VLBA Identification of the Milliarcsecond Active Nucleus in the Seyfert Galaxy NGC 4151

The Seyfert galaxy NGC 4151 has been imaged at resolution better than 0.1 pcusing a VLBI array consisting of the VLBA and three 100m-class telescopes. Aflat-spectrum 3-mJy source with a monochromatic radio power of ~10^{37} ergs/shas been detected, apparently at the location of the active galactic nucleus(AGN) and its central black hole. The radio source has a minimum brightnesstemperature of 2.1 x 10^8 K and a size upper limit of 0.035 pc, about 10 timesthe diameter of the broad-line region and 15,000 times the diameter of theblack hole's event horizon. An additional flat-spectrum component locatedwithin a parsec of the apparent nucleus is likely to be a knot in the innerradio jet. The presence of some steep-spectrum radio emission within 0.1 pc ofthe galaxy nucleus limits the emission measure of a possible ionized torus to amaximum value of 10^8 cm^{-6}pc. If the hard X-ray source in NGC 4151 isassociated with the radio AGN, its radio to X-ray ratio is less than 10^{-5},putting NGC 4151 securely in the radio-quiet class of AGNs. The radio imagereveals a 0.2 pc two-sided base to the well-known arcsecond radio jet. Apparentspeeds of jet components relative to the radio AGN are <0.050c and <0.028c atrespective nuclear distances of 0.16 pc and 6.8 pc. These are the lowest speedlimits yet found for a Seyfert galaxy, and indicate non-relativistic jetmotions, possibly due to thermal plasma, on a scale only an order of magnitudelarger than the broad-line region.Comment: Accepted to Astronomical Journal, 23 pages, 5 figures, 1 color figur