Relativistic Jets in The Radio Reference Frame Image Database I: Apparent Speeds from the First Five Years of Data

摘要

(Abridged) We present the results of an analysis of relativistic jet apparentspeeds from VLBI images in the Radio Reference Frame Image Database (RRFID).The images are snapshot VLBI images at 8 and 2 GHz using the VLBA plus up toten additional antennas that provide global VLBI coverage. We have analyzed the8 GHz images from the first five years of the database (1994-1998), for allsources observed at three or more epochs during this time range. This subsetcomprises 966 images of 87 sources. The sources in this subset have an averageof 11 epochs of observation over the years 1994-1998, with the best-observedsources having 19 epochs. About half of the sources in this RRFID kinematicsurvey have not been previously studied with multi-epoch VLBI observations. Wehave measured apparent speeds for a total of 184 jet components in 77 sources,of which the best-measured 94 component speeds in 54 sources are used in thefinal analysis. The apparent speed distribution shows a peak at low apparentspeeds (consistent with stationary components), a tail extending out toapparent speeds of about 30c, and a mean apparent speed of 3.6c. A total of 36of the sources in this paper are also included in the 2 cm VLBA survey byKellermann et al., with similar angular resolution, sensitivity, and timerange. For those sources, we present a detailed component-by-componentcomparison of the apparent speeds measured by the 2 cm survey and thosemeasured in this paper. Many of the independent apparent speed measurementsagree very well, but for approximately 25% of the components we findsignificant differences in the apparent speeds measured by the two surveys. Theleading cause of these discrepancies are differences in how the two surveyshave identified jet components from epoch-to-epoch.