# Escape speed of stellar clusters from multiple-generation black-hole mergers in the upper mass gap

## TL;DR

This paper investigates how multiple-generation black-hole mergers in dense environments can populate the upper mass gap, and how observing a single black hole in this gap constrains the escape speed of its environment.

## Contribution

It demonstrates that a black hole in the mass gap indicates formation in environments with escape speeds exceeding 50 km/s, such as nuclear star clusters.

## Key findings

- Populating the mass gap requires escape speeds >50 km/s.
- Detection of a mass gap black hole implies dense formation environments.
- Single detections can reveal the nature of black hole formation channels.

## Abstract

Pair instabilities in supernovae might prevent the formation of black holes with masses between $\sim 50 M_\odot$ and $\sim 130 M_\odot$. Multiple generations of black-hole mergers provide a possible way to populate this "mass gap" from below. However this requires an astrophysical environment with a sufficiently large escape speed to retain merger remnants, and prevent them from being ejected by gravitational-wave recoils. We show that, if the mass gap is indeed populated by multiple mergers, the observation of a single black-hole binary component in the mass gap implies that its progenitors grew in an environment with escape speed $v_{\rm esc} \gtrsim 50$ km/s. This is larger than the escape speeds of most globular clusters, requiring denser and heavier environments such as nuclear star clusters or disks-assisted migration in galactic nuclei. A single detection in the upper mass gap would hint at the existence of a much larger population of first-generation events from the same environment, thus providing a tool to disentangle the contribution of different formation channels to the observed merger rate.