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 pc using a VLBI array consisting of the Very Long Baseline Array and three 100 m class telescopes. A flat-spectrum 3 mJy source with a monochromatic radio power of ∼1037 ergs s-1 has been detected, apparently at the location of the active galactic nucleus (AGN) and its central black hole. The radio source has a minimum brightness temperature of 2.1 × 108 K and a size upper limit of 0.035 pc, about 10 times the diameter of the broad-line region and 15,000 times the diameter of the black hole's event horizon. An additional flat-spectrum component located within a parsec of the apparent nucleus is likely to be a knot in the inner radio jet. The presence of some steep-spectrum radio emission within 0.1 pc of the galaxy nucleus limits the emission measure of a possible ionized torus to a maximum value of 108 cm-6 pc. If the hard X-ray source in NGC 4151 is associated 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 image reveals a 0.2 pc two-sided base to the well-known arcsecond radio jet. The apparent speeds of the jet components relative to the radio AGN are less than 0.050c and less than 0.028c at respective nuclear distances of 0.16 and 6.8 pc. These are the lowest speed limits yet found for a Seyfert galaxy and indicate nonrelativistic jet motions, possibly due to thermal plasma, on a scale only an order of magnitude larger than the broad-line region.