Authors:
Enno Middelberg (MPIfR)
Alan Roy (MPIfR)
Neil Nagar (University of Groningen)
Thomas Krichbaum (MPIfR)
Ray Norris (ATNF)
Andrew Wilson (University of Maryland)
Heino Falcke (ASTRON)
Ed Colbert (JHU)
Arno Witzel (MPIfR)
Klaus Fricke (University of Göttingen)
VLBA Project Code: ER008 (observed in November 1999 and February 2000), BF038 (observed in August 1998)
The Seyfert 2 galaxy Mrk 1210 belongs to the >99% of Active Galactic Nuclei (AGN) which are radio-quiet, i.e., they produce greater flux density at optical than at radio wavelengths. However, similar to radio-loud quasars, they harbour super-massive bl ack holes and their quietness is not understood. It may be intrinsic to the mechanism that feeds material into the black hole or it may be that an initially powerful jet is slowed down in interactions with the surrounding medium. Because Seyfert galaxies in general have much lower flux densities than powerful quasars, they are difficult to observe with VLBI, and only in the 1990s have these observations become more frequent and popular.
Mrk 1210 was part of a project to investigate the properties of Seyfert galaxies on pc scales, requiring the use of very long baseline interferometry. Our goals were 1) to measure the speeds at which components are travelling in these objects. High speeds close to the AGN indicate that the jets are launched fast and are slowed down later, low speeds indicate that there may be an intrinsic difference in the core of radio-quiet and radio-loud objects, perhaps the black-hole spin or magnetic field strengths differ. 2) to look for weak, extended radio emission which should arise from collisions of AGN ejecta with the circumnuclear material and 3) to measure the spectral indices, a quantity which describes the ratio of the source's brightnesses at two frequenc ies. Spectral indices are a useful diagnostic of emission processes, source compactness and possible foreground absorbers.
In the case of Mrk 1210, no previous VLBI observations had been made, so our observations now provide an anchor for subsequent observations to measure relative motions. In contrast, for example in NGC 7674, being in the same study, we found two components separated by 516.0 milliarcsec, compared to 508.7 milliarcsec in an EVN observation in 1985. Thus, the two components separate at a speed of 92% of the speed of light. This is, after the short superluminal period in III Zw 2 (Brunthaler et al. 2000), the highest speed ever measured in a Seyfert galaxy and shows that jets in Seyferts can travel large distances while maintaining high speeds.
The structure seen in the 6 cm VLBA image is difficult to interpret. The configuration of the sub-structure in component NW in the 6 cm VLBA image reminded us of the situation in NGC 1068, in which circumnuclear material was found perpendicular to the rad io jet axis. However, the high brightness temperature (the temperature a blackbody would need to have to produce the observed flux density) is more than 10 million degrees, ruling out thermal emission processes. This is confirmed by the spectrum which ind icates non-thermal synchrotron emission. Another possible interpretation is that the AGN is located at component SE and that the arc of components in NW is produced by shocks in a wide-angle outflow. It may also be possible that the AGN is coincident with neither of the components, as has been found in, e.g., the Seyfert 2 galaxy NGC 3079 (a VLBA image of the month (July 2003) by Kondratko et al.) and as we propose for NGC 7674 (Middelberg et al. 2004).
References
Middelberg, E., Roy, A. L., Nagar, N. M., Krichbaum, T. P., Norris, R. P., Wilson, A. S., Falcke, H., Colbert, E. J. M., Witzel, A. and Fricke, K. J., "Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI", 2004, A&A, a ccepted (astro-ph/0402142)
Brunthaler, A., Falcke, H., Bower, G. C., Aller, M. F., Aller, H. D., Teräsranta, H., Lobanov, A. P. Krichbaum, T. P. and Patnaik, A. R., "III Zw 2, the first superluminal jet in a Seyfert galaxy", 2000, A&A, 357, L45
Kondratko, P, Greenhill, L. J and Moran, J. M., "Thick, Flared, and Disorganized Accretion Disk in NGC 3079", VLBA image of the month of July 2003