  The Seyfert galaxy NGC 4151 has been imaged at resolution better than 0.1 pc
using a VLBI array consisting of the VLBA and three 100m-class telescopes. A
flat-spectrum 3-mJy source with a monochromatic radio power of ~10^{37} ergs/s
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 x 10^8 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 10^8 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. Apparent
speeds of jet components relative to the radio AGN are <0.050c and <0.028c at
respective nuclear distances of 0.16 pc and 6.8 pc. These are the lowest speed
limits yet found for a Seyfert galaxy, and indicate non-relativistic jet
motions, possibly due to thermal plasma, on a scale only an order of magnitude
larger than the broad-line region.
