Massive black hole binary systems and the NANOGrav 12.5 yr results

ABSTRACT The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) recently reported evidence the presence of a common stochastic signal across their array pulsars. origin this is still unclear. One possibility that it due to gravitational-wave background (SGWB) in ?1–10 nHz frequency region. Taking NANOGrav observational result at face value, we show would be fully consistent with an SGWB produced by unresolved population in-spiralling massive black hole binaries (MBHBs) predicted current theoretical models. Considering astrophysically agnostic model, MBHB merger rate loosely constrained. Including additional constraints from galaxy pairing fraction and MBH–bulge scaling relations, find ${1.2\times 10^{-5}}{\rm -}{4.5\times 10^{-4}}\, \mathrm{Mpc}^{-3}\, \mathrm{Gyr}^{-1}$ , time-scale $\le 2.7\, \mathrm{Gyr}$, norm MBH?Mbulge relation $\ge 1.2\times 10^{8}\, {\rm M}_\odot$ (all quoted 90 per cent credible intervals). Regardless astrophysical details assembly, imply sufficiently large holes pair up, form merge within Hubble time.