We combine H i 21cm 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 R _ { halo } \sim 50 kpc , a total H i mass in HVCs of \sim 10 ^ { 8 } M _ { \sun } , and a neutral-gas-accretion rate of \sim 0.7 M _ { \sun } yr ^ { -1 } for M31/Milky-Way-type galaxies .
Using a Holmberg-like luminosity scaling of the halo size of galaxies we estimate R _ { halo } \sim 110 kpc for the most massive galaxies .
The total absorption-cross section of HVCs at z \approx 0 most likely is dominated by galaxies with total H i masses between 10 ^ { 8.5 } and 10 ^ { 10 } M _ { \sun } .
Our model indicates that the H i disks of galaxies and their surrounding HVC population can account for 30 - 100 percent of intervening QSO absorption-line systems with log N ( H i ) \geq 17.5 at z \approx 0 .
We estimate that the neutral-gas accretion rate density of galaxies at low redshift from infalling HVCs is dM _ { HI } / dt / dV \approx 0.022 M _ { \sun } 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 for the on-going formation and evolution of galaxies .