High-velocity clouds as streams of ionized and neutral gas in the halo
  of the Milky Way
  High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found
at high galactic latitudes, may play an important role in the evolution of the
Milky Way. The extent of this role depends sensitively on their distances and
total sky covering factor. We search for HVC absorption in HST high resolution
ultraviolet spectra of a carefully selected sample of 133 AGN using a range of
atomic species in different ionization stages. This allows us to identify
neutral, weakly ionized, or highly ionized HVCs over several decades in HI
column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90
km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is
essentially complete, i.e., an undetected population of HVCs with extremely low
N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm
that the predominantly ionized HVCs contain at least as much mass as the
traditional HI HVCs and show that large HI HVC complexes have generally ionized
envelopes extending far from the HI contours. There are also large regions of
the Galactic sky that are covered with ionized high-velocity gas with little HI
emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170
km/s drawn from the AGN and stellar samples are similar. This confirms that
these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs
drops with decreasing vertical height, which is consistent with HVCs being
decelerated or disrupted as they fall to the Milky Way disk. The HVCs with
|v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and
its leading arm at b>0 as well as other large known HI complexes. Therefore
there is no evidence in the Local Group that any galaxy shows a population of
HVCs extending much farther away than 50 kpc from its host, except possibly for
those tracing remnants of galaxy interaction.
