Universit´ede Li`ege Facult´edes Sciences D´epartement d’Astrophysique, G´eophysique et Oc´eanographie AKs–band selected, multi–wavelength survey for quasars in the XMM–LSS field Theodoros Nakos Doctoral Dissertation January 2007 Pr. Jean–Pierre Swings President of the jury Pr. Jean Surdej Thesis co–Adviser Dr.JonWillis Thesisco–Adviser Dr. Jean–Fran¸cois Claeskens Examiner Dr. Damien Hutsem´ekers Examiner Dr. Gr´egor Raw Examiner Dr. Stefano Andreon External Examiner Pr. Herwig Dejonghe External Examiner ii iii Caminante, son tus huellas el camino, y nada mas; caminante, no hay camino, se hace camino al andar. Antonio Machado Canto XXIX, Proverbios y cantares, Campos de Castilla, 1917 Marcheur, ce sont tes traces ce chemin, et rien de plus; marcheur, il n’y a pas de chemin, le chemin se construit en marchant. iv Acknowledgments I would like to express my heartfelt thanks to all who contributed in any way to this work. First of all my thesis co–adviser, Jean Surdej, for his enthusiasm, support and stimulating interaction. A great thanks also goes to Jon Willis, who offered me the main bulk of data on which this work is based and provided valuable advice on many “technical” and scientific issues. Stefano Andreon should also be acknowledged, for providing me part of the material that played a major role in this work and for his suggestions during the course of our collaboration. Starting from my home Institute, there are numerous persons from the University of Li`ege I am grateful to. First of all to Jean–Pierre Swings, for his advices concerning both administrative and scientific matters. An enormous thanks goes to Pierre Riaud, for all the things he taught me, the hours he spent discussing with me and for creating such an amazing atmosphere at the Institute. Olivier Garcet should also be acknowledged, for the productive discussions concerning the properties of the X–ray data and the work on the photometric redshifts. I am also indebted to Jean-Fran¸cois Claeskens, whom I have been “bombarding” with questions since so many years. I have learned a lot from him and I would like to express my gratitude once more. Finally, many thanks also go to Denise, Damien, Eric and Frederic for their advices and solutions regarding practical, administrative or scientific issues. Going back in time, I would like to acknowledge the European Southern Observatory for the ESO studentship and especially Danielle Alloin, who supervised my work during my two–year stay at Santiago, as well as Alain Smette and Poshak Gandhi for the fruitful discussions. Since I spent the early years of my thesis at the Royal Observatory of Belgium, I ought special thanks to a bunch of people there. First of all to Edwin van Dessel, who had been (and still is) supporting me for so many years. Many other colleagues, namely Marijke Burger, Patricia Lampens, David Duval, Jan Cuypers and Henri Boffin, should also be mentioned. Finally, the ex–director of the Observatory, P. Pˆaquet, has to be acknowl- edged, for his strong commitment in supporting me regarding financial and administrative issues. There is a couple of persons I would also like to acknowledge from the National Ob- servatory of Athens. First of all Dimitri Sinachopoulos, for being an excellent teacher and v vi a very good friend. I learned a lot thanks to him and I will always feel grateful. Secondly Panos Boumis, who contributed so much during the preliminary stages of my adventure in astronomy and, by now, ended up being a very good friend. Finally, I would also like to thank Evanthia Hatziminaoglou for sharing with me her expertise during my two–week visit to Tenerife, for collaborating on the last chapter of the thesis and for running the code for modeling the infrared properties of the AGN in my sample. Leaving the scientific environment, there are tens of persons who have not really contributed much that can be cited in this work, but they have surely made my life much more beautiful and helped me overcome the difficult moments. First of all my Greek friends in the homeland and abroad. They are numerous and I can not name them all. Nevertheless, special thanks should go to one of them, Foris, without whom things would surely be very, very different...Of course, Maria, Christina, Dimitris and Yiorgos also played a great role in this. Credits should also go to all the friends with whom I shared so many wonderful moments in Chile: “Daniel–son”, Nuri, Claudito & Lu, Gael, Jon–Jon, Ivo and all the other “chiquillos”. Last, but surely not least, all my love and gratitude go to the persons who have been closest to me during all these years. My parents and my sister, for their everlasting love, support and understanding. Time