The globular cluster populations of giant galaxies: Mosaic imaging of five moderate-luminosity early-type galaxies

Abstract

This paper presents results from wide-field imaging of the globular cluster (GC) systems of five intermediate-luminosity ($M_{V}$ ~ –21 to –22) early-type galaxies. The aim is to accurately quantify the global properties of the GC systems by measuring them out to large radii. We obtained BVR imaging of four lenticular galaxies (NGC 5866, NGC 4762, NGC 4754, NGC 3384) and one elliptical galaxy (NGC 5813) using the KPNO 4 m telescope and Mosaic imager and traced the GC population to projected galactocentric radii ranging from ~20 kpc to 120 kpc. We combine our imaging with Hubble Space Telescope data to measure the GC surface density close to the galaxy center. We calculate the total number of GCs (N GC) from the integrated radial profile and find $N_{GC}$ = 340 ± 80 for NGC 5866, $N_{GC}$ = 2900 ± 400 for NGC 5813, $N_{GC}$ = 270 ± 30 for NGC 4762, $N_{GC}$ = 115 ± 15 for NGC 4754, and $N_{GC}$ = 120 ± 30 for NGC 3384. The measured GC specific frequencies are SN between 0.6 and 3.6 and T in the range 0.9-4.2. These values are consistent with the mean specific frequencies for the galaxies' morphological types found by our survey and other published data. Three galaxies (NGC 5866, NGC 5813, and NGC 4762) had sufficient numbers of GC candidates to investigate color bimodality and color gradients in the GC systems. NGC 5813 shows strong evidence (>3σ) for bimodality and a B – R color gradient resulting from a more centrally concentrated red (metal-rich) GC subpopulation. We find no evidence for statistically significant color gradients in the other two galaxies.