Detection of large-scale intrinsic ellipticity--density correlation from the Sloan Digital Sky Survey and implications for weak lensing surveys

Abstract

The power spectrum of weak lensing shear caused by large‐scale structure is an emerging 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 [the intrinsic ellipticity—intrinsic ellipticity (II) correlation] that could mimic the correlations due to lensing. A related possibility pointed out by Hirata & Seljak is correlation between the intrinsic ellipticities of galaxies and the density field responsible for gravitational lensing shear [the gravitational shear—intrinsic ellipticity (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 none the less statistically consistent with recent detections found using the SuperCOSMOS 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 σ8 with ∼ 10 per cent accuracy, but may still be an issue for future surveys with projected statistical errors below the 1 per cent level unless eliminated using photometric redshifts. In contrast, we have a clear detection of GI correlation in galaxies brighter than L* that persists to the largest scales probed (60 h−1 Mpc) and with a sign predicted by theoretical models. This correlation could cause the existing lensing surveys at z∼ 1 to underestimate 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 cannot be excluded at this point. We propose that cosmic shear surveys should consider rejection of BCGs from their source catalogues 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.

Observational probes of cosmic acceleration

“…This was a clear signal of contamination of non-cosmological origin, probably PSF correction residuals. Also, intrinsic alignments of galaxies were detected at high significance even in the linear regime, at a level that represented a potentially serious systematic error even for then-ongoing surveys (Mandelbaum et al, 2006).…”

“…Note that they predict only E-mode intrinsic alignments, because the alignments are linearly sourced by a scalar field. 58 Observations of LRGs in the SDSS have shown that the galaxy-ellipticity correlation 59 w ge (r p ) has the same power-law slope as the galaxy correlation function w g (r p ) ∝ r −0.7 (Mandelbaum et al, 2006;Hirata et al, 2007), with an amplitude that increases rapidly with LRG luminosity. This is a quantitative success of the linear model.…”

“…Yoo and Seljak (2012) provide an extensive discussion of the cosmological constraints that can be derived from the combination of GGL and galaxy clustering, on small and large scales, in the simplified case where one isolates the population of central galaxies, so that there is one galaxy per dark matter halo. Cluster-galaxy lensing is similar to GGL, but one takes clusters of galaxies rather than individual galaxies as the reference points (Mandelbaum et al, 2006;Sheldon et al, 2009). We will discuss this idea further in §6, arguing that it offers the most reliable route to calibrating cluster mass-observable relations and has the potential to sharpen cosmological parameter constraints significantly.…”

Contamination of early-type galaxy alignments to galaxy lensing–CMB lensing cross-correlation

“…While lensing by the intervening matter creates a tangential alignment of background source galaxies around the lens (Kaiser 1992), tides have the opposite effect, typically leading to a radial alignment of sources towards overdensities. Alignments across large separations were first measured by Brown et al (2002), with a subsequent confirmation by Mandelbaum et al (2006). This signal was shown to arise from the alignment of luminous red galaxies (Hirata et al 2007;Joachimi et al 2011;Singh, Mandelbaum & More 2014), while it has not been detected for late-type galaxies (e.g.…”

Intrinsic alignments of galaxies in the MassiveBlack-II simulation: analysis of two-point statistics

“…Here, we follow the notation of Mandelbaum et al (2006) to give formulae for the calculation of galaxy-intrinsic shear correlation function (ξg+(rp, Π)) and the projected statistic, wg+. Here, rp is the comoving transverse separation of a pair of galaxies in the XY plane and Π is their separation along the Z direction.…”