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J.M. Wrobel, G.B. Taylor, S.T. Myers (NRAO), C.D. Fassnacht (STScI)
The NRAO VLBA was used at 5.0~GHz to image 27 FIRST sources in the Boötes field of the NOAO Deep Wide-Field Survey (NDWFS). FIRST sources were selected as being compact (\Theta\rm M < 5\arcsec) and bright (S\rm peak > 10~mJy~beam-1) at 1.4~GHz, implying a typical position error in one dimension of ±400~mas at 4\sigma. For each FIRST source, this position error defines the VLBA search region in a phase-referenced image with a FWHM resolution of 3.3~mas by 1.5~mas. FIRST images of these 27 sources imply that 24 are candidates for containing active galactic nuclei (AGN), 2 are hotspots within a single extended source, and 1 is a jet-like feature adjacent to an AGN candidate. Among the 24 FIRST sources that are AGN candidates, 5 were detected with the VLBA above a typical 6\sigma threshold of 1.5~mJy~beam-1 and with a position error in one dimension of less than ±2~mas at 1\sigma. Each VLBA detection has a brightness temperature greater than 107~K and must therefore be an AGN. NDWFS images from data release 1.0 suggest that two of these AGN are hosted by galaxies, while two are probable QSOs. NDWFS images are not yet available for the fifth AGN, a possible compact symmetric object. The 19 AGN candidates not detected with the VLBA could still harbor AGN with S\rm peak < 1.5~mJy~beam-1 at 5.0~GHz. Therefore, the AGN content in the radio regime of the Boötes field is at least 21±9%. The statistical accuracy of this result is set by the small numbers of AGN candidates observed and detected in this pilot VLBA survey. When complete, the VLBA survey will target about four times as many AGN candidates. Among the 3 FIRST sources in the pilot survey that are not AGN candidates, none were detected with the VLBA. This work used images provided by the NDWFS (Jannuzi & Dey 1999, Asp Conf.\ Series, 191, 111), which is supported by NOAO. NOAO is operated by AURA, Inc., under a cooperative agreement with the NSF. NRAO is a facility of the NSF operated under cooperative agreement by Associated Universities, Inc.
The author(s) of this abstract have provided an email address for comments about the abstract: jwrobel@nrao.edu