The inner accretion disk in M 87

The observed AGN in M87 is extremely quiescent compared to expectations based upon simple estimates for the mass accretion rate. We examine the possibility that the nal stages of accretion are advection dominated by comparing current observational constraints on the ux from the M87 core to the predictions of an advection dominated accretion ow (ADAF) model. We nd that the observed self-absorbed radio core can be dominated by the ADAF, whereas the optical through to X-ray emission may be dominated by the jet. Speculation on the relevance of ADAFs to the FR-I/FR-II dichotomy is reported. 1 Why is M87 so dim? At the start of this workshop, Roger Blandford raised the question as to why the AGN in M87 is so quiescent. In this contribution, we return to this important question and discuss the possible implications for our understanding of the central, and energetically dominant, regions of the accretion ow. A simple energy budget allows us to place this issue on a quantitative footing. Summing the observed radiative ux (from radio to X-rays) from the non-thermal core of M87 results in a broad-band (isotropic) radiative luminosity of L rad 10 42 erg s ?1 (Biretta, Stern & Harris 1991). If some of this ux originates from the relativistic jet, it will be relativistically beamed and the intrinsic radiative luminosity of the core will be less (unless the lorentz factor of the jet exceeds 10, in which case we may lie outside of the beaming cone). The kinetic luminosity of the AGN is a more diicult quantity to constrain. Models of the global energetics of the radio halo yields a total kinetic luminosity of L kin = 10 43 ? 10 44 erg s ?1 (Bicknell & Begelman 1996; Reynolds et al. 1996), at least an order of magnitude greater than the radiative luminosity. To demonstrate the quiescence of this source, we now estimate the accretion rate. X-ray observations with ROSAT show that M87 possesses an extensive hot interstellar medium (HIM). These X-ray data show that, at the centre of M87, the sound speed of the HIM is c s 500 km s ?1 and the gas density is at least n e 0:5 cm ?3 (C. B. Peres, private communication). Naively, the mass accretion rate onto the central black hole may be estimated from spherical Bondi accretion theory to be _ M 0:2 M yr ?1. If this mass is …