Hibernating black holes revealed by photometric mass functions

We present a novel strategy to uncover the Galactic population of quiescent black holes (BHs). This is based on a new concept, the photometric mass function (PMF), which opens up the possibility of an efficient identification of dynamical BHs in large fields-of-view. This exploits the width of the disc Halpha emission line, combined with orbital period information. We here show that Halpha widths can be recovered using a combination of customized Halpha filters. By setting a width cut-off at 2200 km/s we are able to cleanly remove other Galactic populations of Halpha emitters, including ~99.9% of cataclysmic variables (CVs). Only short period (Porb<2.1 h) eclipsing CVs and AGNs will contaminate the sample but these can be easily flagged through photometric variability and, in the latter case, also mid-IR colours. We also describe the strategy of a deep (r=22) Galactic plane survey based on the concept of PMFs: HAWKs, the HAlpha-Width Kilo-deg Survey. We estimate that ~800 sqr deg are required to unveil ~50 new dynamical BHs, a three-fold improvement over the known population. For comparison, a century would be needed to produce an enlarged sample of 50 dynamical BHs from X-ray transients at the current discovery rate.