Observed Effects of Interactions

Observed Effects of Interactions - Observed Effects of...

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Observed Effects of Interactions Star formation: The following is based on the review by Keel 1990 (IAU Symp. 146, Dynamics of Galaxies and their Molecular Cloud Distributions , p. 243). It is by now part of the lore of galaxy research that galaxy interactions can, among other interesting effects, trigger bursts of star formation. This makes such systems useful laboratories for examining star formation in unusual environments, probing the behavior of a disturbed interstellar medium, and perhaps seeing processes that were important during galaxy formation. This paper reviews the evidence for the presence and scope of enhanced star formation during interactions, and presents several mechanisms that have been proposed to account for this excess. Since we do not have "before and after" views of interacting galaxies, we are driven to perform statistical comparisons of large samples of interacting and non-interacting (sometimes called for brevity "isolated") galaxies. This offers the hope that we might measure shifts in the (already broad) distributions of properties tracing the SFR. Selection of both interacting and comparison samples can involve some subtlety, since the SFR we wish to measure is itself a function of galaxy type and luminosity. Furthermore, selecting program galaxies for obvious morphological signs of interaction biases the sample in favor of certain kinds of interactions seen at certain stages. Conclusions from such samples may not be generalizable to the whole population of encounters. Ideally, then, we should obtain comparable observations of samples of galaxies with the same distribution of Hubble type, luminosity (as measured before any alteration by the interactions), and environment (except, of course, for the presence of companions). Since interactions induce star formation and can therefore change the luminosity of a galaxy, and tidal disturbances can change the morphology, this sort of comparison cannot be attained in practice.
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