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For refcode 2000ApJ...534..165J:
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2000ApJ...534..165J The Radio Jets and Accretion Disk in NGC 4261 Dayton L. Jones Jet Propulsion Laboratory, California Institute of Technology, Mail Code 238-332, 4800 Oak Grove Drive, Pasadena, CA 91109; dj@bllac.jpl.nasa.gov Ann E. Wehrle Jet Propulsion Laboratory, California Institute of Technology, Mail Code 301-486, 4800 Oak Grove Drive, Pasadena, CA 91109; aew@ipac.caltech.edu David L. Meier Jet Propulsion Laboratory, California Institute of Technology, Mail Code 238-332, 4800 Oak Grove Drive, Pasadena, CA 91109; dlm@cena.jpl.nasa.gov and B. Glenn Piner Jet Propulsion Laboratory, California Institute of Technology, Mail Code 238-332, 4800 Oak Grove Drive, Pasadena, CA 91109; glenn@herca.jpl.nasa.gov Received 1999 September 28; accepted 1999 December 14 ABSTRACT The structure of active galactic nucleus (AGN) accretion disks on subparsec scales can be probed through free-free absorption of synchrotron emission from the base of symmetric radio jets. For objects in which both jet and counterjet are detectable with very long baseline interferometry (VLBI), the accretion disk will cover part of the counterjet and produce diminished brightness whose angular size and depth as a function of frequency can reveal the radial distribution of free electrons in the disk. The nearby (41 Mpc, independent of H_0_) FR I radio galaxy NGC 4261 contains a pair of symmetric kiloparsec-scale jets. On parsec scales, radio emission from the nucleus is strong enough for detailed imaging with VLBI. We present new Very Long Baseline Array (VLBA) observations of NGC 4261 at 22 and 43 GHz, which we combine with previous observations at 1.6 and 8.4 GHz to map absorption caused by an inner accretion disk. The relative closeness of NGC 4261 combined with the high angular resolution provided by the VLBA at 43 GHz gives us a very high linear resolution, approximately 2 x 10^-2^ pc ~ 4000 AU ~ 400 Schwarzschild radii for a 5 x 10^8^ M_sun_ black hole. The jets appear more symmetric at 1.6 GHz because of the low angular resolution available. The jets are also more symmetric at 22 and 43 GHz, presumably because the optical depth of free-free absorption is small at high frequencies. At 8.4 GHz, neither confusion effect is dominant and absorption of counterjet emission by the presumed disk is detectable. We find that the orientation of the radio jet axis is the same on parsec and kiloparsec scales, indicating that the spin axis of the inner accretion disk and black hole has remained unchanged for at least 10^6^ (and more likely >10^7^) yr. This suggests that a single merger event may be responsible for the supply of gas in the nucleus of NGC 4261. The jet opening angle is between 0.3^deg^ and 20^deg^ during the first 0.2 pc of the jet and must be less than 5^deg^ during the first 0.8 pc. Assuming that the accretion disk is geometrically and optically thin and composed of a uniform 10^4^ K plasma, the average electron density in the inner 0.1 pc of the disk is 10^3^ - 10^8^ cm^-3^. The mass of ionized gas in the inner pc of the disk is 10^1^ - 10^3^ M_sun_, sufficient to power the radio source for ~10^4^ - 10^6^ yr. Equating thermal gas pressure and magnetic field strength gives a disk magnetic field of ~10^-4^ to 10^-2^ gauss at 0.1 pc. We include an appendix containing expressions for a simple, optically thin, gas-pressure-dominated accretion disk model that may be applicable to other galaxies in addition to NGC 4261. Subject headings: accretion, accretion disks-galaxies: active -galaxies: individual (NGC 4261, 3C 270)-galaxies: jets -galaxies: nuclei
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