Active Galactic Nuclei at the Crossroads of Astrophysics

Over the last five decades, AGN studies have produced a number of spectacular examples of synergies and multifaceted approaches in astrophysics. The field of AGN research now spans the entire spectral range and covers more than twelve orders of magnitude in the spatial and temporal domains. The next generation of astrophysical facilities will open up new possibilities for AGN studies, especially in the areas of high-resolution and high-fidelity imaging and spectroscopy of nuclear regions in the X-ray, optical, and radio bands. These studies will address in detail a number of critical issues in AGN research such as processes in the immediate vicinity of supermassive black holes, physical conditions of broad-line and narrow-line regions, formation and evolution of accretion disks and relativistic outflows, and the connection between nuclear activity and galaxy evolution. 1 Nuclear activity in galaxies Recent years have witnessed substantial progress in studies of active galactic nuclei (AGN). Activity is observed in galaxies throughout the entire range of the electromagnetic spectrum [35]. It is manifested on spatial and temporal scales that span over 12 orders of magnitude, ranging from several Schwarzschild radii to megaparsecs and from minutes to millions of years [141]. On the surface, the activity of galactic nuclei comes in many faces, outlined by the uncomfortably large number of the AGN classes — yet the underlying mechanism and physical conditions in the nuclear regions of all galaxies are probably more similar than may appear at first glance [4,53,178]. Last but not least, AGN are now detected at redshifts of 6 and beyond [40], and they may be closely connected to the formation and evolution of the large-scale structure [124] and to the epoch of reionization of the Universe [39,113,151]. There is growing acceptance of ubiquity of the nuclear activity in galaxies and its connection to the presence of supermassive black holes (SMBH) in the centres of all massive galaxies [3,57,74,75]. Observations in the infrared [52] and sub-millimetre bands [124] indicate that almost every galaxy exhibits certain characteristics previously thought to be related only to a much narrower class of powerful AGN. This implies that virtually all massive galaxies may go through a strong AGN phase in the course of their cosmological evolution. Historically, observational studies of AGN have been concentrated largely within six broadly defined areas. 1. Surveys have been used effectively, first in the optical and radio domains and later throughout much of the electromagnetic spectrum, to find correlations