OBSERVING THE END OF COLD FLOW ACCRETION USING HALO ABSORPTION SYSTEMS

We use cosmological smoothed particle hydrodynamic 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 ∼ 1012 Msun. Before reaching Msh, the galaxies experience cold mode accretion (T < 105 K) and show moderately high covering fractions in accreted gas: fc ∼ 30%-50% for R < 50 comoving kpc and NHi>1016 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 fc 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.