2000ApJ...534..165J

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 H₀) 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 2x10^–2 pc ~4000 AU ~400 Schwarzschild radii for a 5x10⁸ M_☉ 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 10⁶ (and more likely >10⁷) 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 and 20° during the first 0.2 pc of the jet and must be less than 5° during the first 0.8 pc. Assuming that the accretion disk is geometrically and optically thin and composed of a uniform 10⁴ K plasma, the average electron density in the inner 0.1 pc of the disk is 10³-10⁸ cm^–3. The mass of ionized gas in the inner pc of the disk is 10¹-10³ M_☉, sufficient to power the radio source for ∼10⁴-10⁶ 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.