The Supermassive Black Hole and Dark Matter Halo of NGC 4649 (M60)

TL;DR

This study estimates the supermassive black hole and dark matter halo of NGC 4649 using orbit superposition modeling, revealing a larger black hole mass than previous estimates and highlighting the importance of comprehensive data for accurate modeling.

Contribution

The paper introduces an improved dynamical modeling approach incorporating diverse data sets, resulting in a more accurate black hole mass estimate and insights into dark matter halo properties.

Findings

Black hole mass = 4.5 ± 1.0 × 10^9 Msun

Dark matter halo confirmed but less constrained

Dynamical mass profile exceeds X-ray estimates by 60-80%

Abstract

We apply the axisymmetric orbit superposition modeling to estimate the mass of the supermassive black hole and dark matter halo profile of NGC 4649. We have included data sets from the Hubble Space Telescope, stellar, and globular cluster observations. Our modeling gives the black hole mass = 4.5 \pm 1.0 10^9 \Msun and M/L = 8.7 \pm 1.0 (or 8.0 \pm 0.9 after foreground Galactic extinction is corrected). We confirm the presence of a dark matter halo, but the stellar mass dominates inside the effective radius. The parameters of the dark halo are less constrained due to the sparse globular cluster data at large radii. We find that in NGC 4649 the dynamical mass profile from our modeling is consistently larger than that derived from the X-ray data over most of the radial range by roughly 60% to 80%. It implies that either some forms of non-thermal pressure need to be included, the assumed hydrostatic equilibrium may not be a good approximation in the X-ray modelings of NGC 4649, or our assumptions used in the dynamical models are biased. Our new black hole mass is about two times larger than the previous published value; the earlier model did not adequately sample the orbits required to match the large tangential anisotropy in the galaxy center. If we assume that there is no dark matter, the results on the black hole mass and M/L do not change significantly, which we attribute to the inclusion of HST spectra, the sparse globular cluster kinematics, and a diffuse dark matter halo. Without the HST data, the significance of the black hole detection is greatly reduced.