The Luminosity Functions of Old and Intermediate-Age Globular Clusters in NGC 3610

Abstract

The Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST) has been used to obtain high-resolution images of NGC 3610, a dynamically young elliptical galaxy in a group environment. These observations supersede shorter, undithered HST observations in which an intermediate-age population of globular clusters was discovered. The new observations show the bimodal color distribution of globular clusters more clearly, with peaks at V-I = 0.95 and 1.17. The luminosity function of the blue metal-poor population of clusters in NGC 3610 turns over, consistent with a Gaussian distribution with a peak MV ≈ -7.0, similar to old globular cluster populations in elliptical galaxies. The red metal-rich population of clusters has a luminosity function that is more extended toward both the bright and faint ends, as expected for a cluster population of intermediate age. It is well fitted by a power law (L)dL ∝ Lα dL, with an exponent of α = -1.78 ± 0.05, or α = -1.90 ± 0.07 when corrected for observational scatter. A Kolmogorov-Smirnov test confirms the significant difference between the luminosity functions of the red and blue clusters, with a probability of less than 0.1% that they come from the same population. A comparison with the Fall & Zhang cluster disruption models shows marginal agreement with the observed data when comparing both the luminosity functions and the mean color distributions, although there are differences in detail. In particular, there is no clear evidence of the predicted turnover at the faint end, although deeper observations will be required to make a definitive test. A by-product of the analysis is the demonstration that at any given metallicity the peak of the luminosity function should remain nearly constant from 1.5 to 12 Gyr, since the effect of the disruption of faint clusters is almost perfectly balanced by the fading of the clusters. This may help explain the apparent universality of the peak of the globular cluster luminosity function.