2005/05/07 by James S. Ulvestad, J. S. Ulvestad, D. S. Wong +6 · 1 citation
Physics and Astronomy · #Active galactic nucleus #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Astrophysics and Cosmic Phenomena #Brightness #Galaxies: Formation, Evolution, Phenomena #Galaxy #Geometry #Physics #Radio galaxy #Surface brightness #Torus #Very Long Baseline Array #Very-long-baseline interferometry #astro-ph
paper · pdf · doi:10.1086/432034
published as Astron.J.130:936-944,2005 · Accepted to Astronomical Journal, 23 pages, 5 figures, 1 color figure
arxiv created 2005/05/07 · openalex publication_date 2005/09/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
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 ∼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 × 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. The apparent speeds of the jet components relative to the radio AGN are less than 0.050 c and less than 0.028 c 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.