Probing down to early cosmic epochs using limited redshift ($z\lesssim 1$) optical surveys with host galaxy age and lookback time analysis

Abstract

We present a detailed analysis of AGN identification diagnostics and host galaxy evolution using optical spectral diagnostics using ESO-GOODS-S data. We employ traditional Baldwin-Phillips-Terlevich (BPT) diagrams along with their modern extensions, the Mass-Excitation (MEx) and Colour-Excitation (CEx) diagrams, to classify AGNs from among up to 600+ candidates. We extract the spectral properties utilising an indigenous spectral fitting code. In addition, the code also incorporates inputs from a state-of-the-art stellar population synthesis model (Bruzual and Charlot, 2003, MNRAS, 344, 1000), extracting the host galaxy ages and metallicities as model parameters. Redshift values for all the studied spectra are obtained from the literature. Using these redshift values, we are able to assign a 'lookback time' (using standard $\Lambda$CDM cosmological parameters). When this lookback time is combined with the galaxy ages (from spectral fitting), it enables us to study galaxy evolution even up to early cosmological epochs ($\sim$13 Gyr), despite restricting the studied field of galaxies to $z\sim 1$ (lookback time of $\sim$8 Gyr). We conclude that galaxies ultimately evolving into AGN hosts originate earlier ($\sim$8 Gyr ago) than non-AGN host galaxies ($\sim$6 Gyr ago). We relate this fact, that AGN hosts date back to an earlier epoch than non-AGN galaxies, as being likely due to higher cosmic temperatures at that time, probably enhancing black hole accretion. We emphasise that host galaxy age is an additional crucial parameter (apart from cosmological redshift) in defining our understanding of AGN and star formation co-evolution across cosmological time scales.