Orbiting Cool Halo Gas: Observing the End of Cold Mode Gas Accretion onto Galaxies

Details Orbiting Cool Halo Gas: Observing the End of Cold Mode Gas Accretion onto Galaxies Orbiting Cool Halo Gas: Observing the End of Cold Mode Gas Accretion onto Galaxies

Jan 2012

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21942606s&link_type=abstract

American Astronomical Society, AAS Meeting #219, #426.06

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We use cosmological hydrodynamic simulations to study the kinematic signatures of gas accretion onto a pair of well-resolved galaxy halos. We find that cold-flow streams and gas-rich mergers produce a circum-galactic component of cool gas that generally rotates about the galaxy halo before eventually falling in to build the disk. This signature of cosmological accretion should be observable in absorption line studies as gas absorption features that are offset from the galaxy's systemic velocity and that typically co-rotate with the central disk, and has already been seen in studies of intermediate redshift galaxy-absorber pairs. The cool circum-galactic component drops precipitously once the galaxies cross the critical mass to form stable shocks, Mvir = Mshock 1012 Msun. Before reaching Mshock, the galaxies experience cold mode accretion (T < 105 K) and show moderately high covering fractions in this co-rotating, accreted gas: 30-50% for R<50 co-moving kpc and NHI > 1016 cm-2. Within 500 Myr of crossing the Mshock threshold, each galaxy transitions to hot mode gas accretion, and the covering fraction drops to 5%. This sharp transition is primarily a function of halo mass, not redshift. As a result, the fraction of rotating absorption-lines should decrease around bright galaxies at low redshift as cold mode accretion dies out, a trend that should be detectable in absorption system studies that target galaxies of varying host mass. This signature provides a direct observational tracer of the transition from cold flow accretion to hot mode accretion in galaxies.

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