The relationship between Active Galactic Nuclei and the star-forming properties of their host galaxies

It is now accepted that there is a co-evolution between galaxies and their central super massive black holes (SMBHs). Primarily, cosmological simulations suggest that accreting SMBHs (or Active Galactic Nuclei, AGN) must somehow quench their hosts' star-formation rates (SFRs). In contrast, empirical results report that there is no evidence of a strong (anti-)correlation between SFR and X-ray luminosity (a proxy for AGN power). In this thesis, we aim to investigate further this apparent contradiction between AGNs and their host star-forming properties. The hypothesis we test is that a powerful AGN will have a higher impact on a low mass host galaxy than on a high mass one. Therefore, instead of the previously used X-ray luminosity, we investigate the relationship between the X-ray luminosity relative to the host stellar mass (a proxy for Eddington ratio) and the host SFR. We first used a sample of 1620 X-ray selected AGNs, for which we measured Eddington ratios and their host far-infrared luminosities (a proxy for SFR) that we corrected for AGN contamination. By doing this, we found a slight enhancement of SFR at higher Eddington ratios when compared to the SFR of star-forming galaxies with similar stellar masses that do not host AGNs. Furthermore, the change in the star-forming properties at higher Eddington ratio indicates that the Eddington ratio distribution must change with the host star-forming properties. To investigate further how the Eddington ratio distribution of AGNs changes with the star-forming properties of their hosts, we used a model for which we assumed the Eddington ratio distribution simply split between star-forming and quiescent galaxies. Overall, we find that our model is able to reproduce the X-ray luminosity function but fails to reproduce the flat relationship between SFR and X-ray luminosity. Finally, we found that this can be resolved if we introduce a mass-dependency into our model.