The Globular Cluster Kinematics and Dark Matter Content of NGC 4649

Abstract

The globular cluster system (GCS) of the elliptical galaxy NGC 4649 has been examined using the Gemini Multi-Object Spectrograph (GMOS); spectra for 156 candidate globular clusters (GCs) were obtained, extending to a galactocentric radius of 42 kpc. The system was found to have an even 78 GC candidates per population, using a colour of g-i = 0.92 (Faifer et al. 2011) to split the system into sub-populations. The populations refer to their metalicity; a g-i<0.92 is considered metal-poor (MP), and a g-i>0.92 is metal-rich (MR).

Line-of-sight-velocity measurements and subsequent modelling, were used to measure the full GCS rotational velocity as 59+/-28 km/s, with a position angle of 218+/-28 degrees. The MR population was found to have rotational velocity of 81+/-42 km/s with a position angle of 221+/-29 degrees, while the MP population measures a rotational velocity of 30+/-36 km/s with a position angle of 202+/-73 degrees. The average velocity dispersion for the full GCS was calculated at 247+/-61 km/s, the MR population 266+/-94 km/s, and the MP population, 221+/-76 km/s. These findings are consistent (within uncertainties) with previous studies by Hwang et al.(2008), and Bridges et al. (2006). The velocity dispersion profile for all populations is constant with increasing radius, suggesting the presence of a dark matter (DM) halo. A tracer mass estimator was used to measure the mass at 42 kpc as (2.01+/-0.05)X10^{12} solar masses, for an isothermal potential, and (1.21+/-0.05)X10^{12} solar masses if the tracers followed the DM profile. Finally, it was estimated that M/L_{B}=22 - 44, consistent with the presence of considerable amounts of DM for a luminous galaxy.