Low-radiative-efficiency accretion in the nuclei of elliptical galaxies

Abstract

The discovery of hard X-ray emission from a sample of six nearby elliptical galaxies, including the dominant galaxies of the Virgo, Fornax and Centaurus clusters (M87, NGC 1399 and NGC 4696, respectively), and NGC 4472, 4636 and 4649 in the Virgo cluster, has important implications for the study of quiescent supermassive black holes. We describe how the broad-band spectral energy distributions for these galaxies, which accrete from their hot gaseous haloes at rates comparable to their Bondi rates, can be explained by low-radiative-efficiency accretion flows in which a significant fraction of the mass, angular momentum and energy is removed from the flows by winds. The observed suppression of the synchrotron components in the radio band (excluding the case of M87) and the systematically hard X-ray spectra, which are interpreted as thermal bremsstrahlung emission, support the conjecture that significant mass outflow is a natural consequence of systems accreting at low radiative efficiencies. We briefly discuss an alternative model for the observed X-ray emission, in which it is due to non-thermal synchrotron-self-Compton processes in the accretion flows, or outflows. This appears to require implausibly weak magnetic fields. Emission from a collimated jet viewed off-axis should be distinguishable from the bremsstrahlung model by variability and thermal line emission studies. We argue that the difference in radiative efficiency between the nuclei of spiral and elliptical galaxies may arise from the different manner in which interstellar gas is fed into the nuclei. In ellipticals, matter fed from the hot (slowly cooling) interstellar medium (ISM) is likely to be highly magnetized and have low specific angular momentum, which favours low-radiative-efficiency accretion solutions and, possibly, the formation of the observed jets.