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Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment

Published online by Cambridge University Press:  01 October 2007

Y. Y. Kovalev ,
A. P. Lobanov ,
A. B. Pushkarev  and
J. A. Zensus
Y. Y. Kovalev
Affiliation:
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: ykovalev@mpifr-bonn.mpg.de, alobanov@mpifr-bonn.mpg.de, apushkar@mpifr-bonn.mpg.de, azensus@mpifr-bonn.mpg.de Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
A. P. Lobanov
Affiliation:
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: ykovalev@mpifr-bonn.mpg.de, alobanov@mpifr-bonn.mpg.de, apushkar@mpifr-bonn.mpg.de, azensus@mpifr-bonn.mpg.de
A. B. Pushkarev
Affiliation:
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: ykovalev@mpifr-bonn.mpg.de, alobanov@mpifr-bonn.mpg.de, apushkar@mpifr-bonn.mpg.de, azensus@mpifr-bonn.mpg.de Pulkovo Astronomical Observatory, Russia Crimean Astrophysical Observatory, Ukraine
J. A. Zensus
Affiliation:
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: ykovalev@mpifr-bonn.mpg.de, alobanov@mpifr-bonn.mpg.de, apushkar@mpifr-bonn.mpg.de, azensus@mpifr-bonn.mpg.de
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Abstract

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Accurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extragalactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy (“core shift”). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed by Very Long Baseline Interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars could be of the order of 0.1--0.2 mas. This shift exceeds the expected positional accuracy of Gaia and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.

Keywords

galaxies: activegalaxies: jetsradio continuum: galaxiesastrometryreference systems
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 3 , Symposium S248: A Giant Step: from Milli- to Micro-arcsecond Astrometry , October 2007 , pp. 348 - 351
Copyright
Copyright © International Astronomical Union 2008

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