Displacement of X - Ray Sources from Star Clusters in Starburst

We examine the spatial offsets between X-ray point sources and star clusters in three starburst galaxies. We find that the X-ray sources are preferentially located near the star clusters. Because the star clusters are very good tracers of the star formation activity in the galaxies, this indicates that the X-ray sources are young objects associated with current star formation. We find significant displacements of the X-ray sources from the clusters. These displacements are likely due to motion of the X-ray sources and indicates that they are X-ray binaries. We find that brighter X-ray sources preferentially occur closer to clusters. The absence of very bright sources at large displacements from clusters may help constrain models of the sources. 1 INTRODUCTION Starburst galaxies contain prominent populations of X-ray sources (Fabbiano 1989) including unusually luminous ones (Kaaret et al. 2001; Zezas & Fabbiano 2002). The young stellar ages within starbursts suggest that the associated X-ray sources are likely to be young objects (Kilgard et al. 2002). Strong X-ray variability in many sources further suggests those sources are accreting compact objects (Makishima et al. 2000; Kaaret et al. 2001; Fabbiano et al. 2003). In starburst galaxies, a substantial fraction of young stars are found in compact, luminous star clusters (Meurer et al. 1995). Indeed, Tremonti et al. (2001) and Harris et al. (2001) find evidence that most high mass stars in starbursts could form in dense clusters which quickly dissolve to feed the surrounding field. Consistent with this, the young compact cluster R136a contains a rich population of massive stars, evidently following a standard Salpeter-like upper initial mass function (Massey & Hunter 1998). However, there also are indications that some compact young massive clusters have either a flatter than normal upper mass function or a cut-The current data are insufficient to distinguish between these two possibilities (Mengel et al. 2002). In either case, the clusters would con