Low-frequency gravitational waves from massive black hole binaries: Predictions for LISA and pulsar timing arrays

Abstract

The coalescence of massive black hole (BH) binaries due to galaxy mergers provides a primary source of low-frequency gravitational radiation detectable by pulsar timing measurements and by the proposed the Laser Interferometry Space Antenna (LISA) observatory. We compute the expected gravitational radiation signal from sources at all redshifts by combining the predicted merger rate of galactic halos with recent measurements of the relation between BH mass, MBH, and the velocity dispersion of its host galaxy, σ. Our main findings are as follows: (1) the nHz frequency background is dominated by BH binaries at redshifts z≲,2, and existing limits from pulsar timing data place tight constraints o..