The evolution of optically selected QSOs

We report here on a determination of the QSO luminosity function (LF) and its evolution with redshift based on a new catalogue of faint (B<20.9 mag) ultraviolet excess (UVX) QSOs with complete spectroscopic identification for almost 200 QSOs (Boyle et al. in preparation, hereafter BFSP). We find that the steep slope $$(d\,\text{log}\,N/dB\simeq 0.86)$$ exhibited by the number-magnitude relation for UVX QSOs flattens significantly $$(d\,\text{log}\,N/dB\simeq 0.32)$$ beyond B = 19.5 mag, with a UVX QSO surface density at B<20.9 mag of 36±4 per square degree. We demonstrate that this flattening corresponds to a feature in the QSO luminosity function at faint absolute magnitudes. By determining the redshift dependence of this feature we find that the evolution of QSOs at z<2.2 is most simply parameterized by a uniform increase in their luminosity with increasing redshift [$$L\propto (1+z)^{k},\,k=3.5\pm 0.3\,\text{for}\,\text{a}\,{q}_{0}=0.5$$ universe). Such evolution is found to be consistent with a model in which QSOs are long-lived ($$\simeq {10}^{10}\,\text{yr}$$), gradually dimming in luminosity from their epoch of formation at z>2.2 to the present day.