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 \alpha = 14h 29m 27.0s and \delta = + 35°28′30.00″ ( J2000 ) .
The inner 0-2′ of the field reached an unprecedented 1 \sigma rms noise level of 9 ~ { } \mu Jy 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 \sim 16.3 ^ { m } and I \sim 19.6 ^ { m } respectively ) .
Further deep surveys of the inner 2-4′ and 4-6′ of the field , with 1 \sigma rms noise levels of 11-19 \mu Jy 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 \sigma rms noise levels of 37-55 \mu Jy 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 by the double structure of 3 of them .
Combining both the deep and shallow VLBI surveys , optical identifications are available for 8 of the 9 VLBI detections .
Only VLBI J142906.6095 remains unidentified ( I > 25.6 ^ { m } ) , quite unusual as its integrated WSRT flux density is 20 mJy .
Two other sources are not detected in K-band ( K > 18.5 ^ { m } ) 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 ^ { +4 } _ { -5 } % .
The VLBI detection rate for mJy WSRT sources is higher , 29 ^ { +11 } _ { -12 } % .
This observational trend is expected from a rapidly evolving radio source population .
Moreover , this trend supports deep radio surveys , at lower resolution , that infer the radio emission associated with fainter sub-mJy and microJy sources arises via processes associated with extended regions of star formation .
The 9 VLBI detections reported here pin-point the precise location of active nuclei or candidate active nuclei , and their VLBI positions can help to anchor the NOAO Boötes 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 can 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 .