We have reviewed the main spectral properties of quasars, with an emphasis on continuum emission and a brief treatment of the main emission and absorption features.
In the first part, we built an updated SED of quasars, mainly based on continuum emission of local (z < 0.4) PG quasars. The main improvements with respect to the previous reference work in the field, the atlas of Elvis et al. (1994), are: 1) the treatment of selection effects in the X-rays, which allowed us to estimate the correct optical-to-X-ray flux ratio for local quasars, 2) the extension of the SED in the UV at wavelengths shortward of the Lyman break, using HST observations of quasars, 3) the inclusion of the new IR data obtained with ISO, and 4) the use of a larger sample in the computation of the average spectral properties of quasars.
In the second part, we reviewed the main spectral properties of obscured quasars. We also discussed the relation between dust and gas absorption, briefly describing the cases of X-ray obscured objects with no or little optical/near-IR absorption.
In the third and final part, we summarized the methods for finding obscured AGN in the universe, focusing on the problem of disentangling the active nucleus component in infrared luminous sources and disucssing the still open problems. Despite the improvements of the last few years, what we know is most likely only a fraction of the obscured quasars present in the universe.
Acknowledgments
We wish to thank Amy Barger for giving us the opportunity to contribute to this book, and for her really infinite patience with our very long delays in preparing the manuscript.