Retaining Black Holes with Very Large Recoil Velocities

TL;DR

This paper investigates whether supermassive black holes can remain in galaxies despite large recoil velocities from mergers, using simplified and Monte Carlo models to reconcile theoretical predictions with observations.

Contribution

It introduces a simplified merger model and a Monte Carlo simulation to analyze black hole retention, addressing the apparent contradiction with observed galactic centers.

Findings

Even with high ejection probabilities, over 50% of galaxies retain supermassive black holes.

More realistic physics in models likely increases black hole retention.

Monte Carlo simulations support the qualitative analysis and estimate occupation fractions.

Abstract

Recent numerical simulations of binary black hole mergers show the possibility of producing very large recoil velocities (> 3000 km/s). Kicks of this magnitude should be sufficient to eject the final black hole from virtually any galactic potential. This result has been seen as a potential contradiction with observations of supermassive black holes residing in the centers of most galaxies in the local universe. Using an extremely simplified merger tree model, we show that, even in the limit of very large ejection probability, after a small number of merger generations there should still be an appreciable fraction (>50%) of galaxies with supermassive black holes today. We go on to argue that the inclusion of more realistic physics ingredients in the merger model should systematically increase this retention fraction, helping to resolve a potential conflict between theory and observation. Lastly, we develop a more realistic Monte Carlo model to confirm the qualitative arguments and estimate occupation fractions as a function of the central galactic velocity dispersion.