Hypercompact Stellar Systems Around Recoiling Supermassive Black Holes

TL;DR

This paper explores hypercompact stellar systems (HCSSs) ejected by recoiling supermassive black holes, linking their properties to galaxy merger histories and gravitational-wave kicks, and predicts their observable characteristics.

Contribution

It establishes a theoretical framework connecting HCSS properties to host galaxy features and kick velocities, and estimates their detection rates in the local universe.

Findings

HCSSs are similar to globular clusters but can have stellar masses approaching UCDs.

The velocity dispersion of HCSSs encodes the kick velocity of the recoiling black hole.

Approximately 100 HCSSs are predicted to be detectable within 2 Mpc of the Virgo cluster.

Abstract

A supermassive black hole ejected from the center of a galaxy by gravitational wave recoil carries a retinue of bound stars - a "hypercompact stellar system" (HCSS). The numbers and properties of HCSSs contain information about the merger histories of galaxies, the late evolution of binary black holes, and the distribution of gravitational-wave kicks. We relate the structural properties of HCSSs to the properties of their host galaxies, in two regimes: collisional, i.e. short nuclear relaxation times; and collisionless, i.e. long nuclear relaxtion times. HCSSs are expected to be similar in size and luminosity to globular clustersbut in extreme cases their stellar mass can approach that of UCDs. However they differ from all other classes of compact stellar system in having very high internal velocities. We show that the kick velocity is encoded in the velocity dispersion of the bound stars. Given a large enough sample of HCSSs, the distribution of gravitational-wave kicks can therefore be empirically determined. We combine a hierarchical merger algorithm with stellar population models to compute the rate of production of HCSSs over time and the probability of observing HCSSs in the local universe as a function of their apparent magnitude, color, size and velocity dispersion, under two assumptions about the star formation history prior to the kick. We predict that roughly 100 should be detectable within 2 Mpc of the center of the Virgo cluster and that many of these should be bright enough that their high internal velocity dispersions could be measured with reasonable exposure times.