Dynamics of the NGC 4636 globular cluster system

II. Improved constraints from a large sample of globular cluster velocities^⋆,⋆⋆

¹ Argelander-Institut für Astronomie, Universität Bonn Auf dem Hügel 71, 53121 Bonn, Germany
² Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
e-mail: tom@astroudec.cl
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁴ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁵ Friedrich-Ebert-Gymnasium Bonn, Ollenhauerstr. 5, 53113 Bonn, Germany
⁶ Department of Astrophysics, IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

Received: 25 May 2010
Accepted: 26 April 2012

Abstract

We present new radial velocities for 289 globular clusters around NGC 4636, the southernmost giant elliptical galaxy of the Virgo cluster. The data were obtained with FORS2/MXU at the Very Large Telescope. Together with data analysed in an earlier study, we now have a sample of 460 globular cluster velocities out to a radius of 12 arcmin (60 kpc) available – one of the largest of its kind. This new data set also provides a much more complete angular coverage. Moreover, we present new kinematical data of the inner stellar population of NGC 4636. We perform an updated Jeans analysis, using both stellar and GC data, to better constrain the dark halo properties. We find a stellar M/L-ratio of 5.8 in the R-band, higher than expected from single stellar population synthesis. We model the dark halo by cored and cuspy analytical halo profiles and consider different anisotropies for the tracer populations. Properties of NFW halos lie well within the expected range of cosmological simulations. Cored halos give central dark matter densities, which are typical for elliptical galaxies of NGC 4636’s luminosity. The surface densities of the dark matter halos are higher than those of spiral galaxies. We compare the predictions of modified Newtonian dynamics with the derived halo properties and find satisfactory agreement. NGC 4636 therefore falls onto the baryonic Tully-Fisher relation for spiral galaxies. The comparison with the X-ray mass profile of Johnson et al. (2009, ApJ, 706, 980) reveals satisfactory agreement only if the abundance gradient of hot plasma has been taken into account. This might indicate a general bias towards higher masses for X-ray based mass profiles in all systems, including galaxy clusters, with strong abundance gradients.