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J.S. Ulvestad, E.B. Fomalont (NRAO), H. Kimora (NRO-Mitaka)
The VLBA has been used to conduct a 5-GHz snapshot survey of a complete subset of 161 radio sources from the VLA FIRST (Faint Images of the Radio Sky at Twenty cm) survey. The purpose of the observations was to determine the distribution of properties of a set of weak compact radio sources, for comparison to VLBI surveys that have been performed on stronger objects. Selection criteria were (1) total flux density greater than 50~mJy at 4.85 GHz, from the GB6 survey; (2) two-point spectral indices flatter than -0.5 between 1.4 GHz (FIRST) and 4.85 GHz (GB6); and (3) source positions in the range 6h\llap{.}5 < \alpha < 17h\llap{.}5 and 29\circ < \delta < 34\circ. The flux-density limit is seven times fainter than the Caltech-Jodrell (CJ) VLBI surveys, which reached a limit of 0.35~Jy.
The 161 sources in the sample were observed in a single 12-hr VLBA run, using eight frequencies spread across 500~MHz in order to derive source positions via bandwidth synthesis. Each object was observed for 3--3.5 minutes, and all sources were detected and imaged. Source position errors are 10--20~mas in each coordinate, and the dynamic ranges in the final images range from ~40:1 to ~400:1. For the typical beam size of 3.5\times 1.2~mas, only about 20--30% of the sources are resolved, and no more than 10% of the sample sources are larger than two beams across. Preliminary indications are that the typical FIRST sources appear more compact than those in the CJ surveys, although interpretation is complicated by the limited (u,v\/) coverage and dynamic range available for 10-antenna snapshots. Tabulated results and images of the significantly resolved sources will be presented.
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