2004ApJ...612..215M

Very Large Telescope images in BVI are used to identify the ionizing source centered on Sersic 13, the largest H II region of the giant nearby galaxy NGC 5128, with logL_Hα=39.6 ergs/s. This ionizing source turns out to be a close pair of bright and blue star cluster candidates. Spectroscopy obtained with the Magellan I telescope confirms that these are massive young clusters physically associated with the giant H II region Sersic 13. The spectra of both clusters show prominent Wolf-Rayet-type emission features and prominent lines of H I and He II, indicative of a very young age (t~afewx10⁶ yr). Their luminosities make each of them at least as luminous as the massive young cluster R136 in 30 Doradus in the LMC, and their individual masses are estimated to be 1-7.5x10⁵ M_☉. In addition, the projected separation of the cluster pair is 42 pc. The measured velocity difference between the clusters is small, ΔV=49±21 km/s, and within 2 σ of the expected orbital velocity V_orb=5-12 km/s if they are bound. Dynamical models predict that binary clusters with these properties would merge in a short timescale of a few orbital periods, P=20-50x10⁶ yr. The discovery of this binary cluster suggests that mergers of young massive clusters could lead to the formation of the most massive globular clusters, such as ω Cen in our Galaxy and G1 in M31. Alternatively, if they are not gravitationally bound these objects would individually evolve into two normal globular clusters.