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

Hirata, Christopher M. ; Mandelbaum, Rachel ; Ishak, Mustapha ; Seljak, Uroš ; Nichol, Robert ; Pimbblet, Kevin A. ; Ross, Nicholas P. ; Wake, David

In: Monthly Notices of the Royal Astronomical Society, 2007, vol. 381, no. 3, p. 1197-1218

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    Title
    • Intrinsic galaxy alignments from the 2SLAQ and SDSS surveys: luminosity and redshift scalings and implications for weak lensing surveys
    Author
    • Hirata, Christopher M.. Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
    • Mandelbaum, Rachel. Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
    • Ishak, Mustapha. Department of Physics, The University of Texas at Dallas, Richardson, TX 75083, USA
    • Seljak, Uroš. Institute for Theoretical Physics, University of Zurich, 8057 Zurich, Switzerland
    • Nichol, Robert. Institute of Cosmology and Gravitation, Mercantile House, University of Portsmouth, Portsmouth PO1 2EG
    • Pimbblet, Kevin A.. Department of Physics, University of Queensland, Brisbane, QLD 4072, Australia
    • Ross, Nicholas P.. Physics Department, University of Durham, South Road, Durham DH1 3LE
    • Wake, David. Physics Department, University of Durham, South Road, Durham DH1 3LE
    Document Type
    • Postprint
    OAI-PMH Identifier
    • oai:doc.rero.ch:296180
    Summary
    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 σ8 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 σ8 of Δσ8=−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