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

Abstract

We use H i absorption measurements to constrain the amount of cool (≈104 K), photoionized gas in the circumgalactic medium (CGM) of dwarf galaxies with M* = 106.5−9.5M⊙ 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 H i column density profile. We introduce a power-law density profile for the gas distribution and fit our model to archival H i observations to infer the cool CGM gas mass, McCGM, as a function of halo mass. For volume-filling (fV = 1) models, we find McCGM = 5 × 108–2 × 109M⊙, constituting ≲10% of the halo baryon budget. For clumpy gas, with fV = 0.01, the masses are a factor of ≈11 lower, in agreement with our analytic approximation. Our assumption that the measured H i forms entirely in the cool CGM provides a conservative upper limit on McCGM, 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 ≈15% and ≈10%, respectively. Our results show that dwarf galaxies have only ≲15% 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.