Globular Cluster Systems in Brightest Cluster Galaxies. II: NGC 6166
  We present new deep photometry of the globular cluster system (GCS) around
NGC 6166, the central supergiant galaxy in Abell 2199. HST data from the ACS
and WFC3 cameras in F475W, F814W are used to determine the spatial distribution
of the GCS, its metallicity distribution function (MDF), and the dependence of
the MDF on galactocentric radius and on GC luminosity. The MDF is extremely
broad, with the classic red and blue subpopulations heavily overlapped, but a
double-Gaussian model can still formally match the MDF closely. The spatial
distribution follows a Sersic-like profile detectably to a projected radius of
at least $R_{gc} = 250$ kpc. To that radius, the total number of clusters in
the system is N_{GC} = 39000 +- 2000, the global specific frequency is S_N =
11.2 +- 0.6, and 57\% of the total are blue, metal-poor clusters. The GCS may
fade smoothly into the Intra-Cluster Medium of A2199; we see no clear
transition from the core of the galaxy to the cD halo or the ICM. The radial
distribution, projected ellipticity, and mean metallicity of the red
(metal-richer) clusters match the halo light extremely well for R > 15 kpc,
both of them varying as \sigma_{MRGC} ~ \sigma_{light} ~ R^-1.8. By comparison,
the blue (metal-poor) GC component has a much shallower falloff \sigma_{MPGC} ~
R^-1.0 and a more nearly spherical distribution. This strong difference in
their density distributions produces a net metallicity gradient in the GCS as a
whole that is primarily generated by the population gradient. With NGC 6166 we
appear to be penetrating into a regime of high enough galaxy mass and rich
enough environment that the bimodal two-phase description of GC formation is no
longer as clear or effective as it has been in smaller galaxies.
