No Evidence for Accretion around the Intermediate-mass Black Hole in Omega Centauri

Abstract For over a decade, both theoretical predictions and observational studies have suggested that ω Centauri ( ω Cen), the most massive Milky Way globular cluster, might harbor an intermediate-mass black hole (IMBH). 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. We investigate the accretion signature of the IMBH in ω Cen with ultradeep radio continuum observations of the central region of the cluster. Using approximately 170 hr of Australia Telescope Compact Array observations, we achieve a root-mean-square noise of 1.1 μ Jy at 7.25 GHz, making this the most sensitive radio image of the cluster to date. We detect 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. Considering the fundamental plane of black hole activity, our findings indicate that the accretion efficiency around the black hole is exceptionally low (with a conservative 3 σ upper limit of ϵ ≲ 4 × 10 −3 ).