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

Abstract

The growing evidence for supermassive black holes in the centres of relatively nearby galaxies has brought into sharper focus the question of why elliptical galaxies, rich in hot gas, do not possess quasar-like luminosities. Recent studies suggest that the presence of advection-dominated accretion flows (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 systems, 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 obtained using the Very Large Array (VLA) and the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). At these frequencies the predictions are very precise and an ADAF is unequivocally characterized by a slowly rising spectrum with a sharp spectral cut-off produced by thermal synchrotron radiation. Although X-ray analysis of these galaxy cores provides very strong clues for their extreme quiescence (and makes the case of advective accretion plausible), the new radio limits disagree severely with the canonical ADAF predictions which significantly overestimate the observed flux. While the present observations do not rule out the presence of an ADAF in the systems considered here, they do place strong constraints on the model. If the accretion in these objects occurs in an advection-dominated mode, then our radio limits imply that the emission from their central regions must be suppressed. We examine the possibility that the magnetic field in the flow is extremely low, or that synchrotron emission is free-free absorbed by cold material in the accretion flow. We also discuss whether 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.