Context. Dust is commonly present in weakly radio emitting star-forming galaxies and this dust may obscure the signatures of accreting black holes in these objects. Aims. We aim to uncover weak active galactic nuclei, AGN, in the faint radio source population by means of deep high-resolution radio observations. Methods. VLBI observations with a world-wide array at unparallelled sensitivity are carried out to assess the nature of the faint radio source population in the Hubble deep field north and its flanking fields. Results. Images of twelve compact, AGN-driven radio sources are presented. These represent roughly one quarter of the detectable faint radio source sample. Most, but not all of these low power AGN have X-ray detections. Conclusions. The majority of the faint radio source population must be star-forming galaxies. Faint AGN occur in a variety of (distant) host galaxies, and these are often accompanied by a dust-obscured starburst. Deep, high-resolution VLBI is a unique, powerful technique to assess the occurrence of faint AGN.
Aims. Submillimetre-selected galaxies (SMGs) at high redshift (z ∼ 2) are potential host galaxies of active galactic nuclei (AGN). If the local Universe is a good guide, ∼50% of the obscured AGN amongst the SMG population could be missed even in the deepest X-ray surveys. Radio observations are insensitive to obscuration; therefore, very long baseline interferometry (VLBI) can be used as a tool to identify AGN in obscured systems. A well-established upper limit to the brightness temperature of 105 K exists in star-forming systems, thus VLBI observations can distinguish AGN from star-forming systems via brightness temperature measurements. Methods. We present 1.6 GHz European VLBI Network (EVN) observations of four SMGs (with measured redshifts) to search for evidence of compact radio components associated with AGN cores. For two of the sources, e-MERLIN images are also presented. Results. Out of the four SMGs observed, we detect one source, J123555.14, that has an integrated EVN flux density of 201 ± 15.2 μJy, corresponding to a brightness temperature of 5.2 ± 0.7 × 105 K. We therefore identify that the radio emission from J123555.14 is associated with an AGN. We do not detect compact radio emission from a possible AGN in the remaining sources (J123600.10, J131225.73, and J163650.43). In the case of J131225.73, this is particularly surprising, and the data suggest that this may be an extended, jet-dominated AGN that is resolved by VLBI. Since the morphology of the faint radio source population is still largely unknown at these scales, it is possible that with a ∼10 mas resolution, VLBI misses (or resolves) many radio AGN extended on kiloparsec scales.
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
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