Compact Star Clusters in Nearby Dwarf Irregular Galaxies

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Abstract
Nearby dwarf irregular galaxies were searched for compact star clusters using data from the HST archives. Three of the galaxies were found to host both populous clusters (M_V<-9.5 at a fiducial age of 10 Myr) and super-star clusters (M_V<-10.5 at 10 Myr). Four other dwarf galaxies, two of which contain populous and super-star clusters, were drawn from the literature. We find upper limits to integrated galactic M_B and star formation rate for the formation of these clusters. However, galaxies brighter and more active than these limits rarely form them. Yet when they do form, the associated star formation activity is very high, with numerous compact clusters of similar age in the same complex and often evidence for a galaxy-wide perturbation as the trigger. This tendency to concentrate star formation in localized regions of high column density is consistent with previous suggestions that self-gravity must be strong and the pressure must be high to allow a cool phase of gas to exist in equilibrium. Statistical considerations emphasize the peculiarity of super-star clusters in dwarf galaxies, which are too small to sample the cluster mass function to that extreme. We suggest that triggered large-scale flows and ambient gravitational instabilities in the absence of shear make the clouds that form super-star clusters in small galaxies. This is unlike the case in spirals where density wave flows and scale-free compression from turbulence dominate. Further compar- isons with spirals give insight into Larsen & Richtler's relation between star formation rate per unit area and the fraction of young stars in massive dense clusters. We suggest that this relation is the result of a physical connection between maximum cluster mass, interstellar pressure, column density, and star formation rate, combined with a size-of-sample effect.

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