The continuum slopes and evolution of active galactic nuclei

Abstract

Studies of the evolution of active galactic nuclei (AGNs) have mostly used a single continuum slope to compute k-corrections. In this paper it is shown that allowing for the observed diversity of AGN continuum shapes greatly affects measurements of both the continuum slopes and number evolution. Less evolution is required between redshift two and the present, and a spurious correlation between continuum slope and redshift introduced in magnitude-limited samples. Two reported correlations between continuum slope and redshift are shown to be consistent with this spurious origin. However, observations of QSOs from the Large Bright QSO Survey show that luminous highredshift QSOs have intrinsically harder optical-UV continuum slopes. It is concluded that this is an evolutionary effect and not a correlation with luminosity.