A Large Reservoir of Ionized Gas in the Galactic Halo: Ionized Silicon
  in High-Velocity and Intermediate-Velocity Clouds
  The low Galactic halo is enveloped by a sheath of ionized, low-metallicity
gas, which can provide a substantial (1 M_sun/yr) cooling inflow to replenish
star formation in the disk. Using absorption spectra from the HST and FUSE
toward 37 active galactic nuclei at high latitude, we detect widespread
interstellar SiIII 1206.5 absorption: 61 high-velocity clouds (HVCs) along 30
sight lines and 22 intermediate-velocity clouds (IVCs) along 20 sight lines. We
find a segregation of redshifted and blueshifted absorbers across the Galactic
rotation axis at l=180, consistent with a lag in the rotation velocity above
the Galactic plane. The HVC sky coverage is large (81+-5% for 30/37 directions)
with SiIII optical depth typically 4-5 times that of OVI 1032. The mean HVC
column density per sight line, <log N_SiIII>=13.42+-0.21, corresponds to total
column density N_HII~6x10^18)/(Z_Si/0.2Z_sun) of ionized low-metallicity gas,
similar to that inferred in OVI. This reservoir could total 10^8 M_sun and
produce a mass infall rate ~1 M_sun/yr. By modeling SiII, SiIII, SiIV, and HI
in a subset of absorbers, we constrain the mean photoionization parameter in
the low halo, <log U> = -3.0 (+0.3,-0.4), approximately ten times lower than
observed in the low-redshift intergalactic medium. The metallicities in some
HVCs, derived from [SiII/HI], are 10-30% solar, whereas values found from all
three silicon ions are lower in the pure-photoionization models. Thse formally
lower metallicities are highly uncertain, since some of the higher ions may be
collisionally ionized. The mean Si metallicities, <logZ_Si/Z_sun)>=
-2.1(+1.1,-0.3) in 17 HVCs and -1.0(+0.6,-1.0) in 19 IVCs, are somewhat
uncertain owing to ionization modeling, but consistent with the median
photometric metallicity, [Fe/H]=-1.46+-0.30, for ~2e5 halo F/G stars in SDSS.
