Constraints on Black Hole/Host Galaxy Co-evolution and Binary Stalling Using Pulsar Timing Arrays

TL;DR

This paper uses pulsar timing array data to constrain galaxy evolution parameters, especially the black hole-bulge mass relation, and suggests a possible binary black hole stalling time of 1-2 Gyr to reconcile observations.

Contribution

It provides a framework linking PTA limits to galaxy evolution parameters and explores implications for black hole binary evolution models.

Findings

The black hole-bulge mass relation significantly impacts gravitational wave strain predictions.

Current PTA limits challenge existing measurements of the black hole-bulge relation.

A binary black hole stalling time of 1-2 Gyr can reconcile observations with models.

Abstract

Pulsar timing arrays are now setting increasingly tight limits on the gravitational wave background from binary supermassive black holes. But as upper limits grow more constraining, what can be implied about galaxy evolution? We investigate which astrophysical parameters have the largest impact on strain spectrum predictions and provide a simple framework to directly translate between measured values for the parameters of galaxy evolution and PTA limits on the gravitational wave background of binary supermassive black holes. We find that the most influential observable is the relation between a host galaxy's central bulge and its central black hole, $\mbox{$M_{\bullet}$-$M_{\rm bulge}$}$, which has the largest effect on the mean value of the characteristic strain amplitude. However, the variance of each prediction is dominated by uncertainties in the galaxy stellar mass function. Using this framework with the best published PTA limit, we can set limits on the shape and scatter of the $\mbox{$M_{\bullet}$-$M_{\rm bulge}$}$ relation. We find our limits to be in contention with strain predictions using two leading measurements of this relation. We investigate several possible reasons for this disagreement. If we take the $\mbox{$M_{\bullet}$-$M_{\rm bulge}$}$ relations to be correct within a simple power-law model for the gravitational wave background, then the inconsistency is reconcilable by allowing for an additional "stalling" time between a galaxy merger and evolution of a binary supermassive black hole to sub-parsec scales, with lower limits on this timescale of $\sim 1-2$ Gyr.