Highlighting Recent Work by CJ Harris: Tackling a Very Hardening Problem

A professional headshot of CJ Harris, a person with dark curly hair and glasses, wearing a grey button-down shirt and smiling in front of a bright blue background.
CJ Harris, astrophysicist

I am thrilled to highlight the exceptional work of CJ Harris, who has just submitted a significant new paper to AAS Journals. CJ is currently a PhD candidate at the University of Michigan and is officially on the postdoc market (see their CV). Their latest research provides a critical analysis of how stellar scattering is likely to be insufficient on its own to explain how supermassive black hole binaries harden.

Breaking the “Final Parsec” Bottleneck

Although we are now reasonably certain that supermassive black holes form binaries that harden sufficiently to coalesce through gravitational wave emission, the exact solution to the “Final Parsec Problem” is unknown to us, if not to nature.

In their new paper, “Core Scouring Dynamics and Gravitational Wave Consequences: Constraints on Supermassive Black Hole Binary Hardening,” CJ and collaborators (Laura Blecha and I) use a multi-messenger approach to tackle this. By making use of merger trees from the IllustrisTNG cosmological simulations with physically motivated models of core scouring, they were able to see how efficient stellar scattering is in hardening binaries. They were able to reproduce the relationship between mass deficit and galaxy total stellar mass, but only if scattering is more efficient than previously expected. But they were not able to reproduce the nanohertz gravitational wave background with the same efficiency. Meaning that there must be some other source of hardening, most likely gas hardening. If gas hardening plays a dominant role, then we can be far more optimistic about finding binary AGN.

Scientific scatter plot on logarithmic scales. The x-axis represents galaxy log stellar mass ranging from 10 to 13.5 in solar units. The y-axis represents log stellar mass deficit ranging from 6 to 11 in solar units. The plot features numerous colored data points with error bars: green diamonds (Brightest Cluster Galaxies), blue circles (Ellipticals), purple triangles (Lenticulars), a maroon square (Early Type Spirals), and a single yellow circle highlighting M87. These observational points sit atop a underlying density of small black dots and gray contour lines labeled 'TNG Cores,' showing that the simulated merger models closely track the observed trend of increasing mass deficit with increasing galaxy mass.
The figure shows stellar mass deficit as a function of stellar mass for both the observed set of core galaxies and a sample generated using our core scouring model with Ĥ = 1.64. Synthetic data-points are shown in black with the corresponding KDE and are in agreement with the observed data. We reproduce the slope and amplitude of the mass deficit–stellar mass relation, and recover some of the flattening seen in the BCG population at the high mass end. (Fig. 7 from Harris et al. 2026)

Key takeaways from the work:

  • Efficiency Gap: To match observations of real galaxies, the process of binary hardening via stellar scattering must be 1.6x faster than previous N-body experiments suggested.
  • The Role of Gas: Most importantly, the paper demonstrates that even the most efficient stellar scattering cannot fully explain the low-frequency turnover in the gravitational wave background.
A double-logarithmic line plot showing gravitational wave strain versus frequency. Three colored curves with shaded confidence intervals are compared against vertical gray violin plots of observational data. The curves show varying degrees of "turnover" at lower frequencies.
Shown on the y-axis is the characteristic strain amplitude of the GWB, with frequency in Hz on the x-axis. The gray violins show the GW spectrum derived from the NANOGrav HD-w/MP+DP+CURN models (G. Agazie et al. 2023a; W. G. Lamb et al. 2023). The colored curves are GW spectra resulting from SBHB populations generated by the holodeck semi-analytic modeling submodule (SAMs) over 1000 realizations. Shown in green is the spectrum expected if the hardening mechanism that drove all binaries to coalescence was GW emission alone. The orange spectrum is the G. Agazie et al. (2023b) fit to the GWB data. The pink hybrid model is comprised of the phenomenological model where the parameters in equation (24) are set by the results of our model. Shaded regions indicate the 68% confidence intervals. The phenomenological and hybrid models are in agreement in the high frequency regime, but diverge at low frequencies. The discrepancy implies that the attenuation of the GWB at low frequencies is not caused by energy being transferred into the environment through loss-cone scattering alone. Because the attenuation occurs at such low binary separations the effect is most likely caused by gas dynamics.

The “Bright” Side: This strongly suggests that gas dynamics must play a dominant role in bringing these giants together—providing a reason to be optimistic about finding electromagnetically visible binary AGN.

Community Leadership & Presence

Beyond the data, CJ continues to be a leader in the broader physics community. This past November, they presented these findings at the National Society of Black Physicists (NSBP) 2025 Annual Meeting. Their talk, “Multimessenger Constraints on the Supermassive Black Hole Binary Hardening Timescale,” was a deep dive into these results and showcased their ability to bridge complex theory with observational reality.