The globular cluster systems in the Coma ellipticals. II. Metallicity distribution and radial structure in NGC 4874, and implications for galaxy formation
Wf. Harris et al., The globular cluster systems in the Coma ellipticals. II. Metallicity distribution and radial structure in NGC 4874, and implications for galaxy formation, ASTROPHYS J, 533(1), 2000, pp. 137-148
Deep Hubble Space Telescope WFPC2 images in V and I are used to investigate
the globular cluster system (GCS) in NGC 4874, the central cD galaxy of th
e Coma cluster. Although the luminosity function of the clusters displays i
ts normal Gaussian-like shape and turnover level, other features of the sys
tem are surprising. We find the GCS to be (a) spatially extended, with core
radius r(c) similar to 22 kpc, (b) entirely metal poor (a narrow, unimodal
metallicity distribution with [[Fe/H]] similar to -1.5), and (c) modestly
populated for a cD-type galaxy, with specific frequency S-N = 3.7 +/- 0.5.
Model interpretations suggest to us that as much as half of this galaxy mig
ht have accreted from low-mass satellites, but no single one of the three c
lassic modes of galaxy formation (accretion, disk mergers, in situ formatio
n) can supply a fully satisfactory model for the formation of NGC 4874. Eve
n when they are used in combination, strong challenges to these models rema
in. We suggest that the principal anomaly in this GCS is essentially the co
mplete lack of metal-rich clusters. If these were present in normal (M87-li
ke) numbers in addition to the metal-poor ones that are already there, then
the GCS in total would more closely resemble what we see in many other gia
nt E galaxies. This supergiant galaxy appears to have avoided forming globu
lar clusters during the main metal-rich stage of star formation that built
the bulk of the galaxy.