Abstract

Much effort has gone into analysing the now well-known luminosity evolution of the quasar population. Theoretical models, such as the accretion disk model, however, predict a spectral evolution which is more complicated than a simple overall luminosity shift [3], and more detailed spectral information is necessary to compare physical models with the data. We investigate the distribution and evolution of quasars on the color-flux plane in order to test for consistency with the thin accretion disk model.

Introduction

Much effort has gone into analysing the now well-known luminosity evolution of the quasar population. Theoretical models, such as the accretion disk model, however, predict a spectral evolution which is more complicated than a simple overall luminosity shift [3], and more detailed spectral information is necessary to compare physical models with the data. We investigate the distribution and evolution of quasars on the colour-flux plane in order to test for consistency with the thin accretion disk model.

Quasar Colours

It has been known since the first quasar surveys that active galactic nuclei (AGN) occupy a specific region of the two colour diagram, separate from most other galactic and extragalactic sources. Consequently, UBV photometry has been used to select AGN in numerous surveys up to a redshift z ∼ 2.2 where the Lα emission line and absorption edge are redshifted into the observed bands.