Optical Detection of the Hidden Nuclear Engine in NGC~4258

Abstract

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 the most compelling evidence to date for the existence of a massive black hole in the 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 an active galactic nucleus is perhaps best sought using polarized light: probing for light scattered off material surrounding the central source. New polarimetry of NGC~4258 has uncovered a compact polarized nucleus whose spectrum consists of a faint blue continuum similar to those of unobscured quasars ($F_\nu\propto\nu^{-1.1}$), plus broadened (\twid1000\kms) emission lines. 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 result provides substantiating evidence for a weakly active central engine in NGC~4258 and for the existence of obscuring, orbiting tori which impart many of the perceived distinctions between various types of active galaxy.