Deep Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 1316: Evidence for Dynamical Evolution of Second-Generation Globular Clusters

Abstract

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep high-resolution images of the giant early-type galaxy NGC 1316, an obvious merger remnant. These observations supersede previous shallower imagery that revealed the presence of a population of metal-rich globular clusters of intermediate age (~3 Gyr). We detect a total of 1496 cluster candidates, almost 4 times as many as from the previous Wide Field Planetary Camera 2 images. We confirm the bimodality of the color distribution of clusters, even in V - I, with peak colors 0.93 and 1.06. The large number of detected clusters allows us to evaluate the globular cluster luminosity functions as a function of galactocentric radius. We find that the luminosity function of the inner 50% of the intermediate-age 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 clear flattening consistent with a turnover that is about 1.0 mag fainter than the turnover of the blue clusters. This constitutes the first direct evidence that metal-rich cluster populations formed during major mergers of gas-rich galaxies can evolve dynamically (through disruption processes) into the red metal-rich cluster populations that are ubiquitous in normal giant elliptical galaxies.