  We use cosmological SPH simulations to study the cool, accreted gas in two
Milky Way-size galaxies through cosmic time to z=0. We find that gas from
mergers and cold flow accretion results in significant amounts of cool gas in
galaxy halos. This cool circum-galactic component drops precipitously once the
galaxies cross the critical mass to form stable shocks, Mvir = Msh ~ 10^12
Msun. Before reaching Msh, the galaxies experience cold mode accretion (T<10^5
K) and show moderately high covering fractions in accreted gas: f_c ~ 30-50%
for R<50 co-moving kpc and N_HI>10^16 cm^-2. These values are considerably
lower than observed covering fractions, suggesting that outflowing gas (not
included here) is important in simulating galaxies with realistic gaseous
halos. Within ~500 Myr of crossing the Msh threshold, each galaxy transitions
to hot mode gas accretion, and f_c drops to ~5%. The sharp transition in
covering fraction is primarily a function of halo mass, not redshift. This
signature should be detectable in absorption system studies that target
galaxies of varying host mass, and may provide a direct observational tracer of
the transition from cold flow accretion to hot mode accretion in galaxies.
