AAS 201st Meeting, January, 2003
Session 48. Blazars and AGNs
Poster, Tuesday, January 7, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[48.14] VLBI Identification of the Sub-Parsec Nucleus of NGC 4151

J.S. Ulvestad (NRAO), D.S. Wong (Cornell), G.B. Taylor (NRAO), C.G. Mundell (Liverpool John Moores), J.W. Gallimore (Bucknell)

The core of the Seyfert galaxy NGC 4151 has been imaged at 5, 8, and 15 GHz using a VLBI array consisting of the Very Long Baseline Array, the Green Bank Telescope, the phased Very Large Array, and the Effelsberg 100-m radio telescope. At 15 GHz, a compact, flat-spectrum 3-mJy source has been found within component D3, on the western side of the strongest component identified in the MERLIN image of Mundell et al. (1995, MNRAS, 272, 355). This confirms that the AGN is located in D3, as suggested by Mundell et al. (also see astro-ph/0209540), rather than in component E, 5.5 parsecs to the East. The nucleus appears to be the origin of a short, two-sided sub-parsec jet having the same position angle as the faint, highly collimated jet emission identified by Mundell et al. (astro-ph/0209540); this implies that the true jet collimation direction is along PA=85 deg. rather than the value of PA=77 deg. inferred from the well-known arcsecond-scale jet. The core brightness temperature is T\rm B \geq 1\times 108 K, while the components of the apparent sub-parsec jet have T\rm B~107 K.

The separation between the radio core and the jet component E2, located 68 milliarcseconds (mas) to the East, has changed by less than 1 mas (0.08 pc) in the six years between 1996.42 and 2002.37. The corresponding upper limit to the speed of component separation is v\leq 0.04c (or 12,000 km/s) on a 5.5-pc scale. This is the lowest value or limit yet measured for a parsec-scale component speed in a Seyfert galaxy.

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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