The Stellar Kinematic Signature of Massive Black Hole Binaries

TL;DR

This paper demonstrates that massive black hole binaries produce distinct stellar velocity signatures, such as counter-rotating regions and dispersion dips, which can be detected on scales larger than the binary separation, aiding their identification.

Contribution

The study introduces a method to identify massive black hole binaries through their unique stellar kinematic signatures in velocity distribution maps, even when the binary itself is unresolved.

Findings

Counter-rotating stellar torus signature outside BBH Hill sphere.

Dip in velocity dispersion maps near the BBH.

Enhanced higher-order velocity moments indicating BBH presence.

Abstract

The stalling radius of a merging massive binary black hole (BBH) is expected to be below 0".1 even in nearby galaxies (Yu 2002), and thus BBHs are not expected to be spatially resolved in the near future. However, as we show below, a BBH may be detectable through the significantly anisotropic stellar velocity distribution it produces on scales 5-10 times larger than the binary separation. We calculate the velocity distribution of stable orbits near a BBH by solving the restricted three body problem for a BBH embedded in a bulge potential. We present high resolution maps of the projected velocity distribution moments, based on snapshots of ~ 10^8 stable orbits. The kinematic signature of a BBH in the average velocity maps is a counter rotating torus of stars outside the BBH Hill spheres. The velocity dispersion maps reveal a dip in the inner region, and an excess of 20-40% further out, compared to a single BH of the same total mass. More pronounced signatures are seen in the third and fourth Gauss-Hermite velocity moments maps. The detection of these signatures may indicate the presence of a BBH currently, or at some earlier time, which depends on the rate of velocity phase space mixing following the BBH merger.