Deep, wide-field global VLBI observations of the HDF-N

Abstract

We present initial results of wide-field Global VLBI 1.4-GHz observations of the Hubble Deep Field North region (HDF-N) and Flanking Fields surrounding it with a spatial resolution of 4 milliarcseconds. These observations are both deeper and wider than any previous VLBI observations of the field, attaining an r.m.s. noise level of 7.3 µJy/beam in the central field. Four radio sources have been clearly detected in the inner part of the field and a search for additional sources in the adjacent Hubble Flanking Field (HFF) is underway. Among the four detections in the HDF-N, three (VLA J123642+621331, VLA J123644+621133, and VLA J123646+621404) have already been detected in previous EVN 1.6-GHz observations, but the forth one, VLA J123652+621444, is a new detection that clearly shows one-sided, jet-like radio structure. The VLBI observations of VLA J123642+621331 -classified at other wavelengths as a distant, dust-obscured starburst galaxy at z = 4.424 -reveal a resolved jet-like extension emanating from a compact AGN core. These results demonstrate the power of high-resolution VLBI imaging in discriminating between starburst and AGN activity in dust-obscured systems. We plan to analyse 80 additional radio target fields in the HFF over the next few months 1 .8th European VLBI Network Symposium

Using VO tools to investigate distant radio starbursts hosting obscured AGN in the HDF(N) region

“…VLBI results support the AGN interpretation of compact, flat-spectrum radio cores. Extremely compact radio cores (brightness temperature (>10 5 −10 6 ) K) were detected in J123642+621331, J123644+621133, J123646+621404 and J123652+121444 using the European VLBI Network (EVN) and global VLBI (Garrett et al 2001;Chi et al 2006), confirming the presence of an AGN. J123644+621133 is unmistakably an FR 1.…”

“…Four of the 92 sources were detected at (4−20)-mas resolution by the EVN (European VLBI Network) and global VLBI (Garrett et al 2001;Chi et al 2006). At the other extreme, the Westerbork Synthesis Radio Telescope at 15 resolution detected ∼10% more sources than the VLA (Garrett et al 2000).…”

“…The compact core is AGN-dominated; the EVN detected over half the 1.4-GHz flux (Garrett 2002) and global VLBI (Chi et al 2006) resolves a jet component a few tens of pc from the core. J123642+621331 has a measured Γ = 1.35, above the limit for type-II AGN, but Padovani et al (2004) noted that high-redshift sources might be misclassified.…”