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 , M _ { vir } = M _ { sh } \sim 10 ^ { 12 } M _ { \odot } .
Before reaching M _ { sh } , the galaxies experience cold mode accretion ( T < 10 ^ { 5 } K ) and show moderately high covering fractions in accreted gas : f _ { c } \sim 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 \sim 500 Myr of crossing the M _ { sh } threshold , each galaxy transitions to hot mode gas accretion , and f _ { c } drops to \sim 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 .