The NANOGrav 15-year Data Set: Constraints on Supermassive Black Hole Binaries from the Gravitational Wave Background (2306.16220v2)

Abstract: The NANOGrav 15-year data set shows evidence for the presence of a low-frequency gravitational-wave background (GWB). While many physical processes can source such low-frequency gravitational waves, here we analyze the signal as coming from a population of supermassive black hole (SMBH) binaries distributed throughout the Universe. We show that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency gravitational-wave spectrum. While multiple model variations are able to reproduce the GWB spectrum at our current measurement precision, our results highlight the importance of accurately modeling binary evolution for producing realistic GWB spectra. Additionally, while reasonable parameters are able to reproduce the 15-year observations, the implied GWB amplitude necessitates either a large number of parameters to be at the edges of expected values, or a small number of parameters to be notably different from standard expectations. While we are not yet able to definitively establish the origin of the inferred GWB signal, the consistency of the signal with astrophysical expectations offers a tantalizing prospect for confirming that SMBH binaries are able to form, reach sub-parsec separations, and eventually coalesce. As the significance grows over time, higher-order features of the GWB spectrum will definitively determine the nature of the GWB and allow for novel constraints on SMBH populations.

• The paper demonstrates that the 15-year NANOGrav data effectively constrains supermassive black hole binary properties through multifaceted astrophysical models.
• It employs both phenomenological and gravitational-wave–driven models to analyze environmental influences and intrinsic radiation on binary evolution.
• The findings reveal a potential spectral turnover at low frequencies, offering key insights into galaxy merger histories and the dynamics of black hole binaries.

The NANOGrav 15-Year Data Set and Constraints on Supermassive Black Hole Binaries

The paper presents an in-depth analysis of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 15-year data set to explore constraints on supermassive black hole binaries (SMBHBs) from the gravitational wave background (GWB). It provides a comprehensive investigation into astrophysically motivated models that could explain the detected low-frequency GWB signal, suggesting it could originate from a population of SMBHBs distributed throughout the universe.

Analysis of Gravitational Wave Background

The NANOGrav data set reveals a low-frequency GWB indicative of a common-spectrum red noise, which matches predictions for a stochastic GWB from many unresolved SMBHBs. Various models are tested to match the 15-year GWB spectrum with astrophysical expectations. Two primary approaches are used: a phenomenological binary evolution model and a purely gravitational-wave-driven evolution model. The models evaluate key parameters in galaxy merger rates, SMBH masses, and the timescales over which SMBHBs merge.

Phenomenological and Gravitational Wave-Only Models

The phenomenological binary evolution models incorporate environmental interactions to paper their effect on binary hardening timescales. The primary parameters varied are the lifetime of the binary and the SMBH mass distribution, including galaxy stellar-mass functions, pair fractions, and merger times. These factors significantly impact the shape and amplitude of the GWB spectrum, highlighting the influence of binaries' coupling to their galactic environment at sub-parsec separations.

In contrast, the gravitational-wave-driven models assume binary evolution is dominated by gravitational radiation, which simplifies calculations but may not capture the complexity of interactions in actual SMBH binaries.

Results and Implications for Astrophysics

Both models can produce spectra consistent with the observed GWB within current signal-to-noise limitations. Notably, the paper hints at a possible turnover or flattening in the spectral shape at low frequencies, attributed to environmentally driven binary evolution. However, the data remains compatible with a standard power-law model where GW emission predominantly drives binary evolution.

Predictive models suggest that high-mass SMBHBs are most influential in generating observable GW signals. The results emphasize the potential need for high galaxy number densities or rapid binary evolution to achieve the inferred GWB amplitudes. These findings are significant for understanding SMBH formation and evolution, probing galaxy merger histories, and providing targets for future multimessenger astronomical efforts.

Future Directions in Gravitational Wave Astronomy

The NANOGrav collaboration continues to refine its datasets, promising more precise measurements that could enhance constraints on models of SMBH binary populations. Improved sensitivity and longer baselines will aid in distinguishing between GW signals from SMBHBs and other potential sources or new physics models. Multimessenger approaches and collaboration with other PTAs, as well as future missions like LISA, may increase detection capabilities and complement astrophysical observations in unveiling the dynamic history of black holes and galaxies in the cosmos.

Continued cross-validation of astrophysical models with data will leverage GWs as a powerful probe into the high-energy processes governing SMBH dynamics and their roles in galaxy evolution, potentially informing both theoretical models and observational strategies across the broader astronomical community.