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 ( { \hbox { { H } \kern 1.0 pt { \sc i } } } ) – W correlation of Ménard & Chelouche , we computed \Omega ( \hbox { { H } \kern 1.0 pt { \sc i } } ) _ { \hbox { \tiny \hbox { { Mg } \kern 1.0 pt% { \sc ii } } } } \equiv \Omega ( { \hbox { { H } \kern 1.0 pt { \sc i } } } ) _ { halo } = 1.41 ^ { + % 0.75 } _ { -0.44 } \times 10 ^ { -4 } for 0.4 \leq z \leq 1.4 .
We exclude DLAs from our calculations so that \Omega ( H i ) _ { halo } comprises accreting and/or outflowing halo gas not locked up in cold neutral clouds .
We deduce the cosmic H i gas mass density fraction in galactic halos traced by Mg ii absorption is \Omega ( { \hbox { { H } \kern 1.0 pt { \sc i } } } ) _ { halo } / \Omega ( { \hbox { { H } % \kern 1.0 pt { \sc i } } } ) _ { \hbox { \tiny DLA } } \simeq 15 % and \Omega ( { \hbox { { H } \kern 1.0 pt { \sc i } } } ) _ { halo } / \Omega _ { b } \simeq 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 ( \hbox { { H } \kern 1.0 pt { \sc i } } ) _ { infall } is consistent with \Omega ( \hbox { { H } \kern 1.0 pt { \sc i } } ) _ { outflow } for bifurcation at W = 1.23 ^ { +0.15 } _ { -0.28 } Å ; cold accretion would then comprise no more than \sim 7 % of 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 .