Wide-field Washington photometry of the NGC 5128 globular cluster system. II. Large-scale properties of the system

摘要

Building on the CMT1 photometric database presented in Paper I, in this paper we derive the large-scale properties of the globular cluster system (GCS) in NGC 5128, the nearest giant elliptical and the dominant galaxy in the Centaurus group. In global terms, it has a smaller total population than previously thought: we estimate 980 +/- 120 clusters over all magnitudes, yielding a specific frequency S-N = 1.4 +/- 0.2, with a steep projected radial distribution sigma similar to r(-2). The luminosity distribution of the clusters resembles that of an old, normal GC luminosity function (Gaussian-like with peak at M-V similar or equal to -7.4 and dispersion of similar or equal to 1.3 mag), but these parameters are unfortunately quite uncertain because of the system's low population and the heavy field contamination. Using the metallicity-sensitive C - T-1 color index, we discuss the metallicity distribution function (MDF) for a subsample of 211 previously identified clusters, all on a homogeneous photometric system. We find the MDF to be strongly bimodal, with metallicity peaks at Fe/H = -1.55 and -0.55 and with nearly equal numbers of clusters in each of the metal-poor and metal-rich modes. The combined evidence from the system's low specific frequency, the MDF, and the isophotal shell features in the halo light make a "major merger'' a plausible model for the formation history of this giant E galaxy. However, the progenitor galaxies must have been more gas-rich than in any present-day mergers or starbursts. Finally, we present a list of 327 new cluster candidates not identified in any previous surveys; most of these are in the less well studied bulge region of the galaxy and along the minor axis.