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Tidal alignments as a contaminant of redshift space distortions

Hirata, Christopher M. (2009) Tidal alignments as a contaminant of redshift space distortions. Monthly Notices of the Royal Astronomical Society, 399 (2). pp. 1074-1087. ISSN 0035-8711.

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We investigate the effect of orientation-dependent selection effects on galaxy clustering in redshift space. It is found that if galaxies are aligned by large-scale tidal fields, then these selection effects give rise to a dependence of the observed galaxy density on the local tidal field, in addition to the well-known dependences on the matter density and radial velocity gradient. This alters the galaxy power spectrum in a way that is different for Fourier modes parallel to and perpendicular to the line of sight. These tidal galaxy alignments can thus mimic redshift space distortions (RSD), and thus result in a bias in the measurement of the velocity power spectrum. If galaxy orientations are affected only by the local tidal field, then the tidal alignment effect has exactly the same scale and angular dependence as the RSDs in the linear regime, so it cannot be projected out or removed by masking small scales in the analysis. We consider several toy models of tidal alignments and orientation-dependent selection, normalize their free parameter (an amplitude) to recent observations, and find that they could bias the velocity amplitude f(z)G(z) by 5–10 per cent in some models, although most models give much smaller contamination. We conclude that tidal alignments may be a significant systematic error in RSD measurements that aim to test general relativity via the growth of large-scale structure. We briefly discuss possible mitigation strategies.

Item Type:Article
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Hirata, Christopher M.0000-0002-2951-4932
Additional Information:© 2009 The Author. Journal compilation © 2009 RAS. Accepted 2009 July 3. Received 2009 June 25; in original form 2009 April 4. The author wishes to thank Marc Kamionkowski and Ue-Li Pen for useful feedback; and the referee, Andrew Hamilton, for insightful comments and in particular asking the question that inspired Appendix D. CH is supported by the U.S. Department of Energy under contract DE-FG03-02-ER40701, the National Science Foundation under contract AST-0807337, and the Alfred P. Sloan Foundation.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-02-ER40701
Alfred P. Sloan FoundationUNSPECIFIED
Subject Keywords:cosmology: theory; large-scale structure of Universe
Issue or Number:2
Record Number:CaltechAUTHORS:20091102-104706997
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16536
Deposited By: Tony Diaz
Deposited On:03 Nov 2009 22:13
Last Modified:03 Oct 2019 01:13

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