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

6. Conclusions

We have used the HST /WFPC2 to identify 21 GC candidates in a area centered on the nucleus of the nearest giant elliptical galaxy, NGC 5128. There is strong evidence that this galaxy has undergone a merger with a small late-type spiral between and 360 Myr ago. We fit two-dimensional, PSF-convolved, single-mass Michie-King models to each GC candidate and derived core radii, tidal radii, half-mass radii, ellipticities, and position angles for each object. We assumed that the GC candidates were structurally similar to those in the Local Group; i.e., they could be well fit by Michie-King models, and had similar half-mass radii and ellipticities. Therefore, only objects with structural parameters similar to those of GCs in the Local Group were accepted as being GC candidates. It is possible that we have missed NGC 5128 GC candidates that are significantly less centrally concentrated than the GCs in the Local Group. However, since we were primarily interested in using the colors of the GC candidates to estimated their ages we preferred to risk rejecting legitimate GC candidates rather than risk having our sample contaminated with stars and background galaxies. Within this constraint we find no evidence that the NGC 5128 GC candidates have core-, half-mass-, or tidal radii that are distributed differently from those GCs in the Milky Way that do not exhibit central brightness cusps. There is weak evidence that NGC 5128 GC candidates are systematically more elliptical than are the Galactic GCs.

We have obtained V- and I-band photometry for all 21 of our GC candidates. We find no evidence for the bimodal color distribution observed among GCs at larger distances from the center of NGC 5128 (HG92; Zepf & Ashman 1993), although this is likely due to the small sample size (21 objects), the poor metallicity sensitivity of the color index, and confusion due to differential reddening within NGC 5128.

We have identified one very blue GC candidate (#12) with . Using a color-age relation derived from the models of Bruzual & Charlot (1993), we estimate that this object has an age of less than Myr and a mass of between and Solar masses, which is barely consistent with this object being a small GC that formed during the merger event. There are sixteen objects with that have colors and integrated magnitudes that are consistent with their being either young GCs that formed during the merger event or old GCs similar to those found in the Milky Way, depending on what we assume about their iron abundances and differential reddenings. The colors of these GCs are very similar to those measured by HG92, but the amount of reddening for each object is uncertain by mag. There are four GC candidates with , which imply either implausibly high metallicities or very large differential reddenings. We can not rule out the possibility that some of these four objects are background galaxies seen through the central regions of NGC 5128.

Spectroscopic studies will be needed to determine unambiguously if any of the central GC candidates in NGC 5128 are young GCs that may have formed as a result of the merger. If young GCs in NGC 5128 can be unambiguously identified spectroscopically, and their ages determined, this will make it possible to estimate the amount of time required for GCs to form after a galactic merger has occurred.

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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