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Register a new userNowhere to Hide: Radio-faint AGN in the GOODS-N field. I. Initial catalogue and radio properties
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J. F. Radcliffe
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(Abridged) Conventional radio surveys of deep fields ordinarily have arc-second scale resolutions often insufficient to reliably separate radio emission in distant galaxies originating from star-formation and AGN-related activity. Very long baseline interferometry (VLBI) can offer a solution by identifying only the most compact radio emitting regions in galaxies at cosmological distances where the high brightness temperatures (in excess of $10^5$ K) can only be reliably attributed to AGN activity. We present the first in a series of papers exploring the faint compact radio population using a new wide-field VLBI survey of the GOODS-N field. The unparalleled sensitivity of the European VLBI Network (EVN) will probe a luminosity range rarely seen in deep wide-field VLBI observations, thus providing insights into the role of AGN to radio luminosities of the order $10^{22}~mathrm{W,Hz^{-1}}$ across cosmic time. The newest VLBI techniques are used to completely cover an entire 7.5 radius area to milliarcsecond resolutions, while bright radio sources ($S > 0.1$ mJy) are targeted up to 25 arcmin from the pointing centre. Multi-source self-calibration, and a primary beam model for the EVN array are used to correct for residual phase errors and primary beam attenuation respectively. This paper presents the largest catalogue of VLBI detected sources in GOODS-N comprising of 31 compact radio sources across a redshift range of 0.11-3.44, almost three times more than previous VLBI surveys in this field. We provide a machine-readable catalogue and introduce the radio properties of the detected sources using complementary data from the e-MERLIN Galaxy Evolution survey (eMERGE).
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Obtaining a census of active galactic nuclei (AGN) activity across cosmic time is critical to our understanding of galaxy evolution and formation. Many AGN classification techniques are compromised by dust obscuration. However, very long baseline interferometry (VLBI) can be used to identify compact emission that can only be attributed to AGN activity. This is the second in a series of papers dealing with the compact radio population in the GOODS-N field. We review 14 different AGN classification techniques in the context of a VLBI-detected sample, and use these to investigate the nature of the AGN as well as their host galaxies. We find that no single identification technique can identify all VLBI objects as AGN. Infrared colour-colour selection is most notably incomplete. However, the usage of multiple classification schemes can identify all VLBI-selected AGN, independently verifying similar approaches used in other deep field surveys. In the era of large area surveys with instruments such as the SKA and ngVLA, multi-wavelength coverage, which relies heavily upon observations from space, is often unavailable. Therefore, VLBI remains an integral component in detecting AGN of the jetted efficient and inefficient accretion types. A substantial fraction (46%) of the VLBI AGN have no X-ray counterpart, which is most likely due to lack of sensitivity in the X-ray band. A high fraction of the VLBI AGN reside in low or intermediate redshift dust-poor early-type galaxies. These most likely exhibit inefficient accretion. Finally, a significant fraction of the VLBI AGN reside in symbiotic dusty starburst - AGN systems. We present an extensive compilation of the multi-wavelength properties of all the VLBI-selected AGN in GOODS-N in the Appendix.
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