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P. Maloney (CASA, Univ. of Colorado)
Galaxies do not exist in a vacuum. The outer, low-column density edges of galactic disks can provide unique probes of the environments in which they exist. The interactions of these disks with their surroundings can be broadly divided into two classes: photoionization and heating, and direct physical interaction (ram pressure stripping and thermal conduction).
The former process may involve either the extragalactic ionizing background, or photons escaping from star-forming regions in the galaxy itself. In either case there will be a critical column density below which the gas is largely ionized, rather than neutral. These two sources can be distinguished by the variation of emission measure with radius: this is predicted to be essentially independent of radius for the case where the extragalactic background dominates, while it should be a strong function of radius if self-irradiation is important.
Direct physical interactions can also affect the physical state of galactic disks. Galaxies embedded in a hot medium, either the IGM or intragroup or intracluster gas, will be subject to thermal conduction (modulo the influence of the magnetic topology), which can lead to evaporation of cool galactic gas, or to the formation of mixing layers. If the relative velocity of the galaxy and the gas is substantial, ram pressure stripping may be important, which will remove gas below a critical surface density threshold.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.