The Most Exciting Massive Binary Cluster in NGC 5128: Clues to the Formation of Globular Clusters

Abstract

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 log L H ¼ 39:6 ergs s À1 . 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 % a few ; 10 6 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 7:5 ð Þ; 10 5 M . In addition, the projected separation of the cluster pair is 42 pc. The measured velocity difference between the clusters is small, ÁV ¼ 49 AE 21 km s À1 , and within 2 of the expected orbital velocity V orb ¼ 5 12 km s À1 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 ¼ 20 50 ð Þ; 10 6 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 ! 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.

HST photometry of the binary globular cluster Sersic 13N-S in NGC 5128

“…The age we have estimated for the clusters is in agreement with the scenario in which they would merge in the future proposed by Minniti et al (2004). Simulations show that the timescale predicted for the merge is a few orbital periods (Sugimoto & Makino 1989), which in this case corresponds to 8 Myr.…”

“…In the FORS2/VLT images (Minniti et al 2004) the clusters look very similar with FWHM ∼ 3.1 pix. for a pixel scale of 0.2 /pix, which corresponds to ∼11.5 pc, but in our F555W band image the situation is different.…”

“…Hunter et al (1996) derived an R136 IMF from the intermediate mass stars that predicts at least ∼6 stars with M = 50−120 M , which was later confirmed by the Massey & Hunter (1998) spectroscopic classification of the 65 most luminous stars in the cluster as O3 spectral type. Considering that Sersic 13-N and Sersic 13-S are as bright as or brighter than R136 (Minniti et al 2004) this number must be a lower value than the expected for the clusters, so we are probably not defining all the stars in this bin of masses because some of them are missing in the concentrated central core.…”

“…Particularly, an HST spectroscopic classification by Massey & Hunter (1998) showed that the core region in the nearby SSC R136 is populated by O3 supergiants stars and several H-rich WN stars whose masses extend above 120 M , and with magnitudes comparable to our results. Unfortunately, the distance to NGC 5128 does not allow us to obtain individual star spectra, but the presence of this type of star can explain the WR-type emission observed in the integrated spectra of the clusters (Minniti et al 2004). …”

“…The ionizing central source in Sersic 13 was shown by Minniti et al (2004) to be formed by a close pair of bright and blue star clusters. Our goals are to resolve individual stars in each cluster, and to estimate their ages.…”