The Faint End of the z=5 Quasar Luminosity Function from the CFHTLS

TL;DR

This study identifies and characterizes faint quasars at redshift 5, extending the luminosity function measurements and assessing their role in cosmic reionization, with implications for the faint end slope and Lyα emission properties.

Contribution

It provides the first spectroscopic survey of faint z=5 quasars over a large area, extending the luminosity function 1.5 magnitudes fainter than previous work.

Findings

Faint end slope of the luminosity function is not steep.

Faint quasars contribute minimally to hydrogen-ionizing background at z=5.

Observed Lyα emission from faint quasars is weaker than at z~3.

Abstract

We presents results from a spectroscopic survey of $z\sim5$ quasars in the CFHT Legacy Survey (CFHTLS). Using both optical color selection and a likelihood method we select 97 candidates over an area of 105 deg$^2$ to a limit of $i_{\rm AB} < 23.2$, and 7 candidates in the range $23.2 < i_{\rm AB} < 23.7$ over an area of 18.5 deg$^2$. Spectroscopic observations for 43 candidates were obtained with Gemini, MMT, and LBT, of which 37 are $z>4$ quasars. This sample extends measurements of the quasar luminosity function $\sim$1.5 mag fainter than our previous work in SDSS Stripe 82. The resulting luminosity function is in good agreement with our previous results, and suggests that the faint end slope is not steep. We perform a detailed examination of our survey completeness, particularly the impact of the Ly$\alpha$ emission assumed in our quasar spectral models, and find hints that the observed Ly$\alpha$ emission from faint $z\sim5$ quasars is weaker than for $z\sim3$ quasars at a similar luminosity. Our results strongly disfavor a significant contribution of faint quasars to the hydrogen-ionizing background at $z=5$.