Active Galactic Nuclei in galaxy surveys : Empirical paths to the fiery hearts of cosmic beasts

Sammanfattning: Some galaxies emit excessive amounts of light from their centers, caused by accretion of gas onto super-massive black holes (SMBH). These galactic cores are often referred to as Active Galactic Nuclei (AGN) and they come in many different forms, distinguishable by their emission properties. The AGN classes fall into two major categories: those with narrow Balmer lines, and those with broad Balmer lines. The AGN Unification theory of radio-quiet AGN predicts the two classes to differ mainly in the viewing angle of the observer who may, or may not, see the central engine due to dust obscuration in the foreground.In this PhD thesis, I explore the limits of the radio-quiet AGN Unification. In its most famous and simple form, the obscurer is a parsec-sized dust doughnut surrounding the accretion disk. I show that the galaxy neighbours to the two types of AGN are different, in disagreement with the simplest form of unification (Paper I). The two AGN classes differ in their [OIII]5007 luminosity, as well as in their star-formation history (Paper II), suggesting that we must incorporate galactic dust into the concept of AGN Unification, as well as differences in the luminosity of the central engine. I also present a novel, data-driven method to pinpoint the relative spatial origin of certain emission lines in AGN (Paper III). Finally, we conclude the thesis by discussing an anti-transient survey targeted at finding signatures of extra-terrestrial intelligence (Paper IV). This survey can, as a side effect, also be useful to find extreme, variable AGN that challenge both AGN Unification and evolutionary theories.