COLD GAS ACCRETION BY HIGH-VELOCITY CLOUDS AND THEIR CONNECTION TO QSO ABSORPTION-LINE SYSTEMS

We combine H I 21 cm observations of the Milky Way, M31, and the local galaxy population with QSO absorption-line measurements to geometrically model the three-dimensional distribution of infalling neutral-gas clouds ('high-velocity clouds' (HVCs)) in the extended halos of low-redshift galaxies. We demonstrate that the observed distribution of HVCs around the Milky Way and M31 can be modeled by a radial exponential decline of the mean H I volume-filling factor in their halos. Our model suggests a characteristic radial extent of HVCs of Rhalo ∼ 50 kpc, a total H I mass in HVCs of ∼108 M☉, and a neutral-gas accretion rate of ∼0.7 M☉ yr–1 for M31/Milky-Way-type galaxies. Using a Holmberg-like luminosity scaling of the halo size of galaxies we estimate Rhalo ∼ 110 kpc for the most massive galaxies. The total absorption cross-section of HVCs at z ≈ 0 most likely is dominated by galaxies with total H I masses between 108.5 and 1010 M☉. Our model indicates that the H I disks of galaxies and their surrounding HVC population can account for 30%-100% of intervening QSO absorption-line systems with log N(H I) ≥ 17.5 at z ≈ 0. We estimate that the neutral-gas accretion rate density of galaxies at low redshift from infalling HVCs is dMHi/dt/dV∼0.022 Msun yr–1 Mpc–3, which is close to the measured star formation rate density in the local universe. HVCs thus may play an important role in the ongoing formation and evolution of galaxies.