ON THE CONNECTION OF THE APPARENT PROPER MOTION AND THE VLBI STRUCTURE OF COMPACT RADIO SOURCES

Abstract

Many of the compact extragalactic radio sources that are used as fiducial\npoints to define the celestial reference frame are known to have proper motions\ndetectable with long-term geodetic/astrometric Very Long Baseline\nInterferometry (VLBI) measurements. These changes can be as high as several\nhundred micro-arcseconds per year for certain objects. When imaged with VLBI at\nmilli-arcsecond (mas) angular resolution, these sources (radio-loud active\ngalactic nuclei) typically show structures dominated by a compact, often\nunresolved "core" and a one-sided "jet". The positional instability of compact\nradio sources is believed to be connected with changes in their brightness\ndistribution structure. For the first time, we test this assumption in a\nstatistical sense on a large sample, not only for individual objects. We\ninvestigate a sample of 62 radio sources for which reliable long-term time\nseries of astrometric positions as well as detailed 8-GHz VLBI brightness\ndistribution models are available. We compare the characteristic direction of\ntheir extended jet structure and the direction of their apparent proper motion.\nWe present our data and analysis method, and conclude that there is indeed a\ncorrelation between the two characteristic directions. However, there are cases\nwhere the ~1-10-mas scale VLBI jet directions are significantly misaligned with\nrespect to the apparent proper motion direction.\n