DISTANCES TO GALACTIC HIGH-VELOCITY CLOUDS. I. COHEN STREAM, COMPLEX GCP, CLOUD g1

The highand intermediate-velocity interstellar clouds (HVCs/IVCs) are tracers of energetic processes in and around the Milky Way. Clouds with near-solar metallicity about 1 kpc above the disk trace the circulation of material between disk and halo (the Galactic fountain). TheMagellanic Stream consists of gas tidally extracted from the SMC, tracing the dark matter potential of the Milky Way. Several other HVCs have low metallicity and appear to trace the continuing accretion of infalling intergalactic gas. These assertions are supported by the metallicities (0.1 to 1 solar) measured for about 10 clouds in the past decade. Direct measurements of distances to HVCs have remained elusive, however. In this paper we present four new distance brackets, using VLT observations of interstellar Ca ii H and K absorption toward distant Galactic halo stars.We derive distance brackets of 5.0 to 11.7 kpc for the Cohen Stream (likely to be an infalling low-metallicity cloud), 9.8 to 15.1 kpc for ComplexGCP (also known as the Smith Cloud orHVC 40 15+100 andwith still unknown origin), 1.0 to 2.7 kpc for an IVC that appears associatedwith the return flowof the fountain in the Perseus arm, and 1.8 to 3.8 kpc for cloud g1, which appears to be in the outflow phase of the fountain. Ourmeasurements further demonstrate that the Milky Way is accreting substantial amounts of gaseous material, which influences the Galaxy’s current and future dynamical and chemical evolution. Subject headinggs: Galaxy: halo — ISM: clouds — stars: distances — stars: horizontal-branch