Observing the End of Cold Flow Accretion using Halo Absorption Systems

TL;DR

This study uses cosmological simulations to identify a sharp decline in cool gas in galaxy halos at a critical mass, marking the transition from cold to hot mode accretion, which can be observed through absorption systems.

Contribution

It demonstrates that the transition from cold to hot mode accretion causes a significant drop in halo cool gas, providing a potential observational signature of galaxy evolution stages.

Findings

Cool gas in halos drops sharply after galaxies reach Mvir ~ 10^12 Msun.

Pre-shock halos show 30-50% covering fractions, lower than observed, indicating outflows are also important.

Transition to hot mode reduces covering fraction to about 5% within 500 Myr.

Abstract

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.