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. The QSO spectra were extracted from two complete colorselected samples in the magnitude interval m_B ≈ 15–20. The analysis is performed by fitting power-law continua f_ν ∝ ν^α in well-defined rest-frame wavelength intervals after removing regions of the spectrum affected by strong emission lines or weak emission bumps. The average slope in the rest-frame optical-UV region 1200–5500 Å shows a rapid change around the 3000 Å emission bump, with α ≃ 0.15 longward of it and α ≃ -0.65 at shorter wavelengths. Although these average slopes were obtained using spectra of QSOs with different luminosities and redshifts, there are no significant correlations of the average spectral index with these quantities. For a few QSOs in the sample, we were able to measure the same softening of the spectral shape within the individual spectrum. These results have significant consequences on the estimate 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 r bands are computed. The derived average spectral shape in the optical-UV band places interesting constraints on the expected emission mechanisms.