Strong observational constraints on advection-dominated accretion in the cores of elliptical galaxies

The growing evidence for supermassive black holes in the centres of relativ ely nearby galaxies has brought into sharper focus the question of why elli ptical galaxies, rich in hot gas, do not possess quasar-like luminosities. Recent studies suggest that the presence of advection-dominated accretion f lows (ADAFs), with their associated low radiative efficiency, might provide a very promising explanation for the observed quiescence of these systems. Although ADAF models have been applied to a number of low-luminosity syste ms, compelling observational evidence for their existence is still required . Here, we examine new high-frequency radio observations of the three giant , low-luminosity elliptical galaxies NGC 4649, NGC 4472 and NGC 4636 obtain ed using the Very Large Array (VLA) and the Submillimetre Common-User Bolom eter Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT), At these fr equencies the predictions are very precise and an ADAF is unequivocally cha racterized by a slowly rising spectrum with a sharp spectral cut-off produc ed by thermal synchrotron radiation. Although X-ray analysis of these galax y cores provides very strong clues for their extreme quiescence (and makes the case of advective accretion plausible), the new radio limits disagree s everely with the canonical ADAF predictions which significantly overestimat e the observed flux. While the present observations do not rule out the pre sence of an ADAF in the systems considered here, they do place strong const raints on the model. If the accretion in these objects occurs in an advecti on-dominated mode, then our radio limits imply that the emission from their central regions must be suppressed. We examine the possibility that the ma gnetic field in the Row is extremely low, or that synchrotron emission is f ree-free absorbed by cold material in the accretion Bow. We also discuss wh ether slow non-radiating accretion flows may drive winds/outflows to remove energy, angular momentum and mass so that the central densities, pressures and emissivities are much smaller than in a standard ADAF.