{C}\mathrm{G}\mathrm{M}}=5\times {10}^8-2\times {10}^9{M}_{\odot }$, constituting $\lesssim 10\mathrm{\%}$ of the halo baryon budget. For clumpy gas, with $f_V=0.01$, the masses are a factor of $\approx 11$ lower, in agreement with our analytic approximation. Our assumption that the measured HI forms entirely in the cool CGM provides a conservative upper limit on $M_{\mathrm{c}\mathrm{C}\mathrm{G}\mathrm{M}}$, and possible contributions from the intergalactic medium or warm/hot CGM will further strengthen our result. We estimate the mass uncertainties due to the range of redshifts in our sample and the unknown gas metallicity to be $\approx 15\mathrm{\%}$ and $\approx 10\mathrm{\%}$, respectively. Our results show that dwarf galaxies have only $\lesssim 15\mathrm{\%}$ of their baryon budget in stars and the cool CGM, with the rest residing in the warm/hot CGM or ejected from the dark matter halos.