Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: luminosity and redshift scalings and implications for weak 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 Sloan Digital Sky Survey (SDSS) data. Here we present a more detailed characterization of this effect, which can cause anticorrelations between gravitational lensing shear and intrinsic ellipticity (GI correlations). This measurement uses 36 278 luminous red galaxies (LRGs) from the SDSS spectroscopic sample with 0.15 < z < 0.35, split by redshift and luminosity; 7758 LRGs from the 2dF-SDSS LRG and QSO (2SLAQ) survey at 0.4 < z < 0.8; and a variety of other SDSS samples from previous, related work. We find >3σ detections of the effect on large scales (up to 60 h −1 Mpc) for all galaxy subsamples within the SDSS LRG sample; for the 2SLAQ sample, we find a 2σ 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 (for which the 2SLAQ sample, while small, provides crucial constraints due to its longer baseline in redshift). We estimate contamination in the measurement of σ 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 per cent (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, though the central value of predicted contamination is −6.5 per cent. This corresponds to a bias in σ 8 of σ 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, for which we have no detection of the GI signal, but which could dominate the GI contamination if their GI amplitude is near our upper limits. 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.

Observational probes of cosmic acceleration

“…However, one should be cautious about this argument for several reasons, most importantly because there has not yet been any quantitative observational confirmation of the scale and configuration dependence predicted by the quadratic model, and additionally because perturbation theory arguments show that the nonlinear evolution of the tidal field can generate a linear type alignment (Hui and Zhang, 2008). What is clear from observations is that the alignments of late-type galaxies on large scales, at least as measured by w ge (r p ), are consistent with zero and are certainly much less than for LRGs (Hirata et al, 2007;Mandelbaum et al, 2011).…”

“…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.…”

“…Detailed assessments of the intrinsic alignment contamination have been made on the basis of SDSS, 2SLAQ, and WiggleZ observations of w ge (r p ) (Hirata et al, 2007;Mandelbaum et al, 2011;Joachimi et al, 2011). These studies show that for surveys of modest depth (z med ∼ 0.7) the GI contamination may be up to several percent of the expected cosmic shear signal for late-type galaxies if it is near current upper limits, and it could be tens of percent for LRGs.…”

“…Progress was also made on astrophysical systematic errors. We learned that large-scale intrinsic galaxy alignments are strongest for luminous red galaxies (Hirata et al, 2007;Mandelbaum et al, 2011), and that the linear alignment model, once considered a crude analytical tool (Catelan et al, 2001), is in fact an excellent description of the observations of early-type galaxies at ≥ 10h −1 Mpc scales (Blazek et al, 2011).…”

Weak lensing constraints on the dark matter haloes of early type galaxies

“…However, 72 Natarajan the most exciting results on the masses, density profiles and the galaxy-mass correlation function at the present time are from the SDSS (Sloan Digital Sky Survey). Mandelbaum and collaborators (Mandelbaum et al 2006;Hirata et al 2007) find that mass traces light on these large scales as M/L ∼ L 0.5 ; the mass of a typical dark matter (DM) halo that hosts an early-type L * galaxy is 1.5×10 12 h −1 M ; more massive DM haloes host brighter galaxies and that the masses of haloes that host late-types are a factor of 2-4 lower. Fitting parametric models they find that both NFW and Einasto profiles offer good fits over the range 0.2 r v ir < r 1 h −1 Mpc; and the inferred concentration parameter c v ir for NFW profiles is consistent with expectations of a power-law distribution with halo mass from cosmological simulations.…”