A Comparative Study of Knots of Star Formation in Interacting vs. Spiral
  Galaxies
  Interacting galaxies are known to have higher global rates of star formation
on average than normal galaxies, relative to their stellar masses. Using UV and
IR photometry combined with new and published H-alpha images, we have compared
the star formation rates of ~700 star forming complexes in 46 nearby
interacting galaxy pairs with those of regions in 39 normal spiral galaxies.
The interacting galaxies have proportionally more regions with high star
formation rates than the spirals. The most extreme regions in the interacting
systems lie at the intersections of spiral/tidal structures, where gas is
expected to pile up and trigger star formation. Published Hubble Telescope
images show unusually large and luminous star clusters in the highest
luminosity regions. The star formation rates of the clumps correlate with
measures of the dust attenuation, consistent with the idea that regions with
more interstellar gas have more star formation. For the clumps with the highest
star formation rates, the apparent dust attenuation is consistent with the
Calzetti starburst dust attenuation law. This suggests that the high luminosity
regions are dominated by a central group of young stars surrounded by a shell
of clumpy interstellar gas. In contrast, the lower luminosity clumps are bright
in the UV relative to H-alpha, suggesting either a high differential
attenuation between the ionized gas and the stars, or a post-starburst
population bright in the UV but faded in H-alpha. The fraction of the global
light of the galaxies in the clumps is higher on average for the interacting
galaxies than for the spirals. Thus the star forming regions in interacting
galaxies are more luminous, dustier, or younger on average.
