Active Galactic Nuclei and Their Host Galaxies

Active Galactic Nuclei (AGN) play an important role in many aspects of the modern cosmology, and of particular interest is the issue of the interplay between AGN and their host galaxies. The study of AGN host galaxies has shown to be of great importance in order to provide answers to some of the still fundamental questions, including the effect of the AGN on its host galaxy and vice-versa, the environment and clustering properties of AGN host galaxies, the origin of the accretion material, the triggering mechanisms that initiate the active phase in galaxy,  the duration of the active phase, etc., thereby placing important constraints on the models of galaxy formation and evolution.
I will give a basic overview of AGN, their properties, and the fundamental links between the AGN and their host galaxies suggested over the past decade. Finally, I will discuss an anticorrelation between the galaxy light concentration and X-ray-to-optical flux ratio, which has been found recently for the first time and might suggest that early-type galaxies (having higher concentration indexes) have lower Eddington rates than those of late type galaxies.