THE OPTICAL-ULTRAVIOLET CONTINUUM OF A SAMPLE OF QSOS

The average optical-UV continuum shape of QSOs is investigated, using spectra of 62 QSOs that have good relative photometric calibrations. T he QSO spectra were extracted from two complete color-selected samples in the magnitude interval m(B) approximate to 15-20. The analysis is performed by fitting power-law continua f(v) proportional to v(alpha) in well-defined rest-frame wavelength intervals after removing regions of the spectrum affected by strong emission lines or weak emission bu mps. The average slope in the rest-frame optical-UV region 1200-5500 A ngstrom, shows a rapid change around the 3000 Angstrom emission bump, with alpha similar or equal to 0.15 longward of it and alpha similar o r equal to -0.65 at shorter wavelengths. Although these average slopes were obtained using spectra of QSOs with different luminosities and r edshifts, there are no significant correlations of the average spectra l index with these quantities. For a few QSOs in the sample, we were a ble to measure the same softening of the spectral shape within the ind ividual spectrum. These results have significant consequences on the e stimate of the cosmological evolution of the optically selected QSOs, as they affect, for instance, the k-corrections. New k-corrections in the BVR and Gunn I bands are computed. The derived average spectral sh ape in the optical-UV band places interesting constraints on the expec ted emission mechanisms.