An investigation into quasar downsizing

Quasar downsizing was investigated using a sample of 832,521 quasars with redshifts between 0.400 and 3.315, from the Million Quasars catalogue. The sample was divided into bins, and in each bin two reference quasars were identified: the ‘99th percentile most powerful’ quasar and the ‘mode’ quasar, as determined from their absolute optical magnitudes. For three binning methods, absolute optical magnitude was plotted against both redshift and distance for these reference quasars, revealing a linear relationship between absolute optical magnitude and redshift (or distance). The 99th percentile most powerful quasar gave a tighter fit than the mode quasar. Of the other parameter choices, redshift yielded better results than distance, and the preferred binning method used approximately 8,000 quasars per bin. The following relationship was derived between the absolute optical magnitude of the 99th percentile most powerful quasar, Mg(99%), and redshift, z: Mg(99%) = (−1.19 ± 0.07) · z − 24.69 ± 0.11.