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 M star, accounting for intrinsic scatter, measurement uncertainties, and X-ray limits. For early-type galaxies with M star 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.