We report the discovery of an extremely large ( R _ { b } \sim 2 \farcs 77 \approx 4.2 kpc ) core in the brightest cluster galaxy , IC 1101 , of the rich galaxy cluster Abell 2029 .
Luminous core-Sérsic galaxies contain depleted cores—with sizes ( R _ { b } ) typically 20 – 500 pc—that are thought to be formed by coalescing black hole binaries .
We fit a ( double nucleus ) + ( spheroid ) + ( intermediate-scale component ) + ( stellar halo ) model to the Hubble Space Telescope surface brightness profile of IC 1101 , finding the largest core size measured in any galaxy to date .
This core is an order of magnitude larger than those typically measured for core-Sérsic galaxies .
We find that the spheroid ’ s V -band absolute magnitude ( M _ { V } ) of -23.8 mag ( \sim 25 % of the total galaxy light , i.e. , including the stellar halo ) is faint for the large R _ { b } , such that the observed core is 1.02 dex \approx 3.4 \sigma _ { s } ( rms scatter ) larger than that estimated from the R _ { b } - M _ { V } relation .
The suspected scouring process has produced a large stellar mass deficit ( M _ { def } ) \sim 4.9 \times 10 ^ { 11 } M _ { \sun } , i.e. , a luminosity deficit \approx 28 % of the spheroid ’ s luminosity prior to the depletion .
Using IC 1101 ’ s black hole mass ( M _ { BH } ) estimated from the M _ { BH } - \sigma , M _ { BH } - L and M _ { BH } - M _ { * } relations , we measure an excessive and unrealistically high number of “ dry ” major mergers for IC 1101 ( i.e. , \mathcal { N } \ga 76 ) as traced by the large M _ { def } / M _ { BH } ratios of 38 - 101 .
The large core , high mass deficit and oversized M _ { def } / M _ { BH } ratio of IC 1101 suggest that the depleted core was scoured by overmassive SMBH binaries with a final coalesced mass M _ { BH } \sim ( 4 - 10 ) \times 10 ^ { 10 } M _ { \sun } , i.e. , \sim ( 1.7 – 3.2 ) \times \sigma _ { s } larger than the black hole masses estimated using the spheroid ’ s \sigma , L and M _ { * } .
The large core might be partly due to oscillatory core passages by a gravitational radiation-recoiled black hole .