Dynamical Studies of the Globular Cluster Systems around the Giant Elliptical Galaxies NGC4636 and NGC1399

Abstract

Dark matter studies in elliptical galaxies were long hampered by the lack of suitable dynamical tracers. The advent of 8 m-class telescopes equipped with multi-object spectrographs has made it possible to use globular clusters (GCs) as dynamical probes constraining their host galaxy's gravitational potential. <br /> This work presents the two largest samples of globular cluster velocities obtained for giant elliptical galaxies to date: The galaxies studied are NGC 4636 located in the very outskirts of the Virgo cluster of galaxies and NGC 1399, the central galaxy of the Fornax cluster. <br /> NGC 4636 has a very rich GC system and is known for its unusually bright X-ray halo which earned it the reputation of being extremely dark matter dominated. Using 460 velocities of GCs out to a projected galactocentric distance of 60 kpc, we confirm that the metal-poor (blue) GCs have a declining line-of-sight velocity dispersion profile. The corresponding Jeans models require significantly less dark matter than suggested by the X-ray studies, unless the latter incorporate a very strong (and probably unrealistic) metal-abundance gradient. <br /> The extremely populous globular cluster system of NGC 1399 has an extent of at least 250 kpc, which is comparable to the core radius of the Fornax cluster itself. Hence, the question arises whether there exists a population of intra cluster globular clusters (ICGCs). <br /> Using a catalogue of candidate ICGCs from the literature and combining these data with photometry obtained by our group, it is shown that the vast majority of the metal-poor GCs found in between the galaxies of the Fornax cluster have velocities that are compatible with their being members of the very extended NGC 1399 GC system. <br /> The data set used for the dynamical analysis of the NGC 1399 GC system comprises the velocities of about 700 GCs with projected galactocentric radii between 6 and 100 kpc. The most important results are: The metal-rich (red) GCs resemble the stellar field population of NGC 1399 in terms of radial distribution and velocity dispersion. The metal-poor (blue) GCs have a shallower radial distribution and show a more erratic kinematic behaviour. Both subpopulations are kinematically distinct and do not show a smooth transition. It is not possible to find a common dark matter halo which reproduces the properties of both red and blue GCs. The mass estimates obtained from the combined analysis of the red GCs and the stellar velocity dispersion profile agree with the values from X-ray studies in the inner 100 kpc. At larger radii, however, we do not find any evidence for a transition from a galaxy to a cluster halo, as suggested by X-ray work. <br /> Finally, we compare our GC-based NGC 1399 mass profile to the dynamics of the Fornax cluster. The dynamical analysis suggests that the early-type giants form a subsystem which is in dynamical equilibrium. The kinematics of these galaxies agree with the extrapolation of the GC based mass estimate of NGC 1399. The late-type giants, on the other hand, tend to avoid the cluster core and their velocity distribution indicates that these galaxies are an infalling population.