The Globular Cluster System of NGC 3311

The Globular Cluster
System of NGC 3311
Elizabeth Wehner
Utrecht University
in collaboration with:
William Harris
Kristin Woodley
Barry Rothberg
Brad Whitmore
(Wehner & Harris 2007,
Wehner et al. in press)
NGC 3311
Hydra cD galaxy
d = 54 Mpc
v = 3593 km/s
(m-M)0 = 33.62
VT0 = 11.04,
MVT = -22.8
NGC 3309
Globular Cluster System
g’-i’ data, Gemini South, GMOS
3900s in each band
g’(lim) = 26.67
i’(lim) = 26.30
5.5’ x 5.5’ FOV
But, which belong to 3311..?
N3309 contribution estimated
at “a few percent” (Harris et
al. 1983; McLaughlin et al.
1995)
possible to gain more precise
estimate by modeling both
GCSs simultaneously!
--> Generalized Hubble and
Sersic profiles
--> 5 free parameters for fit
--> minimized chi-squared
Best fit: Sersic Model,
n=1.24±0.06, re = 177”±9”
--> NGC 3309 contributes ~ 2.2%
of GCs in the field
Specific Frequency
integrate to get total populations
Choice of Rmax unimportant for NGC
3309 (steeper profile), Rmax ~ 9’ for
NGC 3311 (consistent w/ e.g. Richter
et al. 1982, Vasterberg et al. 1991)
For i’ = 25.4 and brighter: N3311 =
7440, N3309 = 165
Using the GCLF to correct for
completeness,
N3311 ≃ 16500±2000
N3309 ≃ 364±210
GCLF for NGC 3311. Solid circles,
values after background correction
Red line = Gaussian fit with peak at i’
= 25.65 (MV ≃ -7.4) and σi’ = 1.5
GCS Bimodality
Seperated into magnitude
bins, then fit by 2 functions
RMIX (MacDonald 2007)
http://
www.math.mcmaster.ca/
peter/mix/mix.html
Fitting function is userdefined, as are the
dispersions and number of
components
Bimodal fits are strongly
preferred for 2 brightest bins
Mass-Metallicity Relation
Mass-metallicity relation (MMR) (e.g.
Dirsch 2003; Ostrov 1998; Harris
2006, Mieske 2006)
Blue half: 0.3 < (g’-i’) < 0.8
Red half: 0.8 < (g’-i’) < 1.2
Δ(g’-i’)/Δi’ = 0.044 ± 0.011 for blue
sequence
Δ(g’-i’)/Δi’ = 0.012 ± 0.021 for red
sequence
Z ~ M0.6±0.2 for NGC 3311, similar to
Z ~ M0.55 from Harris et al. (2006) and
Z ~ M0.48±0.08 from Mieske et al. (2006)
DGTOs/UCDs in Hydra?
Unusual extension of red,
metal-rich GCs -- fewer seen in
a collection of many BCGs!
Up to M ~ 30 x 106 M⦿ (using
M/L ~ 3)
choice of M/L is difficult!
(e.g Hasegan et al. 2005;
Evstigneeva et al. 2007, Hilker
et al. 2007, Fellhauer & Kroupa
2006, Bruzual & Charlot 2003)
Comparison with WFPC2 data
shows some are marginally
resolved at the 2σ level, with r ~
20pc
<V-I> ~ 1.1
Part of NGC 3311?
Could they be foreground
contamination?
Can use galactic stellar synthesis
model to estimate (e.g. Robin et al.
2003):
http://bison.obs-besancon.fr/modele/
Unlikely to be foreground
contamination
What about a background cluster?
Other background galaxies?
Histogram of field stars within 1
square degree, centered on NGC
3311, versus color
Dashed lines represent typical
locations of the blue and red GC
peaks in B-I (from Brodie et al. 2005)
Radial Distribution
Summary/
Concluding Thoughts
NGC 3311 is one of the largest GCSs in the local
universe, rivaling that of M87 (NGCs ~13000, SN ~
13)
New technique can estimate relative contributions
of companion galaxies
UCDs in the Hydra Cluster?
UCDs = large globular clusters?
(more spectra needed!)
Radial Color Distribution
Azimuthal
Distribution
WFPC2 Data