A Radio Relic and a Search for the Central Black Hole in the Abell 2261 Brightest Cluster Galaxy

TL;DR

This study investigates the core structure of Abell 2261's brightest galaxy to determine if its large flat core results from black hole ejection, using radio and optical observations to search for signs of the central supermassive black hole.

Contribution

The paper provides new high-resolution radio and optical data to test the black hole ejection hypothesis in a galaxy with an unusually large core, exploring the presence and location of the supermassive black hole.

Findings

No active radio core detected at the galaxy's center.

Spectra suggest some knots are infalling galaxies or stripped cluster members.

The black hole ejection hypothesis remains plausible but unconfirmed.

Abstract

We present VLA images and HST/STIS spectra of sources within the center of the brightest cluster galaxy (BCG) in Abell 2261. These observations were obtained to test the hypothesis that its extremely large, flat core reflects the ejection of its supermassive black hole. Spectra of three of the four most luminous "knots" embedded in the core were taken to test whether one may represent stars bound to a displaced massive black hole. The three knots have radial velocity offsets dV < ~150 km/s from the BCG. Knots 2 and 3 show kinematics, colors, and stellar masses consistent with infalling low-mass galaxies or larger stripped cluster members. Large errors in the stellar velocity dispersion of Knot 1, however, mean that we cannot rule out the hypothesis that it hosts a high-mass black hole. A2261-BCG has a compact, relic radio-source offset by 6.5 kpc (projected) from the optical core's center, but no active radio core that would pinpoint the galaxy's central black hole to a tight 10 GHz flux limit <3.6 uJy. Its spectrum and morphology are suggestive of an active galactic nucleus that switched off >48 Myr ago, with an equipartition condition magnetic field of 15 uG. These observations are still consistent with the hypothesis that the nuclear black hole has been ejected from its core, but the critical task of locating the supermassive black hole or demonstrating that A2261-BCG lacks one remains to be done.