Odds Estimation for Supermassive Black Hole Binaries in Local Core Galaxies

Abstract

Supermassive black hole binaries (SMBHBs) are expected products of massive galaxy mergers, but identifying binaries in nearby galaxies remains challenging. We combine simulated massive-galaxy assembly histories from IllustrisTNG with a semi-analytic core-scouring model to connect observable core galaxy properties to the probability of hosting a bound SMBHB at z=0. We (i) construct a core galaxy population using an observationally motivated galaxy stellar mass function, (ii) model binary evolution through dynamical friction, stellar hardening, and gravitational-wave emission, and (iii) perform logistic regressions with five observable predictors: stellar mass deficit, stellar mass, central density, central velocity dispersion, and effective radius. The resulting models perform well on simulated systems, achieving a pseudo-$R^2$ of $0.51\pm0.03$, accuracy $0.95\pm0.01$, precision $0.92\pm0.03$, and recall $0.82\pm0.04$. Uncoalesced binaries are typically compact, with separations peaking near 0.3 pc, though only $\sim0.3\%$ emit in the pulsar timing array band. Applying the model to observed core galaxies, we identify eight nearby galaxies more likely than not to host SMBHBs. Of those eight host candidates, we note Messier 86 (NGC 4406) as a particularly likely example that was similarly identified using independent means and therefore warrants future follow-up.