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

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-tolight ratio, we infer a dark matter mass fraction in NGC 4649 of ∼0.39 at 1R e (10.5 kpc) and ∼0.78 at 4R e . 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.

Constraining the Physical State of the Hot Gas Halos in NGC 4649 and NGC 5846

“…In the same figure we show the velocity profile from Das et al (2011) that best fits their SD, GC, and PN data. This profile follows Shen & Gebhardtʼs up to 7.6 kpc and is flat from that point onward (their VC4 profile); it is compatible with Alabi et alʼs point, as well as with our X-ray HE profiles.…”

Dark matter inside early-type galaxies as function of mass and redshift

“…Studies of the Virgo giant elliptical galaxy NGC 4949 (M60) by Teodorescu et al (2011) reveal that the kinematics of Planetary Nebulae in this object is consistent with the presence of a dark matter halo with fDM ∼ 0.5 for r = 3re. De Bruyne et al (2001) presented three-integral axisymmetric models for NGC 4649 and NGC 7097 and concluded that the kinematic data for NGC 4649 only require a small amount of dark matter, however Das et al (2011) determine fDM ∼ 0.78 at r = 4re for NGC 4649.…”

Made-to-measure galaxy models - II. Elliptical and lenticular galaxies

“…By comparison, the made‐to‐measure (M2M) method formulated by Syer & Tremaine (1996) is less well known but is no less capable, and has been the subject of growing interest recently (e.g. de Lorenzi et al 2007; Jourdeuil & Emsellem 2007; de Lorenzi et al 2008; Dehnen 2009; Long & Mao 2010; Das et al 2011). Both methods achieve their objectives by weighting a system of particles/orbits and superimposing them to reproduce the galactic observations.…”