Description
Observations of massive black holes in the early universe are revolutionising our understanding of galaxy evolution across cosmic time with cutting edge surveys like JADES, CEERS, UNCOVER, RUBIES and many others. Their unprecedented depth has uncovered a population of both type 1 and 2 AGN at high redshifts (out to z~11), through deep NIRSpec MSA spectroscopy. However, standard emission line diagnostics such as BPT or VO87 are insensitive to AGN at z>3 due to the decrease in metallicity of galaxies and lack of detection of high ionisation lines such as He II𝜆4686Å, [Ne V]𝜆3427Å , etc, which are too faint to detect in individual observations. In this talk, I will present our effort to expand these diagnostics through stacking of thousands of galaxies in the JADES survey at z>3, in order to detect these faint lines and investigate the presence of any subtle broad regions around the Balmer lines, indicating AGN activity. I will further discuss the evolution of the narrow line diagnostics with redshift and its effect on the selection of type-2 AGN. I will present a suite of emission line diagnostics, revealing spectral features that may have been hidden in previously studied diagrams of the individual spectra. These stacks of JADES spectra have allowed an evaluation of the high redshift AGN census and have provided novel insights into the average emission line properties of high redshift galaxies.
