Dynamics of the NGC 4636 globular cluster system : An extremely dark matter dominated galaxy?

• Published in: Astronomy and astrophysics (Berlin) 2006-11, Vol.459 (2), p.391-406
• Main Authors: SCHUBERTH, Y, RICHTLER, T, DIRSCH, B, HILKER, M, LARSEN, S. S, KISSLER-PATIG, M, MEBOLD, U
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20053134

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 0\farcm 90 < R < 15\farcm 5, or 0.5{-}9 similar to R_{\rm e} 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 \sigma \approx 200 similar to {\rm km\,s} greater than or equal to The red clusters are found to have a distinctly different behavior: at a radius of about 3 logical or prime, the velocity dispersion drops by similar to 50 similar to {\rm km\,s}to about 170 similar to {\rm km\,s} less than or equal to which then remains constant out to a radius of 7 logical or prime. The cause might be the steepening of the number density profile at similar to 3{\mbox{$ logical or prime$ }} 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 M / L-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.