Tidal alignments as a contaminant of redshift space distortions

Understand

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, and thus result in a bias in the measurement of the velocity power spectrum.