Supermassive black hole binary systems form in galaxy mergers and reside in galactic nuclei with large and poorly constrained concentrations of gas and stars. These systems emit nanohertz gravitational waves that will be detectable by pulsar timing arrays. Here we estimate the properties of the local nanohertz gravitational-wave landscape that includes individual supermassive black hole binaries emitting continuous gravitational waves and the gravitational-wave background that they generate. Using the 2 Micron All-Sky Survey, together with galaxy merger rates from the Illustris simulation project, we find that there are on average 91 ± 7 continuous nanohertz gravitational-wave sources, and 7 ± 2 binaries that will never merge, within 225 Mpc. These local unresolved gravitational-wave sources can generate a departure from an isotropic gravitational-wave background at a level of about 20 per cent, and if the cosmic gravitational-wave background can be successfully isolated, gravitational waves from at least one local supermassive black hole binary could be detected in 10 years with pulsar timing arrays.
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We thank S. Babak, J. Verbiest, D. Kaplan, E. Barr, K. Górski and E. Sheldon for discussions. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation (NSF). C.M.F.M. was supported by a Marie Curie International Outgoing Fellowship within the European Union Seventh Framework Programme. S.R.T was partly supported by appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities and the Universities Space Research Association through a contract with NASA. A.S. is supported by a University Research Fellowship of the Royal Society. Parts of these computations were performed on the Zwicky cluster at Caltech, which is supported by the Sherman Fairchild Foundation and NSF award PHY-0960291. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This work has also been supported by NSF award 1458952, NSF AST-1411945 and 1411642. The NANOGrav project receives support from NSF Physics Frontier Center award number 1430284. The Flatiron Institute is supported by the Simons Foundation.
The authors declare no competing financial interests.
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Mingarelli, C.M.F., Lazio, T.J.W., Sesana, A. et al. The local nanohertz gravitational-wave landscape from supermassive black hole binaries. Nat Astron 1, 886–892 (2017). https://doi.org/10.1038/s41550-017-0299-6
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