Globular Cluster Systems in Brightest Cluster Galaxies. Ii. NGC 6166

Abstract

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 F 475W, F 814W 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 Sérsic-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 gc 15 kpc, both of them varying as σ MRGC ∼ σ light ∼ R −1.8 . By comparison, the blue (metal-poor) GC component has a much shallower falloff σ MP GC ∼ 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.

The difference in metallicity distribution functions of halo stars and globular clusters as a function of galaxy type

“…For instance the MDF of GCs in the MW has peaks at [M/H]=−1.55 ± 0.07 and −0.55 ± 0.10 (Harris et al 2016) and the giant elliptical Cen A has peaks at [M/H]=-1.4 and -0.4 (Beasley et al 2008).…”

The Fornax 3D project: Non-linear colour–metallicity relation of globular clusters

“…The survival of GCs until today allows us to view them as fossil records that have the chemodynamical properties of their origin encapsulated in their stellar population properties and orbital parameters which change only slowly over time (e.g. Beasley et al 2008;Harris et al 2016).…”

Detection of the Stellar Intracluster Medium in Perseus (Abell 426)

“…Since every one of the target fields is quite uncrowded in absolute terms, artificialobject lists were generated randomly in location across the field. The dependence of completeness f (m) (the fraction of successfully detected and measured objects versus magnitude) is modelled with the simple two-parameter function (Harris et al 2016)…”

“…For each of the target fields, the individual exposures (4 in The procedure for detection and photometry of the GC candidates then followed the steps outlined in, e.g., Harris (2009a); Harris et al (2016). SourceExtractor (SE) (Bertin & Arnouts 1996) was used to detect objects and to do a preliminary rejection of nonstellar ones (either single-pixel artifacts or faint background galaxies) from a plot of the SE parameters r 1/2 versus aperture magnitude.…”