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It is well established that MgII absorption lines detected in backgroundquasar spectra arise from gas structures associated with foreground galaxies.The degree to which galaxy evolution is driven by the gas cycling through halosis highly uncertain because their gas mass density is poorly constrained.Fitting the MgII equivalent width (W) distribution with a Schechter functionand applying the N(HI)-W correlation of Menard & Chelouche, we computedOmega(HI)_MgII ~ Omega(HI)_halo =(1.41 +0.75 -0.44)x10^-4 for 0.4<z<1.4. Weexclude DLAs from our calculations so that Omega(HI)_halo comprises accretingand/or outflowing halo gas not locked up in cold neutral clouds. We deduce thecosmic HI gas mass density fraction in galactic halos traced by MgII absorptionis Omega(HI)_halo/Omega(HI)_DLA=15% and Omega(HI)_halo/Omega_b=0.3%. Citingseveral lines of evidence, we propose infall/accretion material is sampled bysmall W whereas outflow/winds are sampled by large W, and find Omega(HI)_infallis consistent with Omega(HI)_outflow for bifurcation atW=1.23^{+0.15}_{-0.28}\AA; cold accretion would then comprise no more than ~7%of of the total HI mass density. We discuss evidence that (1) the total HI masscycling through halos remains fairly constant with cosmic time and that theaccretion of HI gas sustains galaxy winds, and (2) evolution in the cosmic starformation rate depends primarily on the rate at which cool HI gas cyclesthrough halos.

The HI Mass Density in Galactic Halos, Winds, and Cold Accretion as Traced by MgII Absorption