No Evidence for Intermediate-Mass Black Holes in Globular Clusters: Strong Constraints from the JVLA

TL;DR

Deep radio observations of three globular clusters found no evidence of intermediate-mass black holes, setting stringent upper mass limits and suggesting such black holes are rare or inefficiently accreting in these environments.

Contribution

This study provides the most stringent radio-based upper limits on IMBH masses in globular clusters to date, constraining their prevalence and accretion behavior.

Findings

No radio sources detected at cluster centers.

Upper mass limits for IMBHs range from 360 to 980 solar masses.

Results imply IMBHs are either rare or accrete inefficiently in globular clusters.

Abstract

With a goal of searching for accreting intermediate-mass black holes (IMBHs), we report the results of ultra-deep Jansky VLA radio continuum observations of the cores of three Galactic globular clusters: M15, M19, and M22. We reach rms noise levels of 1.5-2.1 uJy/beam at an average frequency of 6 GHz. No sources are observed at the center of any of the clusters. For a conservative set of assumptions about the properties of the accretion, we set 3-sigma upper limits on IMBHs from 360-980 M_sun. These limits are among the most stringent obtained for any globular cluster. They add to a growing body of work that suggests either (a) IMBHs ~> 1000 M_sun are rare in globular clusters, or (b) when present, IMBHs accrete in an extraordinarily inefficient manner.