Probing the Role of AGN Feedback and Galactic Mergers in Galaxy Evolution

Abstract

In this thesis we aim to probe the role of two of the processes that can dictate the evolution of galaxies – feedback from the central active galactic nucleus (AGN) and mergers. To study the importance of AGN feedback and, in particular, its most direct manifestation as galactic-scale cold-gas winds we assembled two carefully matched large samples of nearby galaxies with and without the presence of optical unobscured Seyfert 2 active galactic nucleus activity. To infer and quantify the presence of such galactic kpc-scale outflows we then studied and compared the properties of the interstellar Na i λλ5890,5895 (NaD) absorption-line doublet, present in some of these systems. We detected excess interstellar NaD absorption in a similar fraction of galaxies in both of our samples. We identified only 53 (or 0.5% of the population) of our Seyfert 2 AGN galaxies potentially harbor outflows. Moreover, in a large fraction of these 53 Seyfert 2s, available ancillary radio and infrared data indicated that star-formation may actually be the principal driver of the outflows. Our results suggest that galactic-scale winds at low redshift are no more frequent in Seyfert 2s than they are in their control-sample counterparts and that optical AGNs are not direct significant contributors to the quenching of star formation in the nearby Universe. On the other hand, to investigate the impact that mergers can have on galaxy evolution we have focused on two galaxies that show signatures of embedded counter-rotating components. Such features are believed to be the fossil records of a past gas acquisition events or a merger. We have successfully separated the contributions of the two distinct kinematic components to the spectra in one of them NGC 448. Drawing on this separation we have shown that the two decoupled stellar components in NGC 448 have similar ages, but different chemical compositions. Our findings indicate that the kinematically distinct component in NGC 448 is truly decoupled, has external origin, and was formed through either the acquisition of gas and a subsequent star-formation episode or from the direct accretion of stars from a companion. Conversely, the presence of a kinematically distinct component in NGC 4365 is not associated to a true kinematic decoupling and is instead most likely due to a projection effect stemming from the triaxial nature of this galaxy. We have also used two samples from a large integral-field spectroscopic survey to verify some of our previous finding and study the demographics of galaxies with embedded counterrotating components. We have performed some preliminary analysis of this data. The results of this investigation confirm the validity of our method for the detection of cold-gas flows in our Seyfert 2 and control samples. Finally, we have verified some previously known trends in the demographics and properties of galaxies that display stellar counter-rotation as inferred by integral-field observations. On the other hand, our analysis is in contrast with previous studies that have observed that the presence of a counter-rotating stellar component is associated rather frequently with the presence of counter-rotating gas traced by nebular emission.