The Star Cluster System of the Merger Remnant NGC 7252

Abstract

We present new U, B, V, and I images of the prototypical merger remnant NGC 7252 obtained with the WFPC2 instrument of the Hubble Space Telescope. The photometry reaches about 3 mag deeper than the previous observations with WF/PC1 and we detect 499 cluster candidates, most of them previously undiscovered. We can distinguish three populations of star clusters. We confirm the existence of a very luminous, blue population of clusters with a narrow range in color. Comparisons with Bruzual & Chariot (1996, in preparation) models incorporating a Salpeter stellar IMF show that the mean ages of these clusters are 650 Myr for [Fe/H]=0.0 and 750 Myr if [Fe/H]= -0.3. Therefore, these are the clusters that formed during the merger event. The upper limit to the effective radii of these objects is 4.8±0.4 pc (for D = 64.4 Mpc, Ho = 75), suggesting that they have physical properties like Galactic globular clusters. The second population of clusters is associated with the inner disk. Their (U-B) colors and reddening-free Q values indicate that their light is dominated by O stars and that they have ages less than 10 Myr. However, with (R eff) = 8.3±0.6 pc, these objects may be more like stellar associations and they may not survive for a significant time. Finally, there is also evidence for the presence of the most luminous of the old, metal-poor globular clusters that belonged to the progenitor galaxies. The cluster luminosity function is a single power law with slope — 1.8 down to a limiting magnitude of V= 26. The current specific cluster frequency is S w =0.6±0.3 for old clusters and young clusters with M> 10 5 M 0 , but after the remnant has faded for about 15 Gyr S N will have increased to about 2.5. The central disk is now resolved into much finer detail than in earlier WF/PC1 observations and the light profile of the nuclear region is consistent with a single power law with slope y= -1.26±0.03. After both the main body and the cluster system of NGC 7252 have aged for several Gyr it may have the properties of a field elliptical. © 1997 American Astronomical Society. [80004-6256(97)00512-8]