mJIVE-20: a survey for compact mJy radio objects with the Very Long Baseline Array

Abstract

We present the description and early results of the mJy Imaging VLBA Exploration at 20 cm (mJIVE-20). mJIVE-20 is a large project on the Very Long Baseline Array which uses "filler" time to systematically inspect a large sample of mJy radio sources, pre-selected from the FIRST survey made with the Very Large Array, to identify any compact emission which may be present. The newly available multifield capability of the VLBA makes it possible for us to inspect of order 100 sources per hour of observing time with a 6.75 sigma detection sensitivity of approximately 1 mJy/beam. The results of the mJIVE-20 survey are made publicly available as soon as the data are calibrated. After 18 months of observing, over 20,000 FIRST sources have been inspected, with 4,336 VLBI detections. These initial results suggest that within the range 1 - 200 mJy, fainter sources are somewhat more likely to be dominated by a very compact component than brighter sources. Over half of all arcsecond-scale mJy radio sources contain a compact component, although the fraction of sources which are dominated by milliarcsecond scale structure is smaller at 30-35%, increasing towards lower flux densities. Significant differences are seen depending on the optical classification of the source. Radio sources with a stellar/point-like counterpart in the Sloan Digital Sky Survey (SDSS) are more likely to be detected overall, but this detection likelihood appears to be independent of the arcsecond-scale radio flux density; the trend towards higher radio compactness for fainter sources is confined to SDSS galaxies and non-detections. These results are consistent with a unification model of AGN in which less luminous sources have on average slower radio jets, with lower Doppler suppression of compact core emission over a wider range of viewing angles. (abridged)