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 \sim 10 ^ { 37 } 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 \times 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 < 0.050 c and < 0.028 c 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 .