Bright globular clusters in NGC 5128: the missing link between young massive clusters and evolved massive objects

Abstract

In order to investigate whether the brightest globular clusters (GCs) in the giant elliptical galaxies are similar to the less luminous GCs like those found in Local Group galaxies, we study the velocity dispersion and structural parameter correlations of a sample of bright GCs in the nearest gE galaxy NGC 5128. UVES echelle spectrograph on the ESO VLT, and EMMI on the ESO NTT were used to obtain high resolution spectra of bright GCs in NGC 5128. The velocity dispersions were obtained for all the targets. The structural parameters were either taken from the existing literature, or derived from our VLT FORS1 images using the ISHAPE software. The velocity dispersion and structural parameter measurements were used to obtain masses and M/L_V ratios of 22 clusters. The masses of the clusters in our sample range from M_vir=10^5-10^7 M_sun and the average M/L_V is 3+/-1. The three GCs harbouring X-ray point sources are the second, third and sixth most massive in our sample. The most massive cluster, HCH99-18, is also the brightest and the largest in size. It has the mass (M_vir=1.4x10^7 M_sun) an order of magnitude larger than the most massive clusters in the Local Group, and a high M/L_V ratio (4.7+/-1.2). We discuss briefly possible formation scenarios for this object. The correlations of structural parameters, velocity dispersion, masses and M/L_V for the bright GCs in NGC 5128 extend the properties established for the most massive Local Group clusters towards those characteristic of dE galaxy nuclei and Ultra Compact Dwarfs (UCDs). The detection of the mass-radius and the mass-M/L_V relations for the GCs with masses greater than ~2x10^6 M_sun provides the missing link between ``normal'' old globular clusters, young massive clusters, and evolved objects like UCDs. (Abridged)