CARS: The CFHTLS-Archive-Research Survey

Free access article

Issue		A&A Volume 507, Number 2, November IV 2009


Page(s)		683 - 691
Section		Cosmology (including clusters of galaxies)
DOI		http://dx.doi.org/10.1051/0004-6361/200912655
Published online		24 September 2009

A&A 507, 683-691 (2009)
DOI: 10.1051/0004-6361/200912655

III. First detection of cosmic magnification in samples of normal high-z galaxies

H. Hildebrandt¹, L. van Waerbeke², and T. Erben³

¹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333CA Leiden, The Netherlands
e-mail: hendrik@strw.leidenuniv.nl
² University of British Columbia, Department of Physics and Astronomy, 6224 Agricultural Road, Vancouver, B.C. V6T 1Z1, Canada
³ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany

Received 8 June 2009 / Accepted 15 September 2009

Abstract
Context. 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. WL also introduces coherent magnifications.
Aims. We want to detect this cosmic magnification effect (coherent magnification by the large-scale structure of the Universe) in large samples of high-redshift galaxies selected from the Deep part of the Canada-France-Hawaii-Telescope Legacy Survey (CFHTLS).
Methods. Lyman-break galaxies (LBGs) selected by their colours to be at z= 2.5–5, are used as a background sample and are cross-correlated to foreground lens galaxies, which are selected by accurate photometric redshifts (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.
Results. 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. Avoiding low-redshift ranges in the foreground samples which might contaminate the LBG samples we can make a measurement that is virtually free of systematics. In particular, we detect an anti-correlation between faint LBGs and foreground galaxies which cannot be caused by redshift overlap.
Conclusions. Cross-correlating LBGs (and in future also photo-z selected galaxies) as background sources to well understood foreground samples based on accurate photo-z's will become a powerful cosmological probe in future large imaging surveys.

Key words: large-scale structure of Universe -- cosmology: observations -- dark matter -- cosmological parameters

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