Abstract
To characterize the absorption properties of this circumgalactic medium (CGM)
and its relation to the LG we present the so-far largest survey of metal
absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet
(UV) spectra of extragalactic background sources. The UV data are obtained with
the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST)
and are supplemented by 21 cm radio observations of neutral hydrogen. Along 262
sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and
CIV and associated HI 21 cm emission in HVCs in the velocity range
|v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able
to improve the statistics on HVC covering fractions, ionization conditions,
small-scale structure, CGM mass, and inflow rate. For the first time, we
determine robustly the angular two point correlation function of the
high-velocity absorbers, systematically analyze antipodal sightlines on the
celestial sphere, and compare the absorption characteristics with that of
Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of
the LG. Our study demonstrates that the Milky Way CGM contains sufficient
gaseous material to maintain the Galactic star-formation rate at its current
level. We show that the CGM is composed of discrete gaseous structures that
exhibit a large-scale kinematics together with small-scale variations in
physical conditions. The Magellanic Stream clearly dominates both the cross
section and mass flow of high-velocity gas in the Milky Way's CGM. The possible
presence of high-velocity LG gas underlines the important role of the local
cosmological environment in the large-scale gas-circulation processes in and
around the Milky Way (abridged).
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