No Black Holes in NGC 6397

TL;DR

This paper argues that the central dark objects in NGC 6397 are white dwarfs, not black holes, aligning with theoretical expectations and contrasting with previous claims of black hole presence.

Contribution

It demonstrates that the central dark population in NGC 6397 is due to white dwarfs, challenging prior interpretations of black hole subclusters in such clusters.

Findings

The dark core is explained by white dwarf populations.

Black hole subclusters are absent in core-collapsed clusters.

White dwarf subclusters are a common feature in core-collapsed clusters.

Abstract

Recently, \citet{vitral2021does} detected a central concentration of dark objects in the core-collapsed globular cluster NGC 6397, which could be interpreted as a subcluster of stellar-mass black holes. However, it is well established theoretically that any significant number of black holes in the cluster would provide strong dynamical heating and is fundamentally inconsistent with this cluster's core-collapsed profile. Claims of intermediate-mass black holes in core-collapsed clusters should similarly be treated with suspicion, for reasons that have been understood theoretically for many decades. Instead, the central dark population in NGC 6397 is exactly accounted for by a compact subsystem of white dwarfs, as we demonstrate here by inspection of a previously published model that provides a good fit to this cluster. These central subclusters of heavy white dwarfs are in fact a generic feature of core-collapsed clusters, while central black hole subclusters are present in all {\em non\/}-collapsed clusters.