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 $\sim 3.6\times 10^7$~\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 %($\mathbf{i}=83$°) 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°$\pm$2°) 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.

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.ps

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• Megamaser Disks in Active Galactic Nuclei
• VLBA Continuum Observations of NGC 4258: Constraints on an Advection-Dominated Accretion Flow
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• Polarized Broad H-alpha Emission from the LINER Nucleus of NGC 1052
• Magnetic Fields of Agns and Standard Accretion Disk Model: Testing by Optical Polarimetry
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• The benchmark black hole in NGC 4258: dynamical models from high-resolution two-dimensional stellar kinematics
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• Polarized Narrow-Line Emission from the Nucleus of NGC 4258
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• The BeppoSAX view of the X-ray active nucleus of NGC4258
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• A comprehensive X-ray view of the active nucleus in NGC 4258
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