Star formation rates in normal and peculiar galaxies

Morphologically normal and peculiar galaxies show very different distributions in the (U-B, B-V) diagram. To interpret these differences, an extensive grid of galaxy models with decreasing star formation rates (SFRs) and with bursts on various time scales has been constructed. Normal galaxies have colors that are consistent with a monotonically decreasing SFR, and very few can have experienced large variations in SFR with time scales not exceeding 500 million years. In contrast, the peculiar galaxies have a large scatter in colors that is consistent with bursts as short as 20 million years involving up to about 5% of the total mass. Nearly all of this scatter is associated with galaxies showing evidence of tidal interaction; moreover, interacting systems that are at early stages of dynamical evolution, as inferred from the absence of long tidal tails, have colors consistent with the most recent bursts. These results provide evidence for a 'burst' mode of star formation associated with violent dynamical phenomena.