Accretion Modes, AGN Feedback and Star Formation

Abstract

I study mid-infrared and star formation properties of AGN samples using infrared observations, and star-forming galaxies using radio observations in order to investigate the link between star formation, AGN activity and radio luminosity. I present the results of these investigations in this thesis. I carried out an analysis of four complete samples of radio-loud AGN (3CRR, 2Jy, 6CE and 7CE) using near- and mid-IR data taken by the Wide-Field Infrared Survey Explorer (WISE). The combined sample consists of quasars and radio galaxies, and covers a redshift range 0:003 < z < 3:395. The dichotomy in the mid-IR properties of low- and high-excitation radio galaxies (LERGs - HERGs) is analysed using large complete samples. The results show that a division in the accretion modes of powerful LERGs and HERGs clearly stands out in the mid-IR radio plane. Evaluation of the positions of the sample objects in WISE colour-colour diagrams shows that widely used WISE colour cuts are not completely reliable in selecting AGN. I examined the link between AGN activity and star formation by constructing matched samples of local (0 < z < 0:6) radio-loud and radio-quiet AGN in the Herschel-ATLAS fields. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation properties were derived using Herschel 250-_m and stellar mass measurements are taken from the SDSSð€€€MPA-JHU catalogue. The stacking analyses show that star formation rates (SFRs) and specific star formation rate (SSFRs) of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass. I also investigated the relationship between SFR and low-frequency radio luminosity observed in star-forming galaxies. I used a sample of star-forming galaxies in the 19 local Universe selected from the SDSS-MPA-JHU catalogue. LOFAR observations of the Herschel-ATLAS North Galactic Pole field (NGP) were carried out as part of the LOFAR surveys Key Science Project at an effective frequency of 150 MHz, which provided low-frequency radio luminosity of sample galaxies. SFRs of galaxies in the sample were derived using MAGPHYS spectral energy distribution (SED) fitting. The results of this study show that the slope of L150/SFR is less than unity and not universal for all star-forming galaxies (SFGs) in the local Universe (0 < z < 0:3). The slope of the L150/SFR relation is also found to be steeper than the L1:4/SFR relation, probably due to the contribution from thermal radio emission at 1.4 GHz. If the L150=SFR relation for strongly star-forming objects is explained naively by electron calorimetry, I conclude that low luminosity sources are not ideal calorimeters and differ from the main locus of SFGs at low redshifts. The different gradients we obtain for the far- IR/radio correlation using samples selected at different frequencies reveal the selection effects on relations derived in this thesis.