Accretion onto Nearby Supermassive Black Holes: Chandra Constraints on the Dominant Cluster Galaxy NGC 6166

Chandra observations of low-luminosity supermassive black holes in nearby elliptical galaxies provide tight limits on both their nuclear luminosities and their Bondi accretion rates. We examine Chandra constraints on NGC 6166, the dominant galaxy in the cluster Abell 2199, which hosts an ~10⁹ M_☉ black hole. We measure a nuclear X-ray luminosity of L_{X, 1 keV} ~ 10⁴⁰ ergs s^-1 and show that the density and temperature profiles of the hot interstellar medium imply a Bondi accretion rate of _Bondi ≲ 3 × 10^-2 M_☉ yr^-1. This accretion rate predicts a nuclear luminosity of ~10⁴⁴ ergs s^-1 for a canonical radiative efficiency of 10%. Unless the Bondi estimate is inappropriate and/or the accretion rate onto the black hole is significantly reduced, the observed nuclear flux constrains the radiative efficiency of the accretion flow to be η ~ 10^-5. We show that low radiative efficiency accretion flows can explain the observed nuclear X-ray luminosity but that the power output from the jets in NGC 6166 is also important to the energetics of the system.