Adv. Space Res. Vol.3, No.10-12, pp.175-180, 1984 0273-1177/84 $0.00 + .50 Printed in Great Britain. All rights reserved. Copyright~ CoSPAR OPTICAL AND X-RAY ASPECTS OF QUASARS G. Setti* and G. Zarnorani** *European Southern Observatory, Garching bei Manchen, F.R.G.. **Istituto di Radioastronomia, C.N.R., Bologna, Italy. ABSTRACT We review the present knowledge on the cosmological evolution of quasars, by discussing some of the recent results obtained from studies of optically selected objects. Despite the fast development of prism survey tecniques, the color selection still appears to be the best tecnique for constructing the complete samples which are necessary for statistical studies. It is shown, however, that even the best available complete samples of quasars selected on the basis of ultraviolet excess ( z ~ 2.2 ) are not sufficient to univocally determine the "correct" evolutionary model. Moreover, some preliminary results suggest that the evolution law derived from quasars with m_ < 20 and z <2.2 carl not be extrapolated to fainter magnitudes and higher redshifts. On the Basis of what is known today about the optical and X-ray properties of quasars, we then discuss some of the possible results, relevant to cosmology, which can be achieved with future coordinated optical and X-ray observations of quasars. KEYWORDS Quasars; cosmological evolution; optical and X-ray observations. INTRODUCTION In this paper, we will first briefly discuss a few selected topics related to optical studies of quasars, like optical counts, redshift distribution and luminosity function, which are the main building blocks in our understanding of various aspects of the cosmological evolution of quasars (Section 2). For an extensive coverage of this subject we refer the reader to the recent papers by Woltjer and Setti (1982), Schmidt and Green (1983) and Veron (1983). In Section 3, on the basis of what is known today about the optical and X-ray properties of quasars, we will discuss some of the possible results, relevant to cosmology, which can be achieved with future coordinated optical and X-ray observations of quasars. QUASARS AND COSMOLOGY Under the assumption that the redshift is cosmological and that the Universe can be described by a standard Friedmann model, there is general agreement among astronomers that quasars, as a class, show some kind of cosmological evolution. In fact, the steep slope of the optical counts and the results of the V/V analyses of both radio and X-ray selected quasars suggest max that evolution is a general feature of quasars, independent of the selection criteria. However, this general agreement quickly disappears as soon as one tries to better define the meaning of this evolution. In fact, an even partial inspection of the literature reveals that almost each astronomer working in the field has his own pet evolution. The most popular, and relatively simple, evolution laws are: a) pure density evolution (Schmidt, 1968); b) pure luminosity ewolution (Mathez, 1976; Marshall and others, 1983); c) combined (density and luminosity) evolution (Cheney and Rowan-Robinson, 1981; Marshall and others, 1983; Koo, 1983); 175