HerMES: detection of cosmic magnification of sub-mm galaxies using angular cross-correlation

TL;DR

This study detects cosmic magnification effects on sub-mm galaxies by analyzing their angular cross-correlation with low-redshift galaxy samples, providing robust evidence of gravitational lensing influence in large-scale structure.

Contribution

First robust detection of cosmic magnification of sub-mm galaxies through angular cross-correlation using Herschel data and ancillary low-redshift samples.

Findings

Detected cross-correlation on scales of 1-50 arcmin.

Evidence that the correlation is mainly due to cosmic magnification.

Possible contribution from intrinsic clustering due to redshift overlaps.

Abstract

Cosmic magnification is due to the weak gravitational lensing of sources in the distant Universe by foreground large-scale structure leading to coherent changes in the observed number density of the background sources. Depending on the slope of the background source number counts, cosmic magnification causes a correlation between the background and foreground galaxies, which is unexpected in the absence of lensing if the two populations are spatially disjoint. Previous attempts using submillimetre (sub-mm) sources have been hampered by small number statistics. The large number of sources detected in the {\it Herschel} Multi-tiered Extra-galactic Survey (HerMES) Lockman-SWIRE field enables us to carry out the first robust study of the cross-correlation between sub-mm sources and sources at lower redshifts. Using ancillary data we compile two low-redshift samples from SDSS and SWIRE with <z> ~ 0.2 and 0.4, respectively, and cross-correlate with two sub-mm samples based on flux density and colour criteria, selecting galaxies preferentially at z ~ 2. We detect cross-correlation on angular scales between ~1 and 50 arcmin and find clear evidence that this is primarily due to cosmic magnification. A small, but non-negligible signal from intrinsic clustering is likely to be present due to the tails of the redshift distribution of the sub-mm sources overlapping with those of the foreground samples.