Extra-planar dust as a tracer of cold dense gas in the thick disks of spiral galaxies

The interstellar thick disks of galaxies contain not only gas, but significant quantities of dust. Most of our knowledge of extra-planar dust in disk galaxies comes from direct broadband optical imaging of these systems, wherein the dust is identified due to the irregular extinction it produces against the thick disk and bulge stars. This observational technique is sensitive to only the most dense material, and we argue much of the material identified in this way traces a cold phase of the interstellar thick disks in galaxies. The presence of a cold, dense phase likely implies the interstellar pressures in the thick disks of spiral galaxies can be quite high. This dense phase, of the interstellar medium may also be fueling thick disk star formation, and Ha. observations are now revealing HII regions around newly-formed OB stars associations in several galaxies. We argue that the large quantities of dust and the morphologies of the structures traced by the dust imply that much of the extra-planar material in disk galaxies must have been expelled from the underlying thin disk.