Far-Infrared constraints on the ongoing Star Formation of distant galaxies hosting Active Galactic Nuclei

Abstract

There have been a multitude of observational findings supporting a co-evolution of the central BH and its host galaxy. This co-evolution is expected to be driven by a connection between the two growing mechanisms of active galactic nuclei (AGN; accretion onto the central super-massive black hole), and star formation. In an effort to find more direct evidence for a connection between the two mechanisms of AGN and star formation I investigate the star formation rates (SFRs) of galaxies hosting X-ray, optical, and radio AGN. For the analysis of these samples I have developed methods to calculate the mean and individual SFRs of distant AGN and improve on previous work by simultaneously taking into account redshift and mass when interpreting the results. I use infrared photometry from WISE, Spitzer and Herschel to decompose the infrared (IR) spectral en- ergy distributions (SEDs) into AGN and star formation components and undertake careful treatment of the upper limits in the SED analyses and average SFR calculations. Using these methods I have calculated the mean SFRs of ~2000 X-ray and ∼3000 optically selected AGN spanning the AGN luminosity range of 10^43 <L(AGN)< 5 × 10^47 erg/s at redshifts of 0.2< z <2.5. I have established that the mean SFR as a function of AGN luminosity shows a flat relationship with a slight positive trend at the highest AGN luminosities. Crucially, I show that the positive trends observed here, and in previous work, are strongly influenced by the internal redshift and mass dependencies of average SFRs observed in the global galaxy population. Indeed, the AGN studied in this thesis show mean SFRs consistent to the main sequence of star-forming galaxies, indicating that on average they tend to live in star-forming galaxies. Using new sensitive sub-mm photometry from the ALMA interferometer for a sample ~100 X–ray AGN I obtain up to 10 times improvement on the SFR constraints compared to those obtained using the archival photometry. Furthermore, I demonstrate that combining deep 870μm observations with
mid-infrared photometry is a useful diagnostic for AGN identification.