The HI Mass Density in Galactic Halos, Winds, and Cold Accretion as
  Traced by MgII Absorption
  It is well established that MgII 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 MgII equivalent width (W) distribution with a Schechter function
and applying the N(HI)-W correlation of Menard & Chelouche, we computed
Omega(HI)_MgII ~ Omega(HI)_halo =(1.41 +0.75 -0.44)x10^-4 for 0.4<z<1.4. We
exclude DLAs from our calculations so that Omega(HI)_halo comprises accreting
and/or outflowing halo gas not locked up in cold neutral clouds. We deduce the
cosmic HI gas mass density fraction in galactic halos traced by MgII absorption
is Omega(HI)_halo/Omega(HI)_DLA=15% and Omega(HI)_halo/Omega_b=0.3%. Citing
several lines of evidence, we propose infall/accretion material is sampled by
small W whereas outflow/winds are sampled by large W, and find Omega(HI)_infall
is consistent with Omega(HI)_outflow for bifurcation at
W=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 mass
cycling through halos remains fairly constant with cosmic time and that the
accretion of HI gas sustains galaxy winds, and (2) evolution in the cosmic star
formation rate depends primarily on the rate at which cool HI gas cycles
through halos.
