Dynamics of the Globular Cluster System of NGC 4636
Y. Schuberth1, T. Richtler,
B. Dirsch, K. Gebhardt, M. Hilker,
L. Infante, S. Larsen
1
Sternwarte der Universitaet Bonn, Germany
We present the first dynamical study of the globular cluster system of
NGC 4636, the southernmost giant elliptical galaxy of the Virgo
cluster,
which from X-ray studies has the reputation of being extremely dark
matter dominated. Several hundred medium resolution spectra were
acquired at the VLT with FORS2/MXU. We obtained velocities for 176
globular clusters. 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. Out to this radius, we find
a constant projected velocity dispersion for the blue clusters of
sigma~200 km/s. Applying a spherical,
isotropic model, we calculate a constant circular velocity of
317±25 km/s. The red clusters are found to have a
distinctly different behaviour: at a radius of about
3', the velocity dispersion drops by
~50 km/s to about
170 km/s which then remains constant out to a
radius of 7'. The cause might be the steepening of
the number density profile at ~3' observed for
the red clusters. Adopting a high stellar M/L value from the dynamics
of the inner stellar body (Kronawitter et al.), we find that a NFW
halo with a low dark matter fraction (of about 15% at one effective
radius) approximately yields the required total circular velocity.
Lower stellar M/L values increase the dark matter fraction, but do not
reproduce the circular velocities of the stellar velocity dispersion.
Thus, NGC 4636 does not seem to be extremely dark matter dominated.