A Direct Test of Galaxy Core Formation in BCG2261

We request 8 orbits for STIS spectroscopic observations of three compact knots near the center of the brightest cluster galaxy in Abell 2261 (BCG2261) to directly test for the presence of a recoiling supermassive black hole (SMBH). BCG2261’s exceptionally large, flat core is thought to have been formed by scouring from a binary SMBH inspiral, with additional broadening effects caused by a recoiling SMBH after the binary’s coalescence. A radio jet in the system indicates recent or current active nucleus activity, and appears to point to an origin in one of the off-center stellar concentrations (knots). All of the knots are candidates for a “cloaked” SMBH. Theory predicts that an ejected SMBH should carry a cloak of tightly bound stars with it. The stellar cloak would resemble a globular cluster or dwarf galaxy, but would have a high velocity dispersion. The presence of the off-center knots, the galactic core morphology, and the radio jet tentatively imply that one knot is likely to be cloaking a recoiling SMBH. Our proposed HST/STIS observations will test of the presence of a SMBH in the knots by seeking high-dispersion absorption lines and evidence for active nucleus emission. If any such signature is found, BCG2261 will represent the first direct observational support for three preeminent theoretical speculations: that scouring forms cores, that SMBHs may recoil after coalescence, and that recoil can strongly influence core formation and morphology.