High-velocity clouds and extended gaseous galactic halos
The Role of the Disk-Halo Interaction in Galaxy Evolution: Outflow vs. Infall? Edited by M.A. de Avillez. EAS Publication Series, Volume 56, 2012, pp.257-265
Published in Sep 2012
This contribution summarizes some recent results concerning gas in Galaxy halos. In the past few years, distances have been determined for six high-velocity clouds (HVCs), including the second largest, complex C. The derived distances range from about 2 to 10 kpc, placing these HVCs in the near Galactic halo, but far above the Galactic plane. Distances of ̃10 kpc were determined to three different concentrations in Complex C, implying a mass of ̃5 × 107M☉; this cloud is known to have a metallicity of ̃0.15 solar, and represents an inflow of 0.15-0.25 M☉ yr-1 of low-metallicity material. A statistical analysis of the sky and velocity distribution of the ̃200 small HVCs shows that they probably form a population of clouds orbiting the Milky Way, with distances up to 80 kpc, and a radial inflow component of 50 km s-1. A study of 115 Lyα and 14 O VI absorption lines at redshifts z 〈 0.017 (v 〈 5000 km s-1) in the spectra of 76 AGNs shows that half of the intergalactic Lyα absorption lines originate within 450 kpc of galaxies. The amount of gas inside this distance is a factor 2-4 more than the total amount of mass inside the galaxies.