[Paper Review] The globular cluster system of NGC 1399 V. dynamics of the cluster system out to 80 kpc

Y. Schuberth, Thomas Richtler+5 more|El Servicio de Difusión de la Creación Intelectual (National University of La Plata)|Nov 2, 2009

Scientific Research and Discoveries107 references119 citations

TL;DR

This study analyzes radial velocities of 700 globular clusters in NGC 1399 out to 80 kpc using VLT and Gemini spectroscopy, revealing kinematically distinct red (metal-rich) and blue (metal-poor) subpopulations. The red clusters trace the galaxy's stellar field and require a dark halo with virial mass $8.0 \times 10^{12} M_\odot$, while blue clusters show high-velocity outliers indicating accretion from the Fornax cluster, suggesting a significant population of intracluster globular clusters and a different dynamical history.

ABSTRACT

(Abridged) We use the largest set of globular cluster velocities obtained so far of any elliptical galaxy to revise and extend the previous investigations of the dynamics of NGC 1399, the central dominant galaxy of the nearby Fornax cluster of galaxies. Our sample now comprises velocities for almost 700 GCs with projected galactocentric radii between 6 and 100 kpc. In addition, we use velocities published by Bergond et al. (2007). We study the kinematics of the metal-poor and metal-rich subpopulations and perform spherical Jeans modelling. The most important results are: The metal-rich (red) GCs resemble the stellar field population of NGC 1399 in the region of overlap. Both subpopulations are kinematically distinct and do not show a smooth transition. It is not possible to find a common dark halo which reproduces simultaneously the properties of both subpopulations. Some velocities of blue GCs are only to be explained by orbits with very large apogalactic distances, thus indicating a contamination with GCs which belong to the entire Fornax cluster rather than to NGC 1399. Stripped GCs from nearby elliptical galaxies, particularly NGC 1404, may also contaminate the metal-poor sample. We argue in favour of a scenario in which the majority of the blue cluster population has been accreted during the assembly of the Fornax cluster. The red cluster population shares the dynamical history of the galaxy itself. Therefore we recommend to use a dark halo based on the red GCs alone. The dark halo which fits best is marginally less massive than the halo quoted by Richtler et al. (2004). The comparison with X-ray analyses is satisfactory in the inner regions, but without showing evidence for a transition from a galaxy to a cluster halo, as suggested by X-ray work.

Motivation & Objective

• To investigate the kinematic and dynamical properties of the globular cluster system (GCS) of NGC 1399 out to 80 kpc, extending previous studies beyond 40 kpc.
• To determine whether a single dark matter halo can simultaneously explain the kinematics of both metal-poor (blue) and metal-rich (red) globular cluster subpopulations.
• To assess the contribution of accreted material and intracluster globular clusters to the blue GC population.
• To compare the derived dark matter halo profile with X-ray mass estimates and test for a transition from galaxy-like to cluster-like halo behavior.

Proposed method

• Obtained 477 medium-resolution spectra using VLT FORS2 and Gemini South GMOS, measuring radial velocities for 292 new globular clusters (241 not in prior data sets).
• Revised velocities for existing spectra using consistent templates to create a homogeneous data set of ~700 GCs with projected radii from 6 to 100 kpc.
• Combined with 56 external velocities from Bergond et al. (2007) extending to 200 kpc for extended kinematic modeling.
• Perfomed spherical Jeans modeling to derive mass profiles, fitting NFW dark matter halo models to the velocity dispersion profiles of red and blue GC subpopulations.
• Used wide-field Washington photometry to separate metal-poor (blue) and metal-rich (red) GC subpopulations based on color.
• Compared the derived mass profiles with X-ray-based mass estimates to test for consistency and detect a transition from galaxy to cluster halo behavior.

Experimental results

Research questions

• RQ1 Do the red and blue globular cluster subpopulations share a common dynamical history, or are they kinematically distinct?
• RQ2 Can a single NFW dark matter halo simultaneously reproduce the velocity dispersion profiles of both metal-poor and metal-rich globular clusters?
• RQ3 Are the high-velocity outliers in the blue GC population consistent with orbits in a bound halo, or do they indicate contamination from the intracluster population or nearby galaxies?
• RQ4 Does the mass profile derived from globular cluster kinematics agree with X-ray mass estimates, and is there evidence for a transition from galaxy-like to cluster-like halo structure?
• RQ5 What is the origin of the blue globular cluster population—accretion from dwarf galaxies or in-situ formation?

Key findings

• The red globular cluster subpopulation shows no significant rotation and exhibits a declining velocity dispersion with radius, consistent with orbital isotropy and kinematic similarity to the stellar field population.
• The blue globular cluster subpopulation shows kinematic irregularities, including a weak rotation signal around the minor axis at 22–44 kpc, and high-velocity outliers indicating large apogalactic distances.
• The velocity dispersion profile of blue GCs is not smooth, with extreme velocities suggesting contamination by intracluster or intergalactic globular clusters.
• A best-fitting NFW dark halo for the red GCs has a virial mass of $8.0 \times 10^{12} M_\odot$ and a scale radius of 34 kpc, slightly less massive than in earlier studies.
• Including external velocities from Bergond et al. (2007) yields a slightly more massive halo ($9.5 \times 10^{12} M_\odot$), but no transition to a cluster-like halo is observed in the data.
• The derived mass profile agrees reasonably with X-ray mass estimates in the inner regions but remains significantly below X-ray masses at larger radii, with no evidence for a transition from galaxy to cluster halo behavior.