Distances to the High Velocity Clouds: A Forty-Year Mystery on the Way to Solution

A full understanding of the High Velocity Clouds (HVCs) is still lacking more than four decades after their discovery. Determining the clouds' locations in relation to the Galaxy is an important constraint on hierarchical assembly models of galaxies in a Λ-CDM universe. However, quantifying physical cloud properties such as mass and size have been difficult because of their unknown distances. We report the first definitive distance determinations for three HVCs (Complex C, 6.4-11.3 kpc; the Cohen Stream, 5.0-11.7 kpc; Complex GCP, 9.8-15.1 kpc) and two intermediate-velocity clouds (IVCs: IV-South, 1.0-2.7 kpc; cloud g1, 1.8-3.8 kpc) using the absorption line bracketing method. From the distances to the larger cloud complexes we calculate H I masses ranging from 1.5×106 Msun to 8×106 Msun. In the case of Complex C this implies a mass inflow rate of ˜0.1 Msun yr-1. The measured distances place the HVCs in the hot Galactic Corona and the IVCs in the lowest reaches of the Halo. With this information we may now positively identify at least two roles played by HVCs in the internal and external dynamics of the Milky Way: (1) the infall of low-metallicity gas and (2) the supply and return streams of the Galactic Fountain