TITLE:
Detection of large scale intrinsic ellipticity-density correlation from the Sloan Digital Sky Survey and implications for weak lensing
surveys.
AUTHOR(S):
Rachel Mandelbaum (Princeton), Christopher M. Hirata (Princeton/IAS), Mustapha Ishak (Princeton/U Texas Dallas), Uros Seljak
(Princeton/ICTP), Jonathan Brinkmann (APO).
DATE:
2005 Sep 01 (arXiv, v1, posted); 2005 Sep 01 (MNRAS, submitted); 2005 Dec 01 (MNRAS, accepted);
2006 Feb 21 (MNRAS, published).
AVAILABILITY:
arXiv astro-ph/0509026 (free);
Blackwell Synergy (requires
subscription).
PUBLICATION INFORMATION: Mon. Not. R. Astron. Soc. 367, 611--626 (2006). 16 pages.
ABSTRACT:
The power spectrum of weak lensing shear caused by large-scale structure is an emerg-
ing tool for precision cosmology, in particular for measuring the effects of dark energy
on the growth of structure at low redshift. One potential source of systematic error is
intrinsic alignments of ellipticities of neighbouring galaxies (II correlation) that could
mimic the correlations due to lensing. A related possibility pointed out by Hirata
and Seljak (2004) is correlation between the intrinsic ellipticities of galaxies and the
density field responsible for gravitational lensing shear (GI correlation). We present
constraints on both the II and GI correlations using 265 908 spectroscopic galaxies
from the Sloan Digital Sky Survey (SDSS), and using galaxies as tracers of the mass in
the case of the GI analysis. The availability of redshifts in the SDSS allows us to select
galaxies at small radial separations, which both reduces noise in the intrinsic alignment
measurement and suppresses galaxy-galaxy lensing (which otherwise swamps
the GI correlation). While we find no detection of the II correlation, our results are
nonetheless statistically consistent with recent detections found using the SuperCOS-
MOS survey. Extrapolation of these limits to cosmic shear surveys at z~1 suggests
that the II correlation is unlikely to have been a significant source of error for current
measurements of s8 with ~10 per cent accuracy,
but may still be an issue for future surveys with projected statistical errors below the one per cent level. In contrast,
we have a clear detection of GI correlation in galaxies brighter than L* that persists
to the largest scales probed (60 h-1Mpc) and with a sign predicted by theoretical
models. This correlation could cause the existing lensing surveys at z~1 to underes-
timate the linear amplitude of fluctuations by as much as 20 per cent depending on
the source sample used, while for surveys at z~0.5 the underestimation may reach
30 per cent. The GI contamination is dominated by the brightest galaxies, possibly
due to the alignment of brightest cluster galaxies (BCGs) with the cluster ellipticity
due to anisotropic infall along filaments, although other sources of contamination can-
not be excluded at this point. We propose that cosmic shear surveys should consider
rejection of BCGs from their source catalogs as a test for GI contamination. Future
high precision weak lensing surveys must develop methods to search for and remove
this contamination if they are to achieve their promise.
ADS BIBLIOGRAPHIC CODE: 2006MNRAS.367..611M
COMMENTS: SDSS Publication #539.