The Globular Cluster Systems around NGC 3311 and NGC 3309

Abstract

We present extensive new photometry in (g',i') of the large globular cluster (GC) system around NGC 3311, the central cD galaxy in the Hydra cluster. Our GMOS data cover a 5.5' field of view and reach a limiting magnitude i' = 26, about 0.5 magnitude fainter than the turnover point of the GC luminosity function. We find that NGC 3311 has a huge population of ~16, 000 GCs, closely similar to the prototypical high specific frequency Virgo giant M87. The color-magnitude distribution shows that the metal-poor blue GC sequence and the metal-richer red sequence are both present, with nearly equal numbers of clusters. Bimodal fits to the color distributions confirm that the blue sequence shows the same trend of progressively increasing metallicity with GC mass that has previously been found in many other large galaxies; the correlation we find corresponds to a scaling of GC metallicity with mass of Z ~ M^0.6 . By contrast, the red sequence shows no change of mean metallicity with mass, but it shows an upward extension to much higher than normal luminosity into the UCD-like range, strengthening the potential connections between massive GCs and UCDs. The GC luminosity function, which we measure down to the turnover point at M_I = -8.4, also has a normal form like those in other giant ellipticals. Within the Hydra field, another giant elliptical NGC 3309 is sitting just 100" from the cD NGC 3311. We use our data to solve simultaneously for the spatial structure and total GC populations of both galaxies at once. Their specific frequencies are S_N (NGC 3311) = 12.5 +/- 1.5 and S_N (NGC 3309) = 0.6 +/-0.4. NGC 3311 is completely dominant and entirely comparable with other cD-type systems such as M87 in Virgo.Comment: 15 pages, 15 figures. Accepted to the Astrophysical Journal. Version with higher resolution figures is available at http://www.thewehners.net/astro/papers/wehner_n3311_highres.pd

The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological?

“…The pair NGC 3311/NGC 3309 bears some resemblance to the central pair NGC 1399/NGC 1404 in the Fornax cluster with NGC 1404 being also being underabundant in globular clusters (Richtler et al 1992) and clusters actually belonging to NGC 1404 indeed contaminating the cluster system of NGC 1399 (Schuberth et al 2010). However, our sample of NGC 3311 clusters consists exclusively of very bright objects that are genuinely sparse in normal ellipticals, so we expect the contamination by NGC 3309 to be even lower than the 2% quoted by Wehner et al (2008). An additional difference of NGC 1399/NGC 1404 is visible in the X-ray isophotes.…”

The colour–magnitude relation of globular clusters in Centaurus and Hydra

“…Harris et al 2006Harris et al , 2009aStrader et al 2006;Mieske et al 2006Mieske et al , 2010Spitler et al 2006;Humphrey 2009;Cockcroft et al 2009;Forbes et al 2010;Blom et al 2012;Usher et al 2013). This phenomenon has been nicknamed the blue tilt, and has also been detected through ground-based imaging (Forte et al 2007;Wehner et al 2008;Park 2012). The first strong hint of the existence of such a colour-magnitude relation of GCs was reported in Dirsch et al (2003), who found that the colour distribution of the brightest GCs in NGC 1399 becomes broad and unimodal.…”

“…Value of the blue tilt slope γ = d(U − I)/dI and of the mass-metallicity relation exponent α obtained for Centaurus and Hydra with our data for the range I < 24.0 mag, compared to results of previous studies on these clusters and other environments. (Wehner et al 2008) 0.60 ± 0.20 BCGs (Harris et al 2009a) 0 . 4 8 ± 0.12 −0.11 ± 0.06 M104 0 .…”

Isolated ellipticals and their globular cluster systems

“…which gives a good analytical description of the completeness, and which has been used for many GC studies (e.g., McLaughlin et al 1994;Fleming et al 1995;Wehner et al 2008;Alamo-Martínez et al 2012). The free parameters α and m 50 indicate the speed at which the detection rate falls at the faint level and the 50% completeness, respectively.…”

“…2 as large black points, while resolved sources are depicted will small black dots. Globular cluster candidates were further limited to the color range 0.4 < (g − i) 0 < 1.4, following previous GCS studies with the same filters Wehner et al 2008;Faifer et al 2011), but with a slightly bluer limit to allow studying possible younger clusters (as in NGC 2685 Sikkema et al 2006). The bright limit was fixed at M i = −11.5 following Faifer et al (2011), and the faint limit was given by the 80% completeness limit in both g and i.…”