CARS: The CFHTLS-Archive-Research Survey III. First detection of cosmic magnification in samples of normal high-z galaxies

TL;DR

This paper reports the first detection of cosmic magnification using high-redshift Lyman-break galaxies from CFHTLS, confirming theoretical predictions and demonstrating the potential of magnification as a cosmological probe.

Contribution

It presents the first observational detection of cosmic magnification in normal high-z galaxies, using a novel cross-correlation method with foreground galaxies from CFHTLS data.

Findings

Cosmic magnification detected with high significance.

The observed correlation matches theoretical WL predictions.

Anti-correlation observed between faint LBGs and foreground galaxies.

Abstract

Weak gravitational lensing (WL) has been established as one of the most promising probes of cosmology. So far, most studies have exploited the shear effect of WL leading to coherent distortions of galaxy shapes. But WL also introduces coherent magnifications. We want to detect this cosmic magnification effect in large samples of high-redshift galaxies selected from the Deep part of the CFHTLS. Lyman-break galaxies (LBGs) at z=2.5-5, are used as a background sample and are cross-correlated to foreground lens galaxies selected by photo-z's. The signals of LBGs in different magnitude bins are compared to predictions from WL theory. An optimally weighted correlation function is estimated by taking into account the slope of external LBG luminosity functions. For the first time, we detect cosmic magnification in a sample of normal galaxies. These background sources are also the ones with the highest redshifts so far used for WL measurements. The amplitude and angular dependence of the cross-correlation functions agree well with theoretical expectations and the lensing signal is detected with high significance. In particular, we detect an anti-correlation between faint LBGs and foreground galaxies which cannot be caused by redshift overlap. (abridged)