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O.H. Billett (Yale University), D.A. Hunter (Lowell Observatory), B.G. Elmegreen (IBM T. J. Watson Research Center)
The types of star clusters found in galaxies are an indication of the star formation processes within them. Here we examine compact star clusters in irregular galaxies in order to learn what conditions are necessary for the formation of populous and super-star clusters. We have searched 22 nearby irregular galaxies for compact star clusters using data from the HST archives. Eight of these were found to host compact clusters, 3 of which have populous clusters, which we define as clusters with an MV < -9.5 at an age of 10 Myr. The same three galaxies also contain super-star clusters, which we define as clusters with an MV < -10.5 at 10 Myr. We combine these galaxies with four others from the literature, two of which contain populous and super-star clusters. With the exception of WLM, which contains a globular cluster, we find a galactic magnitude cutoff of MV = -16, fainter than which galaxies do not form compact clusters, although there are many brighter barren galaxies. The galaxies containing populous clusters also have normalized star formation rates of 0.003 solar masses yr-1 kpc-2 or higher, again excluding WLM.
We argue that the formation of super-star clusters in irregular galaxies requires extraordinary circumstances (e.g. a starburst, strong bar, or interaction) but only to bring the star formation rate up to a high level somewhere in the system. We note that super-star clusters often are surrounded by additional compact clusters of similar age, suggesting that the formation of the super-star cluster occurred in a large star-forming complex with several compact star clusters as part of the debris of star formation. This is consistent with previous results suggesting that dwarf galaxies form stars only in localized regions of high gas column density, where the pressure is sufficient to allow a cool phase of gas to exist in equilibrium, and where self-gravity is strong.
The author(s) of this abstract have provided an email address for comments about the abstract: olivia.billett@yale.edu