Imprints of recoiling massive black-holes on the hot gas of early type galaxies

TL;DR

This study uses simulations to explore how recoiling black holes affect hot gas in early-type galaxies, revealing observable X-ray signatures like Mach cones that can inform us about black hole dynamics and galaxy evolution.

Contribution

It demonstrates the impact of recoiling black holes on galactic gas and predicts observable X-ray features, advancing understanding of black hole recoil effects in galaxies.

Findings

Recoiling black holes create gaseous cores and cometary tails.

Mach cones form during super-sonic black hole passages, detectable in X-ray maps.

X-ray signatures can be observed up to hundreds of Mpc, aiding black hole recoil studies.

Abstract

Anisotropic gravitational radiation from a coalescing black hole binary is known to impart recoil velocities of up to ~1000 km/s to the remnant black hole. In this context, we study the motion of a recoiling black hole inside a galaxy modelled as an Hernquist sphere, and the signature that the hole imprints on the hot gas, using N-body/SPH simulations. Ejection of the black hole results in a sudden expansion of the gas ending with the formation of a gaseous core, similarly to what is seen for the stars. A cometary tail of particles bound to the black hole is initially released along its trail. As the black hole moves on a return orbit, a nearly spherical swarm of hot gaseous particles forms at every apocentre: this feature can live up to ~ 100 Myr. If the recoil velocity exceeds the sound speed initially, the black hole shocks the gas in the form of a Mach cone in density near each super-sonic pericentric passage. We find that the X-ray fingerprint of a recoiling black hole can be detected in Chandra X-ray maps out to a distance of Virgo. For exceptionally massive black holes the Mach cone and the wakes could be observed out to a few hundred of Mpc. Detection of the Mach cone is found to become of twofold importance: i) as a probe of high-velocity recoils and ii) as an assessment of the scatter of the mass-sigma relation at large black hole masses.