Constraints on intrinsic alignment contamination of weak lensing surveys using the MegaZ-LRG sample

Abstract

Correlations between the intrinsic shapes of galaxies and the large-scale galaxy density field provide an important tool to investigate galaxy intrinsic alignments, which constitute the major potential astrophysical systematic in cosmological weak lensing (cosmic shear) surveys, but also yield insight into the formation and evolution of galaxies. We measure galaxy position-shape correlations in the MegaZ-LRG sample for more than 800 000 luminous red galaxies for comoving transverse separations of 0.3 < r p < 60 h −1 Mpc, making the first such measurement with a photometric redshift sample. In combination with a re-analysis of several spectroscopic SDSS samples, we constrain an intrinsic alignment model for early-type galaxies over long baselines in redshift (z 0.7) and luminosity (4 mag) with high statistical precision. We develop and test the formalism to incorporate photometric redshift scatter in the modelling of these observations. For r p > 6 h −1 Mpc, the fits to galaxy position-shape correlation functions are consistent with the scaling with r p and redshift of a revised, nonlinear version of the linear alignment model for all samples. An extra redshift dependence ∝ (1 + z) η other is constrained to η other = −0.3 ± 0.8 (1σ). To obtain consistent amplitudes for all data, an additional dependence on galaxy luminosity ∝L β with β = 1.1 +0.3 −0.2 is required. The normalisation of the intrinsic alignment power spectrum is found to be (0.077 ± 0.008) ρ −1 cr for galaxies at redshift 0.3 and r band magnitude of −22 (k-and evolution-corrected to z = 0). Assuming zero intrinsic alignments for blue galaxies, we assess the bias on cosmological parameters for a tomographic CFHTLSlike lensing survey given our new constraints on the intrinsic alignment model parameter space. Both the resulting mean bias and its uncertainty are smaller than the 1σ statistical errors when using the constraints from all samples combined. The addition of MegaZ-LRG data is critical to achieving constraints this strong, reducing the uncertainty in intrinsic alignment bias on cosmological parameters by factors of three to seven.

The alignment of galaxy spin with the shear field in observations

“…We assume that any galaxy's projected short axis is parallel to its spin axis and we note that ample work has been done related to this (Franx et al 1991;Cappellari et al 2011), showing that the majority of the early-type galaxies have small mis-alignments between their shapes and the angular momentum vectors. It is also consistent with the measured shape alignments of ellipticals and the tidal alignment model (Hirata & Seljak 2004) such as in the analysis conducted using the MegaZ-LRG sample (Joachimi et al 2011) and the SDSS data (Hirata et al 2007). This has also been discussed in the recent reviews Kirk et al 2015;Kiessling et al 2015).…”

Observational probes of cosmic acceleration

“…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.…”

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

“…Following Joachimi et al (2011b), we fix C1ρcrit = 0.0134 and use the arbitrary constant AI to describe the amplitude of intrinsic alignments for different samples. D(z) is the linear growth factor, normalized to unity at z = 0.…”