A SUBPARSEC RADIO JET OR DISK IN NGC-4151

We have imaged the classical Seyfert galaxy NGC 4151 with the VLBA at wavelengths of 6 and 18 cm, achieving resolutions of similar or equal to 2 and 5 mas (similar or equal to 0.16 and 0.40 pc), respectively. A t 18 cm, four radio components, spread over similar or equal to 0.'' 5 (similar or equal to 40 pc) are detected. The eastermmost pair compri ses a bright component elongated in position angle similar or equal to 20 degrees (component E) and an extension to the northeast (component Fl which curves into position angle similar or equal to 75 degrees, s imilar to that of the 3.'' 5 (280 pc) scale radio jet and the narrow-l ine region. At the higher resolution achieved at 6 cm, component E is found to be a linear radio source with a length of similar to 13 mas ( 1.0 pc). This source has a length/width ratio of greater than or simil ar to 4, and therefore fulfills one of the classical criteria for a ra dio jet, but its radio luminosity is only similar to 10(38) ergs s(-1) , several orders of magnitude less than the parsec-scale jets in radio galaxies. It is also misaligned by similar or equal to 55 degrees fro m the arcsecond-scale radio jet. Possible reasons for the misalignment include deflection by the inner narrow-line region, buoyancy forces, and a change in the plane of the accretion disk presumably responsible for jet collimation. We also discuss an alternative interpretation, i n which component E is a disk or torus viewed edge-on and emitting fla t-spectrum, possibly thermal, radio emission. Component E appears to b e the only part of the radio emission that contains a significant flat -spectrum component, and we favor identification of the central, brigh test, unresolved subcomponent of E as the ultraviolet (UV) and optical nucleus. In order to reconcile the much smaller column density of H I toward the nucleus found by Lyman absorption than by 21 cm absorption measurements, we argue that a similar or equal to 0.01 pc thick gas d isk surrounds the nucleus and is ionized out to a radius of similar or equal to 2 pc. The large 21 cm absorption column observed then result s from off-nuclear radio components shining through the outer, neutral part of this disk. The flat spectrum of the nuclear radio source may indicate synchrotron self-absorption or free-free absorption by the in ner, ionized part of the accretion disk. Interestingly, the apparent n uclear source and a radio subcomponent similar or equal to 0.2 pc to t he southeast align precisely perpendicular to the arcsec-scale radio j et, suggesting that they may outline the large-scale structure of the accretion disk responsible for jet collimation. Comparison of the 18 c m image with the European VLBI network image acquired by Harrison et a l. in 1984 provides upper limits of 0.14c and 0.25c for the apparent s peeds of the radio components at distances of 7 and 36 pc, respectivel y, from the galaxy nucleus.