Massive Black Hole Binary Mergers in Dynamical Galactic Environments

TL;DR

This paper models the gravitational wave background from massive black hole binaries using comprehensive environmental mechanisms, predicting amplitudes below current PTA detection limits and providing insights into binary evolution and galaxy properties.

Contribution

It presents the first calculation of the GWB including all key environmental mechanisms affecting MBH binaries, offering more realistic amplitude predictions.

Findings

GWB amplitudes are below current PTA limits.

Predicted GWB amplitude at 1/year is about 0.4×10^{-15}.

GWB spectral index is flatter than -2/3 due to environmental effects.

Abstract

Gravitational Waves (GW) have now been detected from stellar-mass black hole binaries, and the first observations of GW from Massive Black Hole (MBH) Binaries are expected within the next decade. Pulsar Timing Arrays (PTA), which can measure the years long periods of GW from MBHB, have excluded many standard predictions for the amplitude of a stochastic GW Background (GWB). We use coevolved populations of MBH and galaxies from hydrodynamic, cosmological simulations ('Illustris') to calculate a predicted GWB. The most advanced predictions so far have included binary hardening mechanisms from individual environmental processes. We present the first calculation including all of the environmental mechanisms expected to be involved: dynamical friction, stellar 'loss-cone' scattering, and viscous drag from a circumbinary disk. We find that MBH binary lifetimes are generally multiple gigayears, and only a fraction coalesce by redshift zero. For a variety of parameters, we find all GWB amplitudes to be below the most stringent PTA upper limit of $A_{\textrm{yr}^{-1}} \approx 10^{-15}$. Our fairly conservative fiducial model predicts an amplitude of $A_{\textrm{yr}^{-1}} \approx 0.4\times 10^{-15}$---less than a factor of three below the current limit. At lower frequencies, we find $A_{0.1\,\textrm{yr}^{-1}} \approx 1.5\times 10^{-15}$ with spectral indices between $-0.4$ and $-0.6$---significantly flatter than the canonical value of $-2/3$ due to purely GW-driven evolution. Typical MBHB driving the GWB signal come from redshifts around $0.3$, with total masses of a few times $10^9\,M_\odot$, and in host galaxies with very large stellar masses. Even without GWB detections, our results can be connected to observations of dual AGN to constrain binary evolution.