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 Very Long Baseline Array and 100 m Green Bank Telescope. Applying wide-field VLBI techniques, a total of 61 sources, selected from a Westerbork Synthesis Radio Telescope (WSRT) image, were surveyed simultaneously with a range of different sensitivities and resolutions. The survey covered a total of 1017 arcmin2 = 0.28 deg2 divided into annular fields centered on α = 14h29m27. ˢ0, δ = +35°28′30. ″00 (J2000.0). The inner 0-2' of the field reached an unprecedented 1 σ rms noise level of 9 μJy beam-1 and yielded two detections. VLBI J142923.6466 and VLBI J142934.7033 have brightness temperatures in excess of 106 K, and we locate the active nuclei of their host galaxies (NDWFS J142923.6+352851 and NDWFS J142934.7+352859, with I ~ 16.3 and 19.6 mag, respectively). Further deep surveys of the inner 2'-4' and 4'-6' of the field, with 1 σ rms noise levels of 11-19 μJy beam-1, detected a previously known source, VLBI J142910.2224, a quasar with a brightness temperature in excess of 109 K, that was also used during these observations as an in-beam phase calibrator. The deep VLBI survey between 0' and 6' thus detected three radio sources, drawn from a total of 24 targets. A shallower VLBI survey, conducted between 6' and 18' of the field center, and with 1 σ rms noise levels of 37-55 μJy beam-1, detected a further six radio sources, drawn from 37 additional targets. Each of those six VLBI detections has a brightness temperature in excess of 105 K; this hints that those six are accretion-powered, a suggestion reinforced by the double structure of three of them. Combining the deep and shallow VLBI surveys, optical identifications are available for eight of the nine VLBI detections. Only VLBI J142906.6095 remains unidentified (I > 25.6 mag), quite unusually, as its integrated WSRT flux density is 20 mJy. Two other sources are not detected in the K-band (K > 18.5 mag), suggesting that some significant fraction of these compact radio sources may be located at z > 1. The VLBI detection rate for sub-mJy WSRT radio sources is 8%. The VLBI detection rate for mJy WSRT sources is higher, 29%. This observational trend is expected from a rapidly evolving radio source population. Moreover, this trend supports deep radio surveys, at lower resolution, which have suggested that the radio emission associated with fainter sub-mJy and μJy sources arises via processes associated with extended regions of star formation. The nine VLBI detections reported here pinpoint the precise location of active nuclei or candidate active nuclei, and their VLBI positions can help to anchor the NOAO Bootes field to the International Celestial Reference Frame. The simultaneous detection of several sub-mJy and mJy radio sources, in a single observation, suggest that their combined response may be used to self-calibrate wide-field VLBI data. There is every prospect that future deep VLBI observations will be able to take advantage of this wide-field technique, in any random direction on the sky, thereby generating large-area, unbiased surveys of the faint radio source population.