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Abstract The nanohertz gravitational wave background (GWB) is believed to be dominated by GW emission from supermassive black hole binaries (SMBHBs). Observations of several dual-active galactic nuclei (AGN) strongly suggest a link between AGN and SMBHBs, given that these dual-AGN systems will eventually form bound binary pairs. Here we develop an exploratory SMBHB population model based on empirically constrained quasar populations, allowing us to decompose the GWB amplitude into an underlying distribution of SMBH masses, SMBHB number density, and volume enclosing the GWB. Our approach also allows us to self-consistently predict the number of local SMBHB systems from the GWB amplitude. Interestingly, we find the local number density of SMBHBs implied by the common-process signal in the NANOGrav 12.5-yr data set to be roughly five times larger than previously predicted by other models. We also find that at most ∼25% of SMBHBs can be associated with quasars. Furthermore, our quasar-based approach predicts ≳95% of the GWB signal comes fromz≲ 2.5, and that SMBHBs contributing to the GWB have masses ≳108M⊙. We also explore how different empirical galaxy–black hole scaling relations affect the local number density of GW sources, and find that relations predicting more massive black holes decrease the local number density of SMBHBs. Overall, our results point to the important role that a measurement of the GWB will play in directly constraining the cosmic population of SMBHBs, as well as their connections to quasars and galaxy mergers.
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This content will become publicly available on July 2, 2026
Quasars Can Signpost Supermassive Black Hole Binaries
Abstract Supermassive black holes (SMBHs) are found in the centers of massive galaxies, and galaxy mergers should eventually lead to SMBH mergers. Quasar activity has long been associated with galaxy mergers, so here we investigate if supermassive black hole binaries (SMBHBs) are preferentially found in quasars. Our multimessenger investigation folds together a gravitational-wave background signal from NANOGrav, a sample of periodic active galactic nucleus candidates from the Catalina Real-Time Transient Survey, and a quasar mass function, to estimate an upper limit on the fraction of quasars which could host an SMBHB. We find at 95% confidence that quasars are at most 5 times as likely to host an SMBHB as a random galaxy. Pulsar timing arrays may therefore be more likely to find SMBHBs by prioritizing quasars over a random selection of galaxies in their searches.
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- Award ID(s):
- 2414468
- PAR ID:
- 10656399
- Publisher / Repository:
- The Astrophysical Journal
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 987
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 106
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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