Massive black hole binary systems and the NANOGrav 12.5 year results

Abstract

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has recently reported evidence for the presence of a common stochastic signal across their array of pulsars. The origin of this signal is still unclear. One of the possibilities is that it is due to a stochastic gravitational wave background (SGWB) in the $\sim 1-10\mathrm{nHz}$ frequency region. Taking the NANOGrav observational result at face value, we show that this signal would be fully consistent with a SGWB produced by an unresolved population of in-spiralling massive black hole binaries (MBHBs) predicted by current theoretical models. Considering an astrophysically agnostic model we find that the MBHB merger rate is loosely constrained to the range $10^{-11}-2$ ${\mathrm{Mpc}}^{-3}{\mathrm{Gyr}}^{-1}$. Including additional constraints from galaxy pairing fractions and MBH-bulge scaling relations, we find that the MBHB merger rate is $10^{-5}-5\times 10^{-4}$ ${\mathrm{Mpc}}^{-3}{\mathrm{Gyr}}^{-1}$, the MBHB merger time-scale is $\le 3\mathrm{Gyr}$ and the norm of the $M_{\mathrm{BH}}-M_{\mathrm{bulge}}$ relation $\ge 1.2\times 10^8{M}_{\odot }$ (all intervals quoted at 90\% confidence). Regardless of the astrophysical details of MBHB assembly, this result would imply that a sufficiently large population of massive black holes pair up, form binaries and merge within a Hubble time.

6 pages, 4 figures, to be submitted

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