Dust and Gas in the Outer Parts of Galaxies

Abstract

We review the observations of dust in the outer disks and the halos of spiral and irregular galaxies. These observations not only yield the abundance of dust and indirectly of heavy elements, but also give indirect information on molecular hydrogen which has been proposed as a constituent of dark matter in galactic disks. Dust can be detected either through its thermal emission or through the reddening and extinction it produces on the light of background objects. Detection of the thermal emission of dust at millimeter wavelengths using bolometers on large telescopes is making considerable progress and we are close to the point where dust will be seen in this way out to the 25-26th mag. arcsec² blue isophote of the disk. Dust has now been detected in these outer regions of the disks from a statistics of the colors of disks as a function of their inclination, and by direct measurements of the reddening of background galaxies. New results concerning M 31 are presented. There is a surprisingly high dust/gas ratio, and also massive star formation, at these very large galactic radii. The evidence for dust in galactic halos is uncertain.