Abstract

Black holes spend the majority of their lives in a low-luminosity, quiescent state whose intrinsic faintness makes it relatively inaccessible to study. While the accretion flow in this state is extremely radiatively inefficient, the distribution of accretion power between radiation, mechanical power and black hole growth is not well understood. I will review our current understanding of low-luminosity accretion, and then outline one promising avenue for making progress, via deep radio surveys of globular clusters. Although several hundred stellar-mass black holes are believed to form from the deaths of the most massive stars early in the lifetime of a globular cluser, it was thought that they would then mass segregate to the cluster centre, causing most of them to be ejected via mutual gravitational interactions. Using deep radio continuum observations, we have recently found the first strong evidence for stellar-mass black holes in galactic globular clusters, challenging this theoretical expectation. I will present our initial results, and outline ongoing follow-up campaigns to verify the black hole nature of these sources, as well as a major radio survey to determine the demographics of this newly-discovered population of candidate black holes, which provide new laboratories for studying quiescent accretion. The known distances to globular clusters can provide more accurate estimates of the physical parameters of these black holes than exist for typical field sources, and the additional formation channels available in a dense cluster should provide a less biased estimate of the black hole mass function than the handful of known systems elsewhere in the galaxy.