The 2dF-SDSS LRG and QSO Survey: the QSO luminosity function at 0.4<z<2.6

TL;DR

This study presents a detailed QSO luminosity function covering a wide range of magnitudes and redshifts, revealing the evolution and downsizing of quasars with unprecedented precision.

Contribution

It combines large survey data to produce the most precise QSO luminosity function across a broad magnitude and redshift range, highlighting significant departures from pure luminosity evolution.

Findings

The luminosity function extends 2.5 magnitudes fainter than previous surveys.

Clear evidence of QSO downsizing with faint QSOs peaking at lower redshift.

The combined data constrains both bright and faint ends of the QSO luminosity function.

Abstract

We present the QSO luminosity function of the completed 2dF-SDSS LRG and QSO (2SLAQ) survey, based on QSOs photometrically selected from Sloan Digital Sky Survey imaging data and then observed spectroscopically using the 2dF instrument on the Anglo-Australian Telescope. We analyse 10637 QSOs in the redshift range 0.4<z<2.6 to a g-band flux limit of 21.85 (extinction corrected) and an absolute continuum magnitude of Mg(z=2)<-21.5. This sample covers an area of 191.9 deg^2. The binned QSO luminosity function agrees with that of the brighter SDSS main QSO sample, but extends ~2.5 mags fainter, clearly showing the flattening of the luminosity function towards faint absolute magnitudes. 2SLAQ finds an excess of QSOs compared to the 2dF QSO Redshift Survey at g>20.0, as found previously by Richards et al. (2005). The luminosity function is consistent with other previous, much smaller, samples produced to the depth of 2SLAQ. By combining the 2SLAQ and SDSS QSO samples we produce a QSO luminosity function with an unprecedented combination of precision and dynamic range. With this we are able to accurately constrain both the bright and faint ends of the QSO LF. While the overall trends seen in the evolution of the QSO LF appear similar to pure luminosity evolution, the data show very significant departures from such a model. Most notably we see clear evidence that the number density of faint QSOs peaks at lower redshift than bright QSOs: QSOs with Mg>-23 have space densities which peak at z<1, while QSOs at Mg<-26 peak at z>2. By fitting simple luminosity function models in narrow Mg intervals we find that this downsizing is significant at the 99.98 per cent level (abridged).