VLA Limits for Intermediate Mass Black Holes in Three Globular Clusters

TL;DR

This study used the VLA to search for radio signals from intermediate mass black holes in three globular clusters but found no significant evidence, setting upper limits on their possible radio emissions.

Contribution

First VLA observations targeting intermediate mass black holes in multiple globular clusters with detailed upper limit analysis.

Findings

No significant radio emission detected in the three clusters.

NGC6266 shows a marginal 2-sigma detection.

Upper limits constrain black hole accretion models.

Abstract

The observational evidence for central black holes in globular clusters has been argued extensively, and their existence has important consequences for both the formation and evolution of the cluster. Most of the evidence comes from dynamical arguments, but the interpretation is difficult, given the short relaxation times and old ages of the clusters. One of the most robust signatures for the existence of a black hole is radio and/or X-ray emission. We observed three globular clusters, NGC6093 (M80), NGC6266 (M62), and NGC7078 (M15), with the VLA in the A and C configuration with a 3-sigma noise of 36, 36 and 25 microJy, respectively. We find no statistically-significant evidence for radio emission from the central region for any of the three clusters. NGC6266 shows a 2-sigma detection. It is difficult to infer a mass from these upper limits due to uncertainty about the central gas density, accretion rate, and accretion model.