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 & Charlot (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, H0=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 0 stars and that they have ages less than 10 Myr. However, with (Reff)=83±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 α≍-1.8 down to a limiting magnitude of V=26. The current specific cluster frequency is SN=0.6±0.3 for old clusters and young clusters with M>105 Msun,but after the remnant has faded for about 15 Gyr SN 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.