ShapePipe: A new shape measurement pipeline and weak-lensing application to UNIONS/CFIS data

Abstract

Context. The Ultraviolet Near-Infrared Optical Northern Survey (UNIONS) is an ongoing collaboration that will provide the largest deep photometric survey of the northern sky in four optical bands to date. As part of this collaboration, the Canada-France Imaging Survey (CFIS) is observing r-band data with an average seeing of 0.65 arcsec, which is complete to magnitude 24.5 and thus ideal for weak-lensing studies. Aims. We perform the first weak-lensing analysis of CFIS r-band data over an area spanning 1 700 deg 2 of the sky. We create a catalogue with measured shapes for 40 million galaxies, corresponding to an effective density of 6.8 galaxies per square arcminute, and demonstrate a low level of systematic biases. This work serves as the basis for further cosmological studies that will use the full UNIONS survey of 4 800 deg 2 when completed. Methods. Here we present ShapePipe, a newly developed weak-lensing pipeline. This pipeline makes use of state-of-the-art methods such as Ngmix for accurate galaxy shape measurement. Shear calibration is performed with metacalibration. We carry out extensive validation tests on the point spread function (PSF) and on the galaxy shapes. In addition, we create realistic image simulations to validate the estimated shear. Results. We quantify the PSF model accuracy and show that the level of systematics is low as measured by the PSF residuals. Their effect on the shear two-point correlation function is sub-dominant compared to the cosmological contribution on angular scales < 100 ′ . The additive shear bias is below 5 × 10 −4 , and the residual multiplicative shear bias is at most 10 −3 as measured on image simulations. Using complete orthogonal sets of E-/B-mode integrals (COSEBIs), we show that there are no significant B-modes present in second-order shear statistics. We present convergence maps and see clear correlations of the E-mode with known cluster positions. We measure the stacked tangential shear profile around Planck clusters at a significance higher than 4σ.

UNIONS: The impact of systematic errors on weak-lensing peak counts

“…MegaPipe provides an astrometric calibration of the survey using Gaia Data Release 2 (Gaia Collaboration 2018), and photometric calibration of the r-band data relative to the Pan-STARRS1 survey (Chambers et al 2016). Both astrometric and photometric calibration are excellent so we do not expect calibration errors to significantly impact weak-lensing measurements (Guinot et al 2022).…”

“…Those measurements are calibrated with metacalibration (Huff & Mandelbaum 2017) to provide shear estimates. The creation and validation of the shear catalog are fully described in Guinot et al (2022). We note that this is a preliminary version, called "version 0", of the catalog where the source density is conservative and does not reflect future versions of the CFIS shear data.…”

“…For both methods, CFIS galaxies were matched to the deeper CFHTLenS on the 50 deg 2 W3 field. First, Guinot et al (2022) approximated the CFIS redshift distribution as the histogram of the best-fit photometric redshifts from CFHTLenS of that matched subsample. Photo-z's for CFHTLenS had been obtained in Hildebrandt et al (2012) (Newman et al 2013) in a 5D space spanned by the u, g, i, r, z photometric bands, to match the density of the matched subsample in that space.…”

“…The CFIS-P3 map is quite large for a projection but peaks are local, and any distortion will rather affect large scales. In our analysis we worked with the E-mode of the convergence map as the B-mode contain mostly noise (Guinot et al 2022). Hereafter, when we speak about convergence, we mean the E-mode of the convergence map.…”

Potential scientific synergies in weak lensing studies between the CSST and Euclid space probes

“…To obtain photo-zs with the required accuracies, the Euclid needs external multi-band optical data (e.g. Rhodes et al 2017;Euclid Collaboration et al 2020b;Guinot et al 2022). For shear measurements using the broad VIS band, the wavelength-dependent point-spread function (PSF) and the differences between the spectral energy distribution (SED) of stars and galaxies can lead to significant biases for galaxy shear measurements (e.g.…”