The LINER Nucleus of M87: A Shock‐excited Dissipative Accretion Disk

Abstract

We present long-baseline Faint Object Spectrograph (FOS) spectra of the nuclear accretion disk in M87 (NGC 4486), o†set from the nucleus by (42.7 pc) in order to avoid the nuclear continuum. Even 0A.6 so close to the nucleus, the optical spectrum has the appearance of a normal LINER galaxy. We show that the presence of strong UV emission lines provides a deÐnitive test of the excitation mechanism; the disk is shock excited, not photoionized by a UV continuum from the central source. The shock velocity inferred (265 km s~1) is about one-half of the Keplerian rotation velocity found earlier by Ford et al. Since shock dissipation appears to be the principal means of increasing the binding energy of the accreting gas, we can use the FOS data and the luminosity proÐle of the accretion disk to estimate the rate of mass accretion as a function of radius. We Ðnd that this rate decreases with decreasing distance from the nucleus, as the material becomes organized into a cool and thin classical accretion disk in the inner regions. In the outer disk, the accretion rate (D4 yr~1) is comparable to that determined for the M _ X-rayEemitting cooling Now, showing that a large fraction of the cooling gas can Ðnd its way into the nuclear regions. The accretion rate near the nucleus (D3 ] 10~2 yr~1) is consistent with the properM _ ties of the relativistic jet and its associated radio emission. Over the lifetime of the jets, about 107 of M _ cool material may have accumulated in the nuclear regions, allowing the formation of a disk that is optically thick to Thomson scattering where it becomes ionized close to the nucleus. We speculate that LINER emission is a general property of the shocked dissipative regions of accretion disks in active galaxies with strongly sub-Eddington accretion and may therefore be used as a diagnostic of these dissipative accretion Nows. Subject headings : accretion, accretion disks E galaxies : individual (M87) E galaxies : kinematics and dynamics E galaxies : nuclei E X-rays : galaxies