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 (≈10 ^4 K), photoionized gas in the circumgalactic medium (CGM) of dwarf galaxies with M _* = 10 ^6.5−9.5 M _⊙ 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, M _cCGM , as a function of halo mass. For volume-filling ( f _V = 1) models, we find M _cCGM = 5 × 10 ^8 –2 × 10 ^9 M _⊙ , constituting ≲10% of the halo baryon budget. For clumpy gas, with f _V = 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 M _cCGM , 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.

Keywords

• Dwarf galaxies
• Galaxy evolution
• Circumgalactic medium