2016ApJ...822...95C

We present new Hubble Space Telescope (HST) optical and near-infrared (NIR) photometry of the rich globular cluster (GC) system NGC 4874, the cD galaxy in the core of the Coma cluster (Abell 1656). NGC 4874 was observed with the HST Advanced Camera for Surveys in the F475W (g₄₇₅) and F814W (I₈₁₄) passbands and with the Wide Field Camera 3 IR Channel in F160W (H₁₆₀). The GCs in this field exhibit a bimodal optical color distribution with more than half of the GCs falling on the red side at g₄₇₅-I₈₁₄ > 1. Bimodality is also present, though less conspicuously, in the optical-NIR I₈₁₄-H₁₆₀ color. Consistent with past work, we find evidence for nonlinearity in the g₄₇₅-I₈₁₄ versus I₈₁₄-H₁₆₀ color-color relation. Our results thus underscore the need for understanding the detailed form of the color-metallicity relations in interpreting observational data on GC bimodality. We also find a very strong color-magnitude trend, or "blue tilt," for the blue component of the optical color distribution of the NGC 4874 GC system. A similarly strong trend is present for the overall mean I₈₁₄-H₁₆₀ color as a function of magnitude; for M₈₁₄ , but the scaling is not a simple power law and becomes much weaker at lower masses. As in other similar systems, the spatial distribution of the blue GCs is more extended than that of the red GCs, partly because of blue GCs associated with surrounding cluster galaxies. In addition, the center of the GC system is displaced by 4±1 kpc toward the southwest from the luminosity center of NGC 4874, in the direction of NGC 4872. Finally, we remark on a dwarf elliptical galaxy with a noticeably asymmetrical GC distribution. Interestingly, this dwarf has a velocity of nearly -3000 km/s with respect to NGC 4874; we suggest it is on its first infall into the cluster core and is undergoing stripping of its GC system by the cluster potential.