CFHQSIR: a Y-band extension of the CFHTLS-Wide survey

TL;DR

This paper presents the addition of Y-band data to the CFHTLS-Wide survey, enhancing the detection of high-redshift quasars and improving photometric redshift accuracy across 130 square degrees.

Contribution

It introduces the CFHQSIR Y-band survey extension, including data acquisition, reduction, calibration, and its integration with existing optical data for better redshift estimation.

Findings

Y-band data reaches a limiting magnitude of ~22.4 (5σ) for point sources.

Adding Y-band data improves photometric redshift accuracy from 86.2% to 91.2%.

Y-band number counts are consistent with the VIDEO survey.

Abstract

The Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) has been conducted over a five-year period at the CFHT with the MegaCam instrument, totaling 450 nights of observations. The Wide Synoptic Survey is one component of the CFHTLS, covering 155 square degrees in four patches of 23 to 65 square degrees through the whole MegaCam filter set (u*, g', r', i', z') down to i'$_{AB}$ = 24.5. With the motivation of searching for high-redshift quasars at redshifts above 6.5, we extend the multi-wavelength CFHTLS-Wide data in the Y-band down to magnitudes of $\sim$ 22.5 for point sources (5$\sigma$). We observed the four CFHTLS-Wide fields (except one quarter of the W3 field) in the Y-band with the WIRCam instrument at the CFHT. Each field was visited twice, at least three weeks apart. Each visit consisted of two dithered exposures. The images are reduced with the Elixir software used for the CFHTLS and modified to account for the properties of near-InfraRed (IR) data. Two series of image stacks are subsequently produced: four-image stacks for each WIRCam pointing, and one-square-degree tiles matched to the format of the CFHTLS data release. Photometric calibration is performed on stars by fitting stellar spectra to their CFHTLS photometric data and extrapolating their Y-band magnitudes. We measure a limiting magnitude of Y$_{AB} \simeq 22.4$ for point sources (5$\sigma$) in an aperture diameter of 0.93", over 130 square degrees. We produce a multi-wavelength catalogue combining the CFHTLS-Wide optical data with our CFHQSIR (Canada-France High-z quasar survey in the near-InfraRed) Y-band data. We derive the Y-band number counts and compare them to the VIDEO survey. We find that the addition of the CFHQSIR Y-band data to the CFHTLS optical data increases the accuracy of photometric redshifts and reduces the outlier rate from 13.8% to 8.8% in the redshift range 1.05 $\lesssim$ z $\lesssim$ 1.2.