To appear in the Proceedings of the Extra-planar Gas
Conference, June 7-11, 2004, Dwingeloo (ASP Conf. Series, ed. R. Braun).
For a PDF version of the article, click
here.
astro-ph/0410363
Abstract. The interstellar thick disks of galaxies contain not
only gas, but
significant quantities of dust. Most of our knowledge of
extraplanar 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
fueling thick disk star formation, and
H
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 extraplanar material in disk galaxies must have been
expelled from the underlying thin disk.
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