1 Introduction

Globular Clusters are among the oldest objects in the Universe. Moreover, they are intrinsically luminous and found around almost all galaxies in which searches have been made. There is growing evidence for globular cluster formation during major star-formation episodes (Schweizer 1997; Larsen & Richtler 1999). This makes them useful tracers of the formation and the evolution of galaxies (Harris 1991,2000; Kissler-Patig et al. 1998; van den Bergh 2000).

The globular cluster luminosity function (GCLF) of a galaxy is the relative number of its globular clusters per luminosity (or magnitude) interval. It is usually unimodal and nearly symmetric with a peak at a characteristic (turn-over) magnitude. Empirically, it was found that turn-over magnitudes for galaxies at the same distance (e.g. in the same galaxy cluster) differ by <0.1, which makes the GCLF a good standard candle (Harris 1991; Jacoby et al.  1992). However, the full calibration of the GCLF is incomplete. The essential problem is that the galaxies at large distances, for which the GCLF method is mostly used, are giant ellipticals in which globular clusters are found in largest numbers and in which there are usually fewer problems with extinction, whilst the primary calibration of the standard candle M_V⁰ rests on the MW and M 31, which are spiral galaxies. In fact, evidence that GCLFs depend on Hubble type and environment has been proposed (Fleming et al. 1995), but the former is probably due to differences in mean metallicity of the globular clusters and the latter was never reliably demonstrated (Kissler-Patig 2000).

There is strong observational evidence that most luminous giant ellipticals have a bimodal globular cluster metallicity distribution (Gebhardt & Kissler-Patig 1999; Kundu 1999). The globular cluster specific frequencies (the number of clusters normalized to the galaxy luminosity; Harris & van den Bergh 1981) are typically higher in ellipticals than in spirals (Harris 1991,2000). To explain the high number of globular clusters and the presence of several populations indicated by the bimodal metallicity distributions, different globular cluster formation mechanisms have been proposed (e.g. Ashman & Zepf 1992; Côté et al. 1998; Forbes et al. 1997). The number, chemical abundance and spatial distribution of a globular cluster system (GCS) put strong constraints on the formation of its parent galaxy.

At the distance of 3.6 Mpc (Soria et al. 1996; Harris et al. 1999) NGC 5128 is the closest giant elliptical galaxy. The first globular cluster in NGC 5128 was discovered by Graham & Phillips (1980). Its brightness was high, with $V=17.2\pm0.1$ mag, which corresponds to M_V=-10.6 mag. Subsequent investigation of photographic plates (van den Bergh et al.  1981; Hesser et al.  1984, 1986) revealed a rich GCS in this galaxy. The estimated number of globular clusters in NGC 5128 is $N\sim 1700$ (Harris et al. 1984) compared with $N\sim 150$ for the MW.

 

 

Table 1: Journal of observations

Field	$\alpha_{(2000)}$	$\delta_{(2000)}$	Date	Telescope &	Exposure	FILTER	Airmass	Seeing
#	(h min s)	($^\circ$ $\hbox{$^\prime$ }$ $\hbox{$^{\prime\prime}$ }$)	dd/mm/yy	Instrument	(s)			$\hbox{$^{\prime\prime}$ }$
1	13 26 23.5	-42 52 00	12/Jul./1999	Antu+FORS1	$2\times 900$	U	1.536	0.52
1	13 26 23.5	-42 52 00	12/Jul./1999	Antu+FORS1	$2\times 900$	V	1.768	0.54
2	13 25 24.0	-43 10 00	11/Jul./1999	Antu+FORS1	$2\times 900$	U	1.283	0.53
2	13 25 24.0	-43 10 00	11/Jul./1999	Antu+FORS1	$2\times 900$	V	1.196	0.44
2	13 25 24.0	-43 09 04	20/Feb./2000	NTT+SOFI	$45\times 6\times10$	K$_{\rm s}$	1.055	0.56
1	13 26 48.1	-42 46 00	28/Feb./2000	2.2 m+WFI	$3\times 210$	V	1.117	1.00
2	13 26 48.1	-43 16 01	28/Feb./2000	2.2 m+WFI	$3\times 210$	V	1.037	0.81
2	13 24 07.6	-43 15 58	29/Feb./2000	2.2 m+WFI	$3\times 210$	V	1.057	1.12

On the basis of the near IR colors for a magnitude-limited sample of 12 globular clusters, Frogel (1980,1984) found one third of the sample to be super metal rich (above solar metallicity) and luminous. However, the Washington photometry of 62 spectroscopically confirmed clusters (Harris et al. 1992) as well as a spectrophotometry for 5 clusters (Jablonka et al. 1996) showed less extreme metallicity. Harris et al. (1992) found a large spread in metallicities with a mean of [Fe/H] $_{C-T_1}=-0.8\pm0.2$ dex. They also found evidence for a weak spatial metallicity gradient, but state that the same could be produced by unknown reddening effects. Zepf & Ashman (1993), using the same photometric data, suggest that the metallicity distribution of globular clusters in NGC 5128 is bimodal, with the higher metallicity peak at ${\rm [Fe/H]}=+0.25$ dex, and proposed that this was due to the formation of a second population of globular clusters in a previous merger which created NGC 5128.

Several photometric studies in the inner regions of NGC 5128 have been conducted, with the aim of constraining the dependence of globular cluster metallicity on galactocentric radius and of finding super-metal-rich clusters (Minniti et al. 1996; Holland et al. 1999). These studies suggest the existence of radial gradients and bimodal distribution in globular cluster metallicity. However, their conclusions are limited due to the restricted samples, possible reddening within NGC 5128 that is especially severe in the central parts around the prominent dust lane, and possible contamination by stars within NGC 5128 as well as background galaxies.

In this paper, I present a sample of globular clusters from two fields in the halo of NGC 5128. Deep images combined with the high resolving power of the VLT and the proximity of NGC 5128 allow the identification of globular clusters primarily on the basis of their non-stellar PSF. Constructing the GCLF for a statistically significant number of clusters that span the whole range of magnitudes, I pursue the question of the uniqueness of GCLF in elliptical galaxies.

This paper is organized as follows. The data, the photometric calibrations and completeness tests are described in Sect. 2. In Sect. 3 the color-magnitude diagram for the NGC 5128 globular clusters is compared to the ones of the MW and M 31. The luminosity function is presented in Sect. 4 and the color distribution in Sect. 5. In the latter, the mean abundances are determined for the metal-poor and metal-rich populations. The results are summarized in Sect. 6.