Compact star clusters in nearby dwarf irregular galaxies

Nearby dwarf irregular galaxies were searched for compact star clusters by using data from the Hubble Space Telescope archives. Eight of the 22 galaxies in our sample were found to host compact clusters of some type. Three of these have populous clusters, with M-V < 9.5 at a fiducial age of 10 Myr, and the same three also have super star clusters, with M-V < -10.5 at 10 Myr. Four other dwarf galaxies, two of which contain populous and super star clusters, are also considered by using data in the literature. The results suggest that galaxies fainter than M-B = 16 or with star formation rates less than 0.003 M-circle dot yr(-1) kpc(-2) do not form populous or super star clusters and that even the brighter and more active dwarf galaxies 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 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 spiral galaxies in which density wave flows and scale-free compression from turbulence seem to dominate. Further comparisons with spiral galaxies give insight into Larsen & Richtler's relation between the 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, interstellar column density, and star formation rate, combined with a size-of-sample effect.