The Effect of Mass Segregation on Gravitational Wave Sources near Massive Black Holes

Abstract

Gravitational waves (GWs) from the inspiral of compact remnants (CRs) into massive black holes (MBHs) will be observable to cosmological distances. While a CR spirals in, two-body scattering by field stars may cause it to fall into the central MBH before reaching a short-period orbit that would give an observable signal. As a result, only CRs very near (~0.01 pc) the MBH can spiral in successfully. In a multimass stellar population, the heaviest objects sink to the center, where they are more likely to slowly spiral into the MBH without being swallowed prematurely. We study how mass segregation modifies the stellar distribution and the rate of GW events. We find that the inspiral rate per galaxy is 30 Gyr-1 for white dwarfs, 6 Gyr-1 for neutron stars, and 250 Gyr-1 for 10 Msolar stellar black holes (SBHs). The high rate for SBHs is due to their extremely steep density profile, nBH(r)~r-2. The GW detection rate will be dominated by SBHs.