Astron. Astrophys. 348, 418-436 (1999)

1. Introduction

Globular star clusters (GCs) are among the oldest stellar systems in the Universe and provide a powerful tracer of the evolutionary history of a galaxy. There is strong evidence that massive star clusters can form during galactic mergers (e.g. Zepf & Ashman 1993; Schweizer et al. 1996), so galaxies that have recently experienced a merger event are ideal places to search for young GCs. Recently several candidates for luminous young GCs have been identified in various merging galaxies, such as NGC 3597 (Lutz 1991), NGC 1275 (Holtzman et al. 1992), and NGC 7252 (Schweizer & Seitzer 1993).

NGC 5128 ( Centaurus A, see Israel 1998for a recent review) is classified as a giant S0pec galaxy. It is composed of a large, dominant spheroid, which itself resembles an E0 galaxy, and a disk that contains large amounts of gas and dust. Soria et al. (1996) used direct observations of resolved stars at the tip of the red giant branch in NGC 5128 to determine a true distance modulus of , which corresponds to Mpc, making NGC 5128 the nearest giant elliptical galaxy to our own. There is strong evidence (see the review by Ebneter & Balick 1983) that it is the product of a recent merger between a large elliptical galaxy and a small late-type spiral. A thick dust band is seen across the center of NGC 5128 and there is evidence for significant star formation occurring in the central regions of the galaxy. G. Harris et al. (1999) used Hubble Space Telescope (HST ) Wide Field Planetary Camera 2 (WFPC2) images to obtain a color-magnitude diagram for the outer halo of NGC 5128. They found a distance of Mpc, consistent with the Soria et al. (1996) estimate, and a population of old stars with iron abundances between and . Their metallicity distribution function is consistent with two bursts of star formation. The first having and producing approximately one-third of the stars, and the second having and producing approximately two-thirds of the stars. They argue that the second burst of star formation occurred at least 1-2 Gyr after the first.

The first observation of a GC in NGC 5128 was by Graham & Philips (1980). The galaxy is now believed to have GCs (H. Harris et al. 1984), with 87 confirmed spectroscopically (see H. Harris et al. 1988; Sharples 1988). Recently G. Harris et al. (1998) used HST WFPC2 images to construct a color-magnitude diagram for C44, a GC in the halo of NGC 5128. They found that this GC was an old, intermediate-metallicity object similar to the GCs in the Milky Way. G. Harris et al. (1992, hereafter referred to as HG92) used Washington photometry to derive metallicities for 62 confirmed GCs in NGC 5128 and found a mean iron abundance of , which suggest that the NGC 5128 GC system is times more metal rich than the Milky Way GC system. They found no evidence for any GCs having metallicities significantly greater than those found in the Milky Way GCs. Such metal-rich GCs might be expected if some of the NGC 5128 GCs had formed recently in a gas-rich merger event. HG92 do, however, suggest that several blue GCs in NGC 5128 may be analogues of the intermediate-age GCs found in the Magellanic Clouds. On the other hand, Zepf & Ashman (1993) suggest that the metallicity distribution of the NGC 5128 GCs is bimodal, with the high-metallicity peak at due to GCs formed in a merger. Hui et al. (1995) analyzed the kinematics of the NGC 5128 GC system and found that the metal-rich GCs are part of a dynamically separate system from the metal-poor GCs. Numerical simulations suggest that the merger event occurred between 160 (Quillen et al. 1993) and 500 ( Mpc) Myr ago, where D is the distance to NGC 5128 in Mpc (Tubbs 1980). This suggests that any GCs that formed in this particular merger should be quite young and, therefore, rather blue (; see Sect. 5).

Minniti et al. (1996) and Alonso & Minniti (1997, hereafter referred to as AM97) used HST Wide-Field/Planetary-Camera 1 (WF/PC-1) images, taken before the corrective optics package was installed in 1993, to search for GCs in the inner regions of NGC 5128. They identified 125 GC candidates, young associations, and open cluster candidates in the inner three kpc of NGC 5128. They also used ground-based RK photometry to estimate metallicities for 47 GC candidates. Schreier et al. (1996) found 74 compact sources along the northern edge of the NGC 5128 dust lane using HST WF/PC-1 images. They estimate that most of these sources are young stars (spectral class A or earlier) but note that some are resolved and may be GCs.

Identifying GC candidates in the inner regions of NGC 5128 is difficult since there is nonuniform extinction, contamination from foreground stars and background galaxies, and confusion with open clusters and blue, star-forming knots in NGC 5128. GC candidates can not be identified based solely on their colors since the large amount of uneven reddening makes it very difficult to determine the dereddened color of an object. A better approach is a scheme to identify GC candidates based solely on their structural parameters. All known GCs in the Local Group can be reasonably well fit by Michie-King models (Michie 1963; King 1966), although % show evidence of having undergone core collapse. The vast majority of the Milky Way's GCs are uniformly old objects with ages of Gyr (Chaboyer et al. 1998), mean King core radii of pc, mean King tidal radii of pc, mean concentrations of , and mean ellipticities of . If the GCs in NGC 5128 are structurally similar to those in the Local Group spiral and dwarf galaxies, then high resolution imaging can be used to identify GC candidates in the inner regions of NGC 5128 without resorting to an identification scheme based upon the integrated colors of the objects.

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
helpdesk.link@springer.de