A population of weak metal-line absorbers surrounding the Milky Way
  We report on the detection of a population of weak metal-line absorbers in
the halo or nearby intergalactic environment of the Milky Way. Using
high-resolution ultraviolet absorption-line spectra of bright QSOs obtained
with the Space Telescope Imaging Spectrograph (STIS), along six sight lines we
have observed unsaturated, narrow absorption in OI and SiII together with
mildly saturated CII absorption at high radial velocities (|v_LSR|=100-320
km/s). The measured OI column densities are small, implying that these
structures represent Lyman-Limit Systems and sub-Lyman-Limit System with HI
column densities < 3x10^18 cm^-2, thus below the detection limits of current
21cm all-sky surveys of high-velocity clouds (HVCs). The absorbers apparently
are not directly associated with any of the large high-column density HVC
complexes, but rather represent isolated, partly neutral gas clumps embedded in
a more tenuous, ionized gaseous medium situated in the halo or nearby
intergalactic environment of the Galaxy. We speculate that this absorber
population represents the local analog of weak MgII systems that are commonly
observed in the circumgalactic environment of low- and high-redshift galaxies.
