Issue |
A&A
Volume 459, Number 2, November IV 2006
|
|
---|---|---|
Page(s) | 391 - 406 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20053134 | |
Published online | 12 September 2006 |
Dynamics of the NGC 4636 globular cluster system
An extremely dark matter dominated galaxy?
1
Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany e-mail: ylva@astro.uni-bonn.de
2
Universidad de Concepción, Departamento de Física, Casilla 160-C, Concepción, Chile
3
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
Received:
26
March
2005
Accepted:
6
April
2006
Context.We present the first dynamical study of the globular cluster system of NGC 4636. It is the southernmost giant elliptical galaxy of the Virgo cluster and is claimed to be extremely dark matter dominated, according to X-ray observations.
Aims.Globular clusters are used as dynamical tracers to investigate, by stellar dynamical means, the dark matter content of this galaxy.
Methods.Several hundred medium resolution spectra were acquired at the VLT
with FORS 2/MXU. We obtained velocities for 174 globular clusters
in the radial range , or
in units of
effective radius. Assuming a distance of 15 Mpc, the clusters are
found at projected galactocentric distances in the range 4 to
70 kpc, the overwhelming majority within 30 kpc. The measured
line-of-sight velocity dispersions are compared to
Jeans-models.
Results.We find some indication of a rotation of the red (metal-rich)
clusters about the minor axis. Out to a radius of 30 kpc, we find
a roughly constant projected velocity dispersion for the blue
clusters of . The red
clusters are found to have a distinctly different behavior: at a radius of about 3′, the velocity dispersion drops
by ~
to about
, which then remains constant out to a radius of 7′. The cause might be the steepening
of the number density profile at ~
observed for the red clusters. Using only the blue clusters as
dynamical tracers, we perform Jeans-analyses for different
assumptions of the orbital anisotropy. Enforcing the model dark
halos to be of the NFW type, we determine their structural
parameters. Depending on the anisotropy and the adopted
-values,
we find that the dark matter fraction within one effective radius
can vary between 20% and 50%, with most a probable range between
20% and 30%. The ambiguity of the velocity dispersion in the
outermost bin is a main source of uncertainty. A comparison with
cosmological N-body simulations reveals no striking disagreement.
Conclusions.Although the dark halo mass still cannot be strongly constrained, NGC 4636 does not seem to be extremely dark matter dominated. The derived circular velocities are also consistent with Modified Newtonian Dynamics.
Key words: galaxies: elliptical and lenticular, cD / galaxies: kinematics and dynamics / galaxies: individual: NGC 4636 / galaxies: halos
© ESO, 2006
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