X-ray constraints on the local supermassive black hole occupation fraction

Distinct seed formation mechanisms are imprinted upon the fraction of dwarf galaxies currently containing a central supermassive black hole. Seeding by Population III remnants is expected to produce a higher occupation fraction than is generated with direct gas collapse precursors. Chandra observations of nearby early-type galaxies can directly detect even low-level supermassive black hole activity, and the active fraction immediately provides a firm lower limit to the occupation fraction. Here, we use the volume-limited AMUSE surveys of 200 optically selected early-type galaxies to characterize simultaneously, for the first time, the occupation fraction and the scaling of L _X with Mstar, accounting for intrinsic scatter, measurement uncertainties, and X-ray limits. For early-type galaxies with M _star < 10¹⁰ M_O, we obtain a lower limit to the occupation fraction of >20% (at 95% confidence), but full occupation cannot be excluded. The preferred dependence of log L _X upon log M _star has a slope of 0.7-0.8, consistent with the "downsizing" trend previously identified from the AMUSE data set, and a uniform Eddington efficiency is disfavored at 2σ. We provide guidelines for the future precision with which these parameters may be refined with larger or more sensitive samples.