ABSTRACT Massive black hole (MBH) binary inspiral time-scales are uncertain, and their spins are even more poorly constrained. Spin misalignment introduces asymmetry in the gravitational radiation, which imparts a recoil kick to the merged MBH. Understanding how MBH binary spins evolve is crucial for determining their recoil velocities, their gravitational wave (GW) waveforms detectable with Laser Interferometer Space Antenna, and their retention rate in galaxies. Here, we introduce a sub-resolution model for gas- and gravitational wave (GW)-driven MBH binary spin evolution using accreting MBHs from the Illustris cosmological hydrodynamic simulations. We also model binary inspiral via dynamical friction, stellar scattering, viscous gas drag, and GW emission. Our model assumes that the circumbinary disc always removes angular momentum from the binary. It also assumes differential accretion, which causes greater alignment of the secondary MBH spin in unequal-mass mergers. We find that 47 per cent of the MBHs in our population merge by z = 0. Of these, 19 per cent have misaligned primaries and 10 per cent have misaligned secondaries at the time of merger in our fiducial model with initial eccentricity of 0.6 and accretion rates from Illustris. The MBH misalignment fraction depends strongly on the accretion disc parameters, however. Reducing accretion rates by a factor ofmore »
An estimate of the stochastic gravitational wave background from the MassiveBlackII simulation
A population of supermassive black hole (SMBH) binaries is expected to generate a stochastic gravitational wave background (SGWB) in the pulsar timing array (PTA) frequency range of 10−9 to $10^{-7}\, {\rm Hz}$. Detection of this signal is a current observational goal and so predictions of its characteristics are of significant interest. In this work, we use SMBH binary mergers from the MassiveBlackII simulation to estimate the characteristic strain of the stochastic background. We examine both a gravitational wave (GW) driven model of binary evolution and a model which also includes the effects of stellar scattering and a circumbinary gas disc. Results are consistent with PTA upper limits and similar to estimates in the literature. The characteristic strain at a reference frequency of $1\, {\rm yr}^{-1}$ is found to be $A_{\rm {yr}^{-1}}= 6.9 \times 10^{-16}$ and $A_{\rm {yr}^{-1}}= 6.4 \times 10^{-16}$ in the GW-driven and stellar scattering/gas disc cases, respectively. Using the latter approach, our models show that the SGWB is mildly suppressed compared to the purely GW-driven model as frequency decreases inside the PTA frequency band.
- Publication Date:
- NSF-PAR ID:
- 10363605
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 511
- Issue:
- 4
- Page Range or eLocation-ID:
- p. 5241-5250
- ISSN:
- 0035-8711
- Publisher:
- Oxford University Press
- Sponsoring Org:
- National Science Foundation
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