Dynamics of the NGC 4636 Globular Cluster System - An extremely dark matter dominated galaxy?

TL;DR

This study investigates the dark matter content of NGC 4636 by analyzing globular cluster velocities, revealing moderate dark matter dominance and challenging previous claims of extreme dark matter domination.

Contribution

First dynamical analysis of NGC 4636's globular clusters, providing insights into its dark matter fraction and challenging prior X-ray based claims.

Findings

Dark matter fraction within one effective radius is 20-50%.

Velocity dispersion of blue clusters remains roughly constant at ~200 km/s.

NGC 4636 does not appear to be extremely dark matter dominated.

Abstract

We present the first dynamical study of the globular cluster system of NGC 4636. This giant elliptical galaxy is claimed to be extremely dark matter dominated, according to X-ray observations. Using the VLT with FORS2/MXU, we obtained velocities for 174 globular clusters. The clusters have projected galactocentric distances in the range 4 to 70 kpc, the overwhelming majority lie within 30 kpc. We find some indication for 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 ~200 km/s. The red clusters exhibit a distinctly different behavior: at a radius of about 13 kpc, the velocity dispersion drops by ~50 km/s to about 170 km/s which then remains constant out to a radius of 30 kpc. Using only the blue clusters as dynamical tracers, we perform Jeans-analyses for different assumptions of the orbital anisotropy. 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%. A main source of uncertainty is the ambiguity of the velocity dispersion in the outermost bin. 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.