Radio observations of NGC 2808 and other globular clusters: constraints on intermediate mass black holes

TL;DR

This study used deep radio observations of NGC 2808 to search for intermediate mass black holes, finding no sources and thus setting constraints on their presence and properties in globular clusters.

Contribution

It provides the first deep radio constraints on intermediate mass black holes in NGC 2808 and compares these with other clusters and accretion-based limits.

Findings

No radio sources detected in NGC 2808's core.

Constraints on the mass of potential intermediate black holes.

Most globular clusters do not follow the $M_{BH}-\sigma$ relation.

Abstract

We present the results of a deep radio observation of the globular cluster NGC 2808. We show that there are no sources detected within the core of the cluster, placing constraints on both the pulsar population of the cluster and the mass of a possible intermediate mass black hole in NGC 2808. We compare the results for this cluster with other constraints on intermediate mass black holes derived from accretion measures. With the exception of G1 in M~31 which has previously shown radio emission, even with considerably more conservative assumptions, only the clusters with the poorest of observational constraints are consistent with falling on the $M_{BH}-\sigma$ relation. This result is interpreted in terms of the fundamental differences between galaxies and globular clusters.