X-ray observations and gravitational wave detections have identified a population of stellar-mass black holes with masses up to 200 times that of the Sun. This complement the well-established population of supermassive black holes (SMBHs) at the centre of galaxies and quasars. By contrast, we have comparatively little observational evidence for a population of intermediate-mass black holes (IMBHs, with masses between ∼200 and 10⁵ solar masses), which are the missing link in our understanding of black hole evolution. IMBHs observed today offer critical insights into the origin and early growth channels of SMBHs.

For over a decade, both theoretical predictions and observational studies have suggested that Omega Centauri (ω Cen), the most massive Milky Way globular cluster, might harbour an intermediate-mass black hole. Recently, identification of fast-moving stars in the core of ω Cen provided the strongest evidence to date for the presence of such an IMBH. One of the key questions in the study of IMBHs is their accretion efficiency, which determines their radio and X-ray signatures. Mahida et al. investigate the accretion signature of the IMBH in ω Cen with ultra-deep radio continuum observations of the central region of the cluster. Using approximately 170 hr of Australia Telescope Compact Array observations, they achieved a root-mean-square noise of 1.1 μJy at 7.25 GHz, making this the most sensitive radio image of the cluster to date. They detected no radio emission at any of the proposed centers of the cluster, imposing stringent constraints on the presence of an accreting IMBH in ω Cen. Their findings indicate that the accretion efficiency around the black hole must be exceptionally low. The image above left shows the core region of ω Cen (white circle). The zoomed-in region in the right image is indicated by the cyan box. The right image shows the recent centers proposed by other researchers for ω Cen.