Mass function of binary massive black holes in Active Galactic Nuclei
by Hayasaki, Kimitake and Ueda, Yoshihiro and Isobe, Naoki
9 pages, 11 figures, submitted to Publications of the Astronomical Society of Japan
If the activity of active galactic nuclei (AGNs) is predominantly induced by major galaxy mergers, then a significant fraction of AGNs should harbor binary massive black holes in their centers. We study the mass function of binary massive black holes in nearby AGNs based on the theory of evolution of binary massive black holes interacting with ambient gaseous disks proposed by Hayasaki (2009). The timescale of orbital decay is estimated as the order of $latex 10^8 yr$, being independent of the black hole mass but only dependent on the mass ratio and Eddington ratio. This makes it possible for any binary massive black holes to merge within a Hubble time. We find that $latex 1.3%–1.7%$ of the total number of nearby AGNs have close, binary massive black-holes with orbital period less than ten-years, detectable with on-going highly sensitive X-ray monitors such as Monitor of All-sky X-ray Image and/or Swift/Burst Alert Telescope. Close binaries with total black-hole masses of $latex 10^{6.5-7}M_sun$ are the most frequent in massive binary black-hole populations of nearby AGNs.