A Photometric Survey of Globular Cluster Systems in Brightest Cluster Galaxies

Hubble Space Telescope imaging for 26 giant early-type galaxies, all drawn from the MAST archive, is used to carry out photometry of their surrounding globular cluster (GC) systems. Most of these targets are Brightest Cluster Galaxies (BCGs) and their distances range from 24 to 210 Mpc. The catalogs of photometry, completed with DOLPHOT, are publicly available. The GC color indices are converted to [Fe/H] through a combination of 12-Gyr SSP (Single Stellar Population) models and direct spectroscopic calibration of the fiducial color index (F475W-F850LP). All the resulting metallicity distribution functions (MDFs) can be accurately matched by bimodal Gaussian functions. The GC systems in all the galaxies also exhibit shallow metallicity gradients with projected galactocentric distance that average $Z \sim R_{gc}^{-0.3}$. Several parameters of the MDFs including the means, dispersions, and blue/red fractions are summarized. Perhaps the most interesting new result is the trend of blue/red GC fraction with galaxy mass, which connects with predictions from recent simulations of GC formation within hierarchical assembly of large galaxies. The observed trend reveals two major transition stages: for low-mass galaxies, the metal-rich (red) GC fraction $f(red)$ increases steadily with galaxy mass, until halo mass $M_h \simeq 3 \times 10^{12} M_{\odot}$. Above this point, more than half the metal-poor (blue) GCs come from accreted satellites and $f(red)$ starts declining. But above a still higher transition point near $M_h \simeq 10^{14} M_{\odot}$, the data hint that $f(red)$ may start to increase again because the metal-rich GCs also become dominated by accreted systems.