The H I Mass Density in Galactic Halos, Winds, and Cold Accretion as Traced by Mg II Absorption

It is well established that Mg II absorption lines detected in background quasar spectra arise from gas structures associated with foreground galaxies. The degree to which galaxy evolution is driven by the gas cycling through halos is highly uncertain because their gas mass density is poorly constrained. Fitting the Mg II equivalent width (W) distribution with a Schechter function and applying the N(H I)-W correlation of Ménard & Chelouche, we computed Ω(H I)_{Mg II} ≡ Ω(H I)_halo = 1.41^+0.75 _-0.44 × 10^-4 for 0.4 <= z <= 1.4. We exclude damped Lyα's (DLAs) from our calculations so that Ω(H I)_halo comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds. We deduce that the cosmic H I gas mass density fraction in galactic halos traced by Mg II absorption is Ω(H I)_halo/Ω(H I)_DLA ~= 15% and Ω(H I)_halo/Ω _b ~= 0.3%. Citing several lines of evidence, we propose that infall/accretion material is sampled by small W whereas outflow/winds are sampled by large W, and find that Ω(H I)_infall is consistent with Ω(H I)_outflow for bifurcation at W = 1.23^+0.15 _{- 0.28} Å cold accretion would then comprise no more than ~7% of the total H I mass density. We discuss evidence that (1) the total H I mass cycling through halos remains fairly constant with cosmic time and that the accretion of H I gas sustains galaxy winds, and (2) evolution in the cosmic star formation rate depends primarily on the rate at which cool H I gas cycles through halos.