Optical Detection of the Hidden Nuclear Engine in NGC 4258

The sub-parsec masing disk recently found to be orbiting a central mass of$\sim3.6\times10^7$~\msun\ in the Seyfert/LINER galaxy NGC~4258 provides themost compelling evidence to date for the existence of a massive black hole inthe nucleus of a galaxy. The disk is oriented nearly edge-on %($\bf i=83$\deg)and the X-ray spectrum %(Makishima \etal\ 1994) is heavily absorbed. Therefore,in this galaxy, the optical emission-line spectrum generally exhibited by anactive galactic nucleus is perhaps best sought using polarized light: probingfor light scattered off material surrounding the central source. Newpolarimetry of NGC~4258 has uncovered a compact polarized nucleus whosespectrum consists of a faint blue continuum similar to those of unobscuredquasars ($F_\nu\propto\nu^{-1.1}$), plus broadened (\twid1000\kms) emissionlines. The lines are strongly linearly polarized ($5-10$\%) at a position angle(85\deg$\pm$2\deg) coincident with the plane of the maser disk. This resultprovides substantiating evidence for a weakly active central engine in NGC~4258and for the existence of obscuring, orbiting tori which impart many of theperceived distinctions between various types of active galaxy.Comment: Accepted by ApJ Letters, 16 pages including 3 figures; uuencoded, gzipped postscript, also available via anon ftp at ftp://head-cfa.harvard.edu/pub/transfer/belinda/ngc4258_apjl.p