Limits on the Accretion Rates onto Massive Black Holes in Nearby Galaxies

The radio emission from supermassive black holes in nearby early-typegalaxies can be used to test possible explanations for their low luminosities.We calculate the expected contribution from thermal synchrotron emission fromhot accretion flows to the high radio frequency observations of NGC 2300, NGC1399, NGC 4278 and NGC 4594. We find that, in all cases, and in accordance withour previous findings, hot flows accreting close to their Bondi ratesoverestimate significantly the observed fluxes. This implies that simplyassuming a low radiative efficiency for the accreting gas is not enough toexplain their low luminosities. Smaller central densities and accretion rates,as expected in the presence of strong mass loss or convection in the flows, canhelp reconcile the models with observations. We also show that a significantcontribution to the high-frequency radio spectra can be due to non-thermalsynchrotron emission from the small scale radio jets observed in these systems,allowing for even lower accretion rates in the inflows. We suggest that theseoutflows or jets may dump significant energy into the surronding medium closeto the accretion radius and so reduce the accretion rates onto these systems.We discuss the relationship between the radio flux and black hole mass for theobserved sample and its potential importance for probing accretion models.Comment: 14 pages, 5 figures; submitted to Ap