Sizes, Half-Mass Densities, and Mass Functions of Star Clusters in the Merger Remnant NGC 1316: Clues to the Fate of Second-Generation Globular Clusters

Abstract

We study mass functions of globular clusters derived from HST/ACS images of the early-type merger remnant galaxy NGC 1316 which hosts a significant population of metal-rich globular clusters of intermediate age (~3 Gyr). For the old, metal-poor (`blue') clusters, the peak mass of the mass function M_p increases with internal half-mass density rho_h as (M_p proportional to rho_h^0.44) whereas it stays approximately constant with galactocentric distance R_gal. The mass functions of these clusters are consistent with a simple scenario in which they formed with a Schechter initial mass function and evolved subsequently by internal two-body relaxation. For the intermediate-age population of metal-rich ("red") clusters, the faint end of the previously reported power-law luminosity function of the clusters with R_gal > 9 kpc is due to many of those clusters having radii larger than the theoretical maximum value imposed by the tidal field of NGC 1316 at their R_gal. This renders disruption by two-body relaxation ineffective. Only a few such diffuse clusters are found in the inner regions of NGC 1316. Completeness tests indicate that this is a physical effect. Using comparisons with star clusters in other galaxies and cluster disruption calculations using published models, we hypothesize that most red clusters in the low-rho_h tail of the initial distribution have already been destroyed in the inner regions of NGC 1316 by tidal shocking, and that several remaining low-rho_h clusters will evolve dynamically to become similar to "faint fuzzies" that exist in several lenticular galaxies. Finally, we discuss the nature of diffuse red clusters in early-type galaxies.