Cold gas accretion by high-velocity clouds and their connection to QSO absorption-line systems

TL;DR

This study models the three-dimensional distribution of high-velocity clouds around low-redshift galaxies, linking them to QSO absorption systems and suggesting they significantly contribute to galaxy gas accretion and evolution.

Contribution

It introduces a geometric model connecting HVCs with QSO absorption-line systems, estimating their extent, mass, and role in galaxy gas accretion at low redshift.

Findings

HVCs have a characteristic radial extent of ~50 kpc around galaxies.

HVCs contribute significantly to QSO absorption-line systems at z=0.

The neutral-gas accretion rate from HVCs matches local star-formation rates.

Abstract

We combine HI 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 (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 HI volume filling factor in their halos. Our model suggests a characteristic radial extent of HVCs of R_halo=50 kpc, a total HI mass in HVCs of ~10^8 M_sun, and a neutral-gas-accretion rate of ~0.7 M_sun/yr for M31/Milky-Way-type galaxies. Using a Holmberg-like luminosity scaling of the halo size of galaxies we estimate R_halo=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 HI masses between 10^8.5 and 10^10 M_sun. Our model indicates that the HI disks of galaxies and their surrounding HVC population can account for 30-100 percent of intervening QSO absorption-line systems with log N(HI)>17.5 at z=0. We estimate that the neutral-gas accretion rate density of galaxies at low redshift from infalling HVCs is dM_HI/dt/dV=0.022 M_sun/yr/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.