Deep VLBI Imaging of Faint Radio Sources in the NOAO Bootes Field

We have conducted a deep, very long baseline interferometry (VLBI)observation at 1.4 GHz of an area of sky located within the NOAO Bootes field,using the NRAO VLBA and 100-m Green Bank Telescope. Applying wide-field VLBItechniques, a total of 61 sources, selected from a WSRT image, were surveyedsimultaneously with a range of different sensitivities and resolutions. Theinner 0-2', of the field reached an unprecedented 1-sigma rms noise level ~ 9uJy/beam and yielded 2 detections. A further 7 sources were detected in therest of the field. All of the sources have a brightness temperature in excessof 10^5 K; suggesting they are AGN. Optical identifications are available for 8of the 9 VLBI detections - only VLBI J142906.6095 remains unidentified (I >25.6m). Two sources are not detected in K-band (K > 18.5m) suggesting that somesignificant fraction of these compact radio sources may be located at z > 1.The VLBI detection rate for sub-mJy radio sources is 8^{+4}_{-5} %. The VLBIdetection rate for mJy sources is higher, 29^{+11}_{-12}%. This trend isexpected if the radio emission associated with fainter sub-mJy and microJysources increasingly arises from extended regions of star formation. The 9 VLBIdetections pin-point the precise location of AGN or candidate AGN, and theirVLBI positions can help to anchor the NOAO Bootes field to the ICRF. Thesimultaneous detection of several sub-mJy and mJy radio sources, in a singleobservation, suggest that their combined response may be used to self-calibratewide-field VLBI data. Future VLBI observations of faint sub-mJy and microJyradio sources can take full advantage of this "full-beam" calibrationtechnique.Comment: ApJ in press, 36 pages. Reduced abstract presented her