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

¹ ESO, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: mrejkuba@eso.org
² Observatoire de Geneve, ch. des Maillettes 51, 1290 Sauverny, Switzerland
³ INTEGRAL Science Data Centre, ch. d'Ecogia 16, 1290 Versoix, Switzerland e-mail: pierre.dubath@obs.unige.ch
⁴ Department of Astronomy and Astrophysics, Pontificia Universidad Católica de Chile, Vicuña Mackenna 4860, Santiago 22, Chile e-mail: dante@astro.puc.cl
⁵ Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland e-mail: georges.meylan@epfl.ch

Received: 2 October 2006
Accepted: 13 March 2007

Abstract

Context.Globular clusters are the simplest stellar systems in which structural parameters are found to correlate with their masses and luminosities.

Aims.To investigate whether the brightest globular clusters in the giant elliptical galaxies are similar to the less luminous globular clusters like those found in Local Group galaxies, we study the velocity dispersion and structural parameter correlations of a sample of bright globular clusters in the nearest giant elliptical galaxy NGC 5128 (Centaurus A).

Methods.The UVES echelle spectrograph on the ESO Very Large Telescope (VLT) was used to obtain high-resolution spectra of 23 bright globular clusters in NGC 5128, and 10 clusters were observed with EMMI in echelle mode with the ESO New Technology Telescope. The two datasets have 5 clusters in common, while one cluster observed with UVES had too low a signal-to-noise ratio. Hence the total number of clusters analysed in this work is 27, more than doubling the previously known sample. Their spectra were cross-correlated with template spectra to measure the central velocity dispersion for each target. The structural parameters were either taken from the existing literature, or in cases where this was not available, we derived them from our VLT FORS1 images taken under excellent seeing conditions, using the ISHAPE software. The velocity dispersion and structural parameter measurements were used to obtain masses and mass-to-luminosity ratios () for 22 clusters.

Results.The masses of the clusters in our sample range from M_vir = 10⁵-10⁷ , and the average is 3 ± 1. The three globular clusters 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. It has a mass (M_vir = 1.410⁷ ) that is an order of magnitude higher than the most massive clusters in the Local Group and a high ratio (4.7 ± 1.2). We briefly discuss possible formation scenarios for this object.

Conclusions.The correlations of structural parameters, velocity dispersion, masses, and for the bright globular clusters in NGC 5128 extend the properties established for the most massive Local Group clusters towards those characteristic of dwarf elliptical nuclei and ultra-compact dwarf galaxies (UCDs). The detection of the mass-radius and the mass- relations for the globular clusters with masses higher than ~ 2 10⁶  provides the missing link between “normal” old globular clusters, young massive clusters, and evolved objects like UCDs.