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Where are NANOGrav's big black holes? (2312.06756v1)

Published 11 Dec 2023 in astro-ph.CO, astro-ph.HE, and gr-qc

Abstract: Multiple pulsar timing array (PTA) collaborations have recently reported the first detection of gravitational waves (GWs) of nanohertz frequencies. The signal is expected to be primarily sourced by inspiralling supermassive black hole binaries (SMBHBs) and these first results are broadly consistent with the expected GW spectrum from such a population. Curiously, the measured amplitude of the GW background in all announced results is a bit larger than theoretical predictions. In this work, we show that the amplitude of the stochastic gravitational wave background (SGWB) predicted from the present-day abundance of SMBHs derived from local scaling relations is significantly smaller than that measured by the PTAs. We demonstrate that this difference cannot be accounted for through changes in the merger history of SMBHs and that there is an upper limit to the boost to the characteristic strain from multiple merger events, due to the fact that they involve black holes of decreasing masses. If we require the current estimate of the black hole mass density -- equal to the integrated quasar luminosity function through the classic Soltan argument -- to be preserved, then the currently measured PTA result would imply that the typical total mass of SMBHs contributing to the background should be at least $\sim 3 \times 10{10} M_\odot$, a factor of $\sim 10$ larger than previously predicted. The required space density of such massive black holes corresponds to order $10$ $3 \times 10{10} M_\odot$ SMBHs within the volume accessible by stellar and gas dynamical SMBH measurements. By virtue of the GW signal being dominated by the massive end of the SMBH distribution, PTA measurements offer a unique window into such rare objects and complement existing electromagnetic observations.

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Citations (6)
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Summary

  • The paper finds that gravitational wave amplitudes from pulsar timing arrays exceed model predictions, implying supermassive black holes are at least ten times more massive than expected.
  • It employs robust mathematical modeling to contrast predicted amplitudes from established SMBH scaling relations with observed nanohertz-frequency signals.
  • The study suggests that revising the SMBH mass function is essential, as current models may overlook a population of extremely massive black holes.

Analyzing the Constraints from NANOGrav on Supermassive Black Holes

The paper under review focuses on the analysis of gravitational wave (GW) data from pulsar timing arrays (PTAs), specifically addressing discrepancies related to the black hole masses derived from such data. The paper critiques the identification of a stochastic GW background (SGWB) predominantly originating from supermassive black hole binaries (SMBHBs). The authors, Gabriela Sato-Polito, Matias Zaldarriaga, and Eliot Quataert, observe that the amplitude of the nanohertz-frequency GW signal detected by PTAs, including the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), is significantly higher than theoretical expectations based on the observed Supermassive Black Hole (SMBH) population.

The methodology employed by the authors relies on contrasting the predicted GW amplitude, based on existing local-galactic SMBH scaling relations, with the reported values from multiple PTA collaborations. These discrepancies suggest that existing models may underestimate the abundance or mass distribution of SMBHs. According to the authors, the GW signals primarily arise from SMBHs with masses far exceeding what current models predict. Specifically, the paper deduces that the SMBH mass contributing to the SGWB should be at least ten times larger than previous estimates, requiring typical contributing masses to be around 3×1010M⊙3 \times 10^{10} M_\odot.

In their analysis, the authors assess whether this inconsistency could be resolved by assuming a different merger history for SMBHs. They explore potential enhancements in characteristic strain due to repeated SMBH mergers but illustrate that such scenarios are insufficient to explain the observed results. Consequently, the authors suggest that the characteristic masses might be underpredicted, potentially due to an underestimated high-mass end of the SMBH mass function. They reason that PTA measurements offer a unique observational window into the population of such massive SMBHs, complementary to electromagnetic observations, which might overlook certain high-mass ranges due to rarity and opacity at requisite wavelengths.

The paper's core contribution lies in its quantitative analysis of the GW amplitudes' mismatch, offering an original interpretation of the PTA measurements. The authors deploy robust mathematical modeling to demonstrate that allowed modifications to the SMBH mass function are commensurate with maintaining established mass density constraints derived from the Soltan argument. However, these modifications need invoke an increase in the mass estimates of the most massive SMBHs, suggesting the existence of a previously unobserved population of extremely massive black holes. Proposals such as increasing scatter in the M−σM-\sigma relation or adopting a steeper high-velocity dispersion galaxy slope are examined. They conclude that constraints from direct kinematic measurements of nearby SMBHs do not corroborate such proposed changes.

The implications of this research connect GW observations with cut-edge quasar and galactic investigations, presenting critical questions about the high-mass tail of the SMBH distribution. Foremost, it engenders new hypotheses about the cosmic history and behavior of SMBHs, questioning factors like merger rates and their correspondence with observed galaxy properties. Future telescopic advancements and observational campaigns geared towards SMBHs could corroborate or refute these inferred mass extents, refining the theoretical models in line with GW signal data.

In conclusion, this paper underscores a vital intersection of gravitational wave astronomy and astrophysical SMBH research, challenging established conventions and prompting further investigative thrust into the behavioral patterns of the Universe's most massive black holes. Such analytical convergence propounds a deeper cosmological understanding of black hole evolution and their consequential influence on the Universe's large-scale structure.

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