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Active galaxies

In the local Universe about 10% of galaxies posses an extremely luminous and compact nucleus, a so-called Active Galactic Nucleus (AGN). In many cases, the AGN is more luminous than the entire stellar population of the galaxy (~100 billion stars) and the luminosity is generated in a region approximately the size of the Solar System. Such emission cannot be explained in terms of models of stellar emission, but it is thought that AGN are fueled by the accretion of mass onto a massive black hole (up to a billion times the mass of the Sun). As the interstellar gas possesses angular momentum it tends to form a rotating accretion disk around the black hole. In the disk, viscosity allows the gas to loose angular momentum and fall towards the black hole, converting gravitational energy into electromagnetic radiation, and producing particles accelerated to relativistic velocities.

 

AGN classification is complex and not unambiguous, and often depends on the region of the spectrum in which they are studied for the first time. Nonetheless, there are three main AGN classes: Seyfert galaxies, Quasars and radio galaxies. Seyfert galaxies are the low luminosity, nearby members of the Quasar population. Radio galaxies are AGN characterised by strong radio emission that can extend to scales larger than the galaxy itself, that is, over 100 kiloparsecs. Most of these sources in fact posses enormous symmetric radio lobes, from which a large part of the radiation is emitted, and some show one or two highly collimated jets that exit directly from the nucleus and travel towards the lobes. Some AGN are peculiar in the sense that they are characterised by powerful and compact radio sources (without lobes), the so-called Blazars.

 

In recent years the astrophysical community has made a great effort to understand the fundamental aspects of the structure, physics and geometry of the inner-most zones of AGN in all their diverse phenomenologies, and to shed light on the physical mechanisms that regulate the galaxy/black hole interaction, and their evolution as a function of cosmic time.

 

One of the most important discoveries in recent years has, in fact, been the discovery of a relation between the mass of the black hole and some structural parameters of the host galaxy, that clearly shows how the evolution of the galaxy and the growth of the central black hole are closely related. One speaks, then, of "co-evolution" to indicate that, in some way, galaxies and super-massive black holes grow and evolve together. It is hypothesised that interactions between galaxies could be responsible for the activation of star formation and accretion onto the black hole. Since, during the early phases of activity, large quantities of gas and dust are present in the galaxies, it is thought that most of the primary radiation due to star formation and nuclear activity is heavily obscured and almost invisible at optical and ultraviolet wavelengths. As time goes by, the power of the radiation associated with the growth of the super-massive black hole becomes ever more intense, to the point of being able to efficiently heat, and eventually expel, the interstellar gas from the galaxy itself. It is thought that this phase is associated with bright, unobscured emission in the optical. Such "feedback" by the AGN slows accretion, and it is hypothesised that it is also a necessary phase to explain the cessation of new star formation in large mass elliptical galaxies. When most of the cold gas is expelled from the system, the accretion onto the black hole and star formation cannot proceed further. At the end of this process, a passive galaxy with an inactive super-massive black hole at its centre remains.

 

AGN then aren't just super-energetic sources, but constitute a fundamental phase in the global evolution of galaxies. Awareness of this has led to a new interaction between these two sectors of research, once completely separate, leading also to large "survey" projects that combine optical and infrared observations with those at X-ray wavelengths. In these projects, the Italian community has an acknowledged, centre-stage role.