Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: luminosity and redshift scalings and implications for weak lensing surveys

TL;DR

This work characterizes the GI intrinsic-alignment contamination in weak lensing by measuring the density–intrinsic shear correlation $w_{g+}(r_p)$ across SDSS Main, SDSS LRG, and 2SLAQ LRG samples. It finds strong GI signals for red, luminous galaxies with a scale dependence $w_{\delta+}(r_p) \propto r_p^{\alpha}$ where $\alpha \approx -0.7$ and a redshift evolution parameter $\gamma$ near the passive value, while blue galaxies show little to no signal. The authors provide empirical fits tying GI amplitude to luminosity and redshift, and translate these into projections of cosmic-shear contamination, showing potentially significant biases on $\sigma_8$ if unaccounted for. They propose a practical, probabilistic marginalization framework to incorporate GI uncertainties into current and near-future cosmic shear analyses, while highlighting the need for better understanding of blue-galaxy GI and of small-scale behavior. The results offer concrete scaling relations and contamination models to guide mitigation strategies in lensing surveys.

Abstract

Correlations between intrinsic shear and the density field on large scales, a potentially important contaminant for cosmic shear surveys, have been robustly detected at low redshifts with bright galaxies in SDSS data. Here we present a more detailed characterization of this effect, which can cause anti-correlations between gravitational lensing shear and intrinsic ellipticity (GI correlations). This measurement uses 36278 Luminous Red Galaxies (LRGs) from the SDSS spectroscopic sample with 0.15<z<0.35, split by redshift and luminosity; 7758 LRGs from the 2SLAQ Survey at 0.4<z<0.8; and a variety of other SDSS samples from previous, related work. We find >3sigma detections of the effect for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ sample, we find a 2sigma detection for a bright subsample, and no detection for a fainter subsample. Fitting formulae are provided for the scaling of the GI correlations with luminosity, transverse separation, and redshift. We estimate contamination in the measurement of sigma_8 for future cosmic shear surveys on the basis of the fitted dependence of GI correlations on galaxy properties. We find contamination to the power spectrum ranging from -1.5 (optimistic) to -33 per cent (pessimistic) for a toy cosmic shear survey using all galaxies to a depth of R=24 using scales l~500. This corresponds to a bias in sigma_8 of Delta sigma_8=-0.004 (optimistic), -0.02 (central), or -0.10 (pessimistic). We provide a prescription for inclusion of this error in cosmological parameter estimation codes. The principal uncertainty is in the treatment of the L<=L* blue galaxies. Characterization of the tidal alignments of these galaxies, especially at redshifts relevant for cosmic shear, should be a high priority for the cosmic shear community. (Abridged)

Figures (11)

• Figure 1: The GI correlation functions for the SDSS Main subsamples, split into colour and luminosity bins.
• Figure 2: Confidence contours for power-law fits to $w_{g+}(r_p)$ for Main sample galaxies. Contours are shown for various subsets of data labelled on the plots; in each case, $1\sigma$, $2\sigma$, and $3\sigma$ contours are shown.
• Figure 3: The density-shape correlation function $w_{g+}(r_p)$ from $1$--$60$$h^{-1}\,$Mpc with the full spectroscopic LRG sample and various subsamples as labeled on the plot. Errors are $1\sigma$ but are somewhat correlated on large scales.
• Figure 4: Confidence contours for power-law fits to $w_{g+}(r_p)$ for SDSS spectroscopic LRGs. Contours are shown for various subsets of data labelled on the plots; in each case, $1\sigma$, $2\sigma$, and $3\sigma$ contours are shown.
• Figure 5: A comparison of the $r$-band absolute magnitudes and colours of the SDSS and 2SLAQ LRG samples. The upper left panel shows the absolute magnitudes as a function of redshift. The upper right shows the distribution of $g-i$ rest frame colour, and the lower right shows the absolute magnitude histograms over the full samples.