Quasar spectra and the K correction

A new determination of the optical K(z) relation for QSOs is presented, derived from multiwavelength photometry of 25 low-redshift QSOs. The relation deviates strongly from the simple picture of single power law spectra as an overall description, but also from previous empirical determinations based on composite spectra. The deviations are particularly significant at low redshifts where K(z) drops steeply, bending over towards a flatter slope around z ≈ 0.6. The shape of K(z) does not show any discernible dependency on either QSO luminosity or radio power. There is evidence that the derived relation might also be representative for QSOs at higher redshift up to z ' 2. Under that hypothesis, the resulting rest-frame absolute magnitudes of QSOs are systematically fainter than usually estimated up to now. The inferred global evolution of the optical luminosity function of quasars is substantially reduced. This modification may reconcile the hitherto discrepant evolution rates of optical and X-ray luminosity functions.