Nowhere to Hide: Radio-faint AGN in the GOODS-N field. II. Multi-wavelength AGN selection techniques and host galaxy properties

Abstract: 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.