Weighing the Quiescent Central Black Hole in an Elliptical Galaxy with X‐Ray‐Emitting Gas

Abstract

We present a Chandra study of the hot interstellar medium (ISM) in the giant elliptical galaxy NGC 4649. In common with other group-centered ellipticals, its temperature profile rises with radius in the outer parts of the galaxy, from ~0.7 keV at 2 kpc to ~0.9 keV by 20 kpc. However, within the central ~2 kpc the trend reverses, and the temperature peaks at ~1.1 keV within the innermost 200 pc. Under the assumption of hydrostatic equilibrium, we demonstrate that the central temperature spike arises due to the gravitational influence of a quiescent central supermassive black hole. We constrain the black hole mass (MBH) to (3.35+ 0.67−0.95) × 109 M☉ (90% confidence), in good agreement with stellar kinematics measurements. This is the first direct measurement of MBH based on studies of hydrostatic X-ray-emitting gas, which are sensitive to the most massive black holes, and is a crucial validation of both mass-determination techniques. This agreement clearly demonstrates that the gas must be close to hydrostatic, even in the very center of the galaxy, which is consistent with the lack of morphological disturbances in the X-ray image. NGC 4649 is now one of only a handful of galaxies for which MBH has been measured by more than one method. At larger radii, we were able to decompose the gravitating mass profile into stellar and dark matter (DM) components. Unless one accounts for the DM, a standard virial analysis of the stars dramatically overestimates the stellar mass of the galaxy. We find that the measured J-band stellar mass-to-light ratio, 1.37 ± 0.10 M☉ L−1☉, is in good agreement with simple stellar population model calculations for this object.