A Low Metallicity Molecular Cloud In The Lower Galactic Halo
  We find evidence for the impact of infalling, low-metallicity gas on the
Galactic disk. This is based on FUV absorption line spectra, 21-cm emission
line spectra, and FIR mapping to estimate the abundance and physical properties
of IV21 (IVC135+54-45), a galactic intermediate-velocity molecular cloud (IVMC)
that lies ~300 pc above the disk. The metallicity of IV21 was estimated using
observations toward the sdB star PG1144+615, located at a projected distance of
16 pc from the cloud's densest core, by measuring ion and HI column densities
for comparison with known solar abundances. Despite the cloud's bright FIR
emission and large column densities of molecular gas as traced by CO, we find
that it has a sub-solar metallicity of log(Z/Z_Sun)=-0.43 +/- 0.12dex. IV21 is
thus the first known sub-solar metallicity cloud in the solar neighborhood. In
contrast, most intermediate-velocity clouds (IVC) have near-solar metallicities
and are believed to originate in the Galactic Fountain. The cloud's low
metallicity is also atypical for Galactic molecular clouds, especially in the
light of the bright FIR emission which suggest a substantial dust content. The
measured I_100mu/N(HI) ratio is a factor of three below the average found in
high latitude \HI clouds within the solar neighborhood. We argue that IV21
represents the impact of an infalling, low-metallicity high-velocity cloud
(HVC) that is mixing with disk gas in the lower Galactic halo.
