A Shining Death of Unequal Supermassive Black Hole Binaries

In the ΛCDM scenario, small galaxies merge to produce larger entities. Since supermassive black holes (SMBHs) are found in galaxies of all sizes, SMBH binaries (SMBHBs) are generally expected to form during the amalgamation of galaxies. It is unclear what fraction of these binaries could eventually merge, but a general consensus is that initially the orbital decay is mediated by the surrounding gas and stars. In this Letter, we show that in active galactic nuclei (AGNs) the radiation field also causes the orbits of the accreting SMBHs to shrink. The corresponding mechanism, known as the "Poynting-Robertson drag" (PR drag), takes effect on a well-defined timescale CT_Sal, where T_Sal is the Salpeter timescale of the AGN, presumably coinciding with the primary SMBH, and $C=4{\XI }^{-1}{\epsilon }^{-1}{q}^{1/3}{(1+q)}^{2/3}(1-\epsilon )$ is a constant determined by the radiative efficiency ɛ, the mass ratio q of the two black holes, and a parameter ξ characterizing the size of the circumsecondary accretion disk. We find that when q ≲ a few × 10^-5, the PR drag is more efficient in shrinking the binary than many other mechanisms, such as dynamical friction and type-I migration. Our finding points to a possible new channel for the coalescence of unequal SMBHBs and the clearing of intermediate-massive black holes in AGNs.