Using NMAGIC to probe the dark matter halo and orbital structure of the X-ray bright, massive elliptical galaxy, NGC 4649

TL;DR

This study models the mass distribution and orbital structure of galaxy NGC 4649 using NMAGIC, revealing insights into dark matter content, potential uncertainties in hot gas measurements, and orbital anisotropy.

Contribution

It applies the NMAGIC dynamical modeling technique to NGC 4649, comparing multiple mass profiles from X-ray, globular clusters, and stellar data, highlighting uncertainties and orbital characteristics.

Findings

Dark matter fraction ~0.39 at 1Re and ~0.78 at 4Re.

PN velocities favor a less massive halo than GCs suggest.

Stellar orbits are mostly isotropic to mildly radial in the center.

Abstract

We create dynamical models of the massive elliptical galaxy, NGC 4649, using the N-body made-to-measure code, NMAGIC, and kinematic constraints from long-slit and planetary nebula (PN) data. We explore a range of potentials based on previous determinations from X-ray observations and a dynamical model fitting globular cluster (GC) velocities and a stellar density profile. The X-ray mass distributions are similar in the central region but have varying outer slopes, while the GC mass profile is higher in the central region and on the upper end of the range further out. Our models cannot differentiate between the potentials in the central region, and therefore if non-thermal pressures or multi-phase components are present in the hot gas, they must be smaller than previously inferred. In the halo, we find that the PN velocities are sensitive tracers of the mass, preferring a less massive halo than that derived from the GC mass profile, but similar to one of the mass distributions derived from X-rays. Our results show that the GCs may form a dynamically distinct system, and that the properties of the hot gas derived from X-rays in the outer halo have considerable uncertainties that need to be better understood. Estimating the mass in stars using photometric information and a stellar population mass-to-light ratio, we infer a dark matter mass fraction in NGC 4649 of ~0.39 at 1Re (10.5 kpc) and ~0.78 at 4Re. We find that the stellar orbits are isotropic to mildly radial in the central ~6 kpc depending on the potential assumed. Further out, the orbital structure becomes slightly more radial along R and more isotropic along z, regardless of the potential assumed. In the equatorial plane, azimuthal velocity dispersions dominate over meridional velocity dispersions, implying that meridional velocity anisotropy is the mechanism for flattening the stellar system.