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    Alloin D. Johnson R., Lira P. (2006) Physics of active galactic nuclei at all scales.

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    The study of the physical processes at work in Active Galactic Nuclei (AGN) has kept a large number of astronomers busy since the discovery, in the early sixties, of the first radio galaxies and quasars. After a few decades of debate there is now a clear consensus about the source of energy in AGN, namely gravitational energy released through matter accretion onto a massive black hole.

    The most commonly accepted ideas regarding AGN are pictured in the so-called AGN unified model, although one should remember that this is still a working model. Tremendous progress has been made in unveiling, analyzing, and modeling the different components in AGN: the accretion disk, the jets of relativistic particles, the x-ray absorber close to the central engine, the torus that funnels the ionizing radiation, the surrounding clouds of dense material in the broadline region and narrowline region, the cool molecular and dusty material around the torus, as well as jet-induced effects on larger scales. Yet, it remains to incorporate all these components and physical processes into a self-consistent AGN model that would take into account, and predict, all the facets of AGN activity. The mere process of matter fueling is far from being totally understood and neither is the link of the AGN with its host galaxy. The birth and growth of massive black holes in the universe is a related question to be investigated and is of prime interest for cosmology. A wealth of new observational constraints about the physical processes taking place in AGN has recently been made available, thanks to ongoing key space missions (probing in particular their x-ray emission), large ground-based telescopes, and high spatial resolution techniques such as adaptive optics and interferometry. Therefore, it is both exciting and timely to see how these additional constraints fit in with model predictions and to explore the new perspectives they bring to the field.

    In order to contribute to this goal, we organized the International Workshop, Physics of Active Galactic Nuclei at all Scales, which was held at the European Southern Observatory in Santiago, Chile, from December 3 to 6, 2003. The meeting was sponsored jointly by the Conicyt/FONDAP Center for Astrophysics, the European Southern Observatory, and the Calan Observatory of Universidad de Chile. The Workshop was intended for training young researchers and students: it included a series of invited reviews, each followed by oral contributions of topical relevance, and was completed by a rich display of posters. Although the emphasis was on providing a solid theoretical base, appropriate results from recent observations across the electromagnetic spectrum were discussed, which made this Workshop extremely enlightening. The reviews have been prepared for publication and constitute the bulk of this volume. We are grateful to the reviewers for their generous and enthusiastic contributions.

    The Workshop was opened with a tutorial by Hagai Netzer, providing a comprehensive picture of AGN physics. He pointed out that in spite of the remarkable insights into the AGN phenomenon at present, some key questions are still open. One of them is the energy budget problem, which is the large discrepancy between the energy output required from the observed emission line spectrum and that extrapolated from the observed continuum energy distribution. Another question refers to the detailed understanding of the physical processes hidden within the so-called alpha parameter used to parameterize the viscosity in the accretion disk.

    Following the tutorial, and in the framework of the AGN unified model, subsequent reviews and discussions about AGN subsystems were given by specialists in the field. Diana Worrall and Bradley Peterson focused on the physics of material in the close vicinity of the central source and on related radiative processes, discussing relativistic jets and the broadline region. Moving to larger scales, Bob Fosbury, Jack Gallimore, and Moshe Elitzur reviewed the physics of gas further away but still exposed to the intense ultraviolet radiation field from the AGN, as well as of molecular and dusty material surviving such a harsh environment.

    Questions about the AGN fueling and the AGN relationship with its host galaxy were discussed by Sharda Jogee. Very recent results from space xray missions are shedding new light on the evolution of the AGN luminosity function with redshift and on the AGN-galaxy formation scenario: these were presented by Niel Brandt and Omar Almaini.

    Thorough reviews on the forefront instrumentation available today in Chile-based observatories were given by Chris Lidman and Malcolm Smith, which, however, are not provided in this volume.

    The workshop was attended by around seventy participants, with a remarkable delegation from South America. It provided the ideal format and size to boost exchanges and discussions and in this, it met the original goals of the organizers, Danielle Alloin, Poshak Gandhi, and Rachel Johnson at ESO; and Paulina Lira, Sebastian Lopez, and Jose Maza at Calan Observatory.

    Alloin D. Johnson R., Lira P. (2006) Physics of active galactic nuclei at all scales.

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