A candidate SMBBH system in the brightest cluster galaxy of RBS 797 (Gitti et al.)

Vol. 557
In section 1. Letter to the Editor

A candidate supermassive binary black hole system in the brightest cluster galaxy of RBS 797

In the hierarchical view of the Universe, galaxies merge to form bigger objects. Since a supermassive black hole (SMBH) exists in each nucleus, we would expect to find binary SMBH very often. However, evidence of their existence is rare. The authors have discovered a possible binary SMBH in the cool-core galaxy cluster RBS 797 at z=0.35. With the VLA, this radio source was already known to have ~ kpc-scale jets that are almost orthogonal to the radio emission which extends for tens of kpc. They have performed new high-resolution observations with the European VLBI Network (EVN) reaching an angular resolution of ~ 9 x 5 mas^2, and detected two compact radio sources with a projected separation of ~ 77 pc. Together with the two pairs of radio jets, misaligned by ~ 90 degrees on the same ~ kpc, this suggests the existence of a binary SMBH, which will be confirmed by future VLBI experiments making. If confirmed, RBS 797 would be the first supermassive binary black hole system observed at medium-high redshift at VLBI resolution.