High-velocity clouds and extended gaseous galactic halos

Department of Astronomy, University of Wisconsin, 475 N. Charter St., Madison, WI 53705, USA ;
e-mail: wakker@astro.wisc.edu

Abstract

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.