Star Clusters in Irregular Galaxies in the Local Group

Abstract

I summarize our knowledge of star clusters and associations in irregular galaxies other than the Magellanic Clouds in the Local Group. Surveys affording complete area coverage at high angular resolution are still lacking. Confirmed globular clusters are known only in NGC 6822 and WLM. Very few dIrrs contain populous or sparse open clusters. There is a pronounced deficiency of intermediate-age and young clusters. Apart from parent galaxy mass, the lack of interactions may be a key reason for the lack of cluster formation in the dIrrs. All dIrrs have one or several short-lived OB associations in the star-forming regions in their centers.

Three newly discovered globular clusters in NGC 6822

“…The galaxy has also been found to have an extended stellar spheroid (Battinelli, Demers, & Kunkel 2006) (see Figure 1). The central regions of NGC 6822 contain many young massive clusters (Chandar, Bianchi, & Ford 2000); however, until very recently, it was believed that there was only one truly old globular cluster in the system (Grebel 2002), known as Hubble VII -one of his original list of "nebulae" 2 in NGC 6822 (Hubble 1925). Cohen & Blakeslee (1998) undertook a spectroscopic study of this object, reporting an age of 11 Gyr and [Fe/H] = -1.95 dex.…”