The accretion history of galactic haloes

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We propose a systematic study of the physical properties of neutral and ionized gas in the inner and outer halo of the Milky Way. Beside the large neutral gas complexes in the halo (intermediateandhighvelocity clouds; IVCs and HVCs, respectively) there exists a population of partly ionized gaseous structures with low gas column densities that have a substantial area filling factor. The nature of these structures and their role in the Milky Way’s gas accretion history is nearly unexplored. In this project we will combine new, deep HI 21cm data with ultraviolet and optical spectra to study the filling factor and size distribution of such gaseous filaments in the halo at HI column densities below 1019 cm2. For this, we want to establish a comprehensive statistical data base of Milky Way halo absorbers, comprising more than 400 lines of sight. This will reduce any systematically bias naturally introduced by distance ambiguities towards the clouds of interest. Our unique data sample will allow us to realistically estimate the gas accretion rate of the Galaxy and to clarify the currently unknown relation between Milky Way halo clouds and intervening quasar metal absorptionline systems at low and high redshift.