has not only made us older, but wiser, and we have benefited so much from this... My daughter, Carolina, who never lets me forget that the happiness in life is found in the most simple things. Finally, Alejandra, for her unconditional love, who made me see the world with different eyes. This thesis is dedicated to them, they all deserve it... Thodori Nakos Preface The phenomenon of Active Galactic Nuclei (AGN) is still puzzling astronomers, despite the fact that the very first discovery of an active galaxy took place a century ago (in 1908, to be more precise). The term AGN refers to a special type of galaxies, whose radiation is mainly produced by non–thermal processes in a small, compact region in the nucleus of the galaxy. Astronomers are now confident that this region, of the size of our solar system, consists of a super–massive black hole, constantly fed with matter from an accretion disk surrounding it. The AGN “zoo” is populated by BL Lac objects, Type–1 and Type–2 quasars and Seyfert galaxies, etc. Although the AGN classification is mainly based on the amount of radiation emitted by the compact central source, the distinction between them is to some degree a matter of definition. The numerous sub–categories into which AGN have been classified is a self–proof on how incomplete is our knowledge regarding the processes taking place in the active nucleus. The “holy grail” for AGN astronomers, a unified model that will explain, using simple geometrical and physical solutions, the different features we observe in AGN, is far away from complete. The quasars (quasi–stellar objects, or QSOs), one of the two main sub–categories of AGN, are among the most luminous and distant objects. The light of the most distant quasars registered on our detectors was emitted when the Universe was only a fraction of its current age. Thanks to their high luminosities, quasars serve as cosmological light- houses: even at the far edges of the Universe there are still some shinning, and capturing their light traces back to the very early cosmic history. Once the importance of quasars was recognized, numerous QSO surveys were initiated for their discovery. However, their identification had to be based on some selection criteria. Objects not respecting these criteria were excluded, thus introducing a bias in the selected sample. The most striking example is the coincidental discovery of the first QSOs, as radio sources, in the late 1950s. We now know that the percentage of “radio–loud” quasars is only 5 10% of the total QSO known population. Working on sub–samples of the parent population∼ − prevents us from building up a single model that can successfully describe the features we observe in the various sub–classes. For this reason it is essential to understand the selection effects related to each survey and how the observed populations fit in a more general AGN context. vii viii One of the most commonly used optical techniques for the discovery of quasars is the so–called Ultra–Violet excess (UVX), implemented for the first time in the mid 1960s (Sandage 1965), and more systematically in the early 1980 (Schmidt & Green 1983). This technique is based on the fact that, in the (U B) versus (B V ) color–color plane, quasars occupy a locus different from that of stars,− because of their− bluer colors. These colors are attributed (a) to the quasar blue continuum, and (b) to the Lya emission line (1216A),˚ which, for QSOs found until a given redshift limit, enters the U–band. The UVX method works quite well up to redshifts z 2. At higher redshifts, however, the ≈ U B color starts becoming less effective in isolating quasars, and for redshifts higher than− 2.2, when the Lya line is shifted to wavebands redder than the U–filter, the method fails in detecting high redshift quasars. As mentioned before, each survey suffers from selection effects. Understanding the properties of a sample, selected using specific criteria, is fundamental for properly de- scribing the parent population. Because of the filter combination, the UVX favors the selection of blue quasars. Hence, intrinsically red, and reddened, due to dust, QSOs are not selected by UVX. As a result, the question whether we are missing a “hidden” quasar population has been troubling astronomers since about a decade (Webster et al. 1995). A new approach to tackle the weak points of the UV excess was introduced by Warren − et al. (2000). The K–excess (KX), similar in its concept to the UVX, suggested to use infrared filters (among which the K–band, at 2.2 µm) for detecting high z, red and dusty QSOs. The quasar spectral energy distribution∼ (SED) is described by a− power–law, while stellar SEDs have a convex shape.