Astronomy and Astrophysics – Astrophysics
Scientific paper
2008-07-29
Astrophys.J.696:1798-1822,2009
Astronomy and Astrophysics
Astrophysics
60 preprint pages with 15 figures, ApJ accepted version
Scientific paper
10.1088/0004-637X/696/2/1798
The supermassive black holes (SMBHs) massive enough to power the bright redshift ~6 quasars observed in the Sloan Digital Sky Survey (SDSS) are thought to have assembled by mergers and/or gas accretion from less massive "seed" BHs. If the seeds are the ~100 Msol remnant BHs from the first generation of stars, they must be in place well before redshift z=6, and must avoid being ejected from their parent proto-galaxies by the large (few hundred km/s) kicks they suffer from gravitational-radiation induced recoil during mergers with other BHs. We simulate the SMBH mass function at redshift z=6 using dark matter (DM) halo merger trees, coupled with a prescription for the halo occupation fraction, accretion histories, and radial recoil trajectories of the growing BHs. Our purpose is (i) to map out plausible scenarios for successful assembly of the z~6 quasar BHs by exploring a wide region of parameter space, and (ii) to predict the rate of low-frequency gravitational wave events detectable by the Laser Interferometer Space Antenna (LISA) for each such scenario. Our main findings are as follows: (1) ~100 Msol seed BHs can grow into the SDSS quasar BHs without super--Eddington accretion, but only if they form in minihalos by z~30 and subsequently accrete ~60% of the time; (2) the scenarios with optimistic assumptions required to explain the SDSS quasar BHs overproduce the mass density in lower--mass (10^5 to 10^7 Msol) SMBHs by a factor of 100-1000, unless seeds stop forming, or accrete at a severely diminished rates or duty cycles (e.g. due to feedback), at z < 20-30. We also present several successful assembly models and their LISA detection rates, including a "maximal" model that gives the highest rate (~30/yr at z~6) without overproducing the total SMBH density.
Haiman Zoltan
Tanaka Takamitsu
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