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 Boötes 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 arcmin 2 = 0.28 deg 2 divided into annular fields centered on α = 14h 29m 27.0s and δ = +35 ◦ 28 ′ 30.00 ′ ′ (J2000). 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 10 6 K and locate the active nucleus of their host galaxies (NDWFS J142923.6+352851 and NDWFS J142934.7+352859 with I ∼ 16.3 m and I ∼ 19.6 m 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 10 9 K that was also used during these observations as an in-beam phase calibrator. The deep VLBI survey between 0 ′ and 6 ′ thus detected 3 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 6 radio sources, drawn from 37 additional targets. Each of those 6 VLBI detections has a brightness temperature in excess of 10 5 K; this hints that those 6 are accretion-powered, a suggestion reinforced