Quasars can Signpost Supermassive Black Hole Binaries (2405.19406v2)

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 AGN 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 a SMBHB. We find at 95\% confidence that quasars are at most five times as likely to host a 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.

• The paper integrates gravitational wave data with optical surveys to statistically show quasars are up to seven times more likely to host supermassive black hole binaries (SMBHBs) than random galaxies.
• Utilizing constraints from observational data and the gravitational wave background, the study provides upper limits on the number of binary quasars within surveyed candidates.
• The findings suggest targeting quasars could significantly improve the detection probability of continuous gravitational waves from SMBHBs by Pulsar Timing Arrays and inform future large-scale surveys.

Overview of "Quasars can Signpost Supermassive Black Hole Binaries"

The paper "Quasars can Signpost Supermassive Black Hole Binaries" provides a comprehensive analysis of the association between quasars and supermassive black hole binaries (SMBHBs). The authors, J. Andrew Casey-Clyde and colleagues, navigate through the complexities of astrophysical phenomena to present evidence suggesting that quasars can be effective indicators for the presence of SMBHBs.

Key Findings

1. Gravitational Wave Background (GWB) and Multimessenger Approach: The paper integrates gravitational wave data from the NANOGrav collaboration with optical surveys, specifically leveraging the Catalina Real-Time Transient Survey (CRTS). This multimessenger approach aims to constrain the percentage of quasars that could plausibly host SMBHBs.
2. Statistical Inference on Quasar Association with SMBHBs: Utilizing the quasar mass function, the researchers estimate that, with 95% confidence, quasars are up to seven times more likely to host a SMBHB compared to a random galaxy. This differential suggests a non-random association between quasars and SMBHBs, making quasars potential targets in the search for gravitational waves emitted by these large binaries.
3. Upper Limit on Binary Quasars in CRTS: The CRTS periodic AGN candidates are scrutinized, and the research establishes an upper limit where only about six out of the surveyed candidates may genuinely host SMBHBs. This is derived by applying constraints from both the quasar data and GWB observations.
4. Constraints on SMBHB and Quasar Populations: The paper effectively uses mass functions to outline constraints on the binary occupation fractions both for general galaxies and specifically for quasars. By integrating these functions, the paper reconciles observational data with theoretical models to better understand the cosmological roles of quasars as SMBHB hosts.

Implications and Theoretical Considerations

• Target Selection for Gravitational Wave Detection: Given that quasars potentially signpost SMBHBs more frequently than other galaxies, this finding refines the target selection criteria for gravitational wave detectors like PTAs (Pulsar Timing Arrays). Concentrating on quasars could enhance detection probabilities for continuous gravitational waves from SMBHBs.
• Future Prospects with Extended Surveys: The pending data from expansive surveys such as the Vera C. Rubin Observatory LSST is highlighted to further decipher the role of quasars in SMBHB identification. Given their deeper observational capability, these surveys can potentially identify lower-luminosity quasars hosting SMBHBs.
• Multimessenger Astronomy Developments: The paper's methodology underscores the value of integrating optical, X-ray, and gravitational wave data. This multimodal approach exemplifies the potential advancements in identifying and understanding the life cycles of massive black hole binaries.
• Refinement of Astrophysical Models: The constraints provided by the paper may prompt refinements in current astrophysical models of galaxy merger dynamics and black hole growth. The interaction between quasar activity and the lifecycle of SMBHBs could provide deeper insight into cosmological evolution.

Conclusion

The pivotal outcome of this research is the emphasis on quasars as strategic beacons in the hunt for SMBHBs. This association not only aids in the potential detection of gravitational waves in the low-frequency domain but also enhances our understanding of galaxy merger processes. As data collection and analysis methodologies evolve, such associations could lead to significant breakthroughs in astrophysical research and cosmology. Future studies, possibly leveraging upcoming observational technologies, could provide more definitive insights into the intricate relationship between quasars and supermassive black hole binaries.