A Dynamical Test for Cooling-Induced Entrainment in a Runaway Supermassive Black Hole Tail

TL;DR

This paper uses observations and simulations of a runaway supermassive black hole's tail to test the physics of radiative turbulent mixing layers in hot astrophysical flows.

Contribution

It provides the first quantitative dynamical test of radiative mixing-layer theory using a real astrophysical system, RBH-1.

Findings

Observed tail deceleration matches accretion-induced drag from radiative mixing layers.

Without radiative cooling, a coherent cold tail does not form.

The tail deceleration correlates with cooling luminosity, enabling future observational tests.

Abstract

Radiative turbulent mixing layers are widely invoked to explain the survival, growth, and entrainment of cold gas in hot astrophysical flows, but quantitative dynamical tests have remained scarce. RBH-1, the first confirmed runaway supermassive black hole, offers a rare opportunity to test this framework: JWST observations show a 62 kpc tail of cold H$\alpha$ and [O III]-emitting gas behind a source moving at ~950 km/s through the hot circumgalactic medium, with a coherent velocity gradient of ~200 km/s along the tail. Using 3D hydrodynamical simulations together with turbulent mixing-layer theory, we model the coherent downstream tail. We find that the observed downstream deceleration is well reproduced by accretion-induced drag from radiative mixing layers, and that without radiative cooling no coherent cold tail forms. We also derive a direct connection between the tail deceleration and the cooling luminosity, yielding predictions for future measurements of the cooling luminosity profile. RBH-1 therefore provides a rare quantitative dynamical stress test of radiative mixing-layer physics in an astrophysical system.