Dynamical Evolution of Globular Cluster Systems formed in Galaxy Mergers: Deep HST/ACS Imaging of Old and Intermediate-Age Globular Clusters in NGC 3610

摘要

(ABRIDGED) The ACS camera on board the Hubble Space Telescope has been usedto obtain deep images of the giant elliptical galaxy NGC 3610, awell-established dissipative galaxy merger remnant. These observationssupersede previous WFPC2 images which revealed the presence of a population ofmetal-rich globular clusters (GCs) of intermediate age (~1.5-4 Gyr). We detecta total of 580 GC candidates, 46% more than from the previous WFPC2 images. Thenew photometry strengthens the significance of the previously found bimodalityof the color distribution of GCs. Peak colors in V-I are 0.93 +/-0.01 and 1.09+/- 0.01 for the blue and red subpopulations, respectively. The luminosityfunction (LF) of the inner 50% of the metal-rich (`red') population of GCsdiffers markedly from that of the outer 50%. In particular, the LF of the inner50% of the red GCs shows a flattening consistent with a turnover that is about1.0 mag fainter than the turnover of the blue GC LF. This is consistent withpredictions of recent models of GC disruption for the age range mentioned aboveand for metallicities that are consistent with the peak color of the red GCs aspredicted by population synthesis models. We determine the specific frequencyof GCs in NGC 3610 and find a present-day value of S_N = 1.4 +/- 0.6. Weestimate that this value will increase to S_N = 3.8 +/- 1.7 at an age of 10Gyr, which is consistent with typical S_N values for `normal' ellipticals. Ourfindings constitute further evidence in support of the notion that metal-richGC populations formed during major mergers involving gas-rich galaxies canevolve dynamically (through disruption processes) into the red, metal-rich GCpopulations that are ubiquitous in `normal' giant ellipticals.