Dynamical Evolution of Globular Cluster Systems Formed in Galaxy Mergers: DeepHubble Space TelescopeAdvanced Camera for Surveys Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610

Abstract

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the giant elliptical galaxy NGC 3610, a well-established remnant of a dissipative galaxy merger. These observations supersede previous, shallower observations that revealed the presence of a population of metal-rich globular clusters of intermediate age (~1.5-4 Gyr). We detect a total of 580 cluster candidates, 46% more than from the previous WFPC2 images. The new photometry strengthens the significance of the previously found bimodality of the color distribution of clusters. Peak colors in V - I are 0.93 ± 0.01 and 1.09 ± 0.01 for the blue and red subpopulations, respectively. The luminosity function of the inner 50% of the metal-rich (red) population of clusters differs markedly from that of the outer 50%. In particular, the luminosity function of the inner 50% of the red clusters shows a flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue clusters. This is consistent with predictions of recent models of cluster disruption for the age range mentioned above and for metallicities that are consistent with the peak color of the red clusters, as predicted by population synthesis models. The radial surface density profile of red clusters follows that of the underlying galaxy light more closely than in elliptical galaxies, which is consistent with the intermediate-age nature of the red clusters. We determine the specific frequency of clusters in NGC 3610 and find a present-day value of SN = 1.4 ± 0.6. Using published age estimates for the diffuse light of NGC 3610, as well as cluster disruption models, we estimate that this value will increase to SN = 3.8 ± 1.7 at an age of 10 Gyr, which is consistent with typical SN values for elliptical galaxies. Our findings constitute further evidence in support of the notion that metal-rich cluster populations formed during major mergers involving gas-rich galaxies can evolve dynamically (through disruption processes) into the red, metal-rich cluster populations that are ubiquitous in normal giant ellipticals.