Intrinsic Alignment as an RSD Contaminant in the DESI Survey

TL;DR

This paper investigates how intrinsic galaxy alignments can contaminate redshift-space distortion measurements in the DESI survey, quantifying the effect and demonstrating its impact on cosmological parameter estimation.

Contribution

It introduces a method to measure intrinsic galaxy alignments and forecasts their impact as a systematic bias in RSD analyses for DESI.

Findings

Intrinsic alignments cause a 0.5% decrease in the RSD quadrupole.

The effect is reproducible using cosmological simulations.

Orientation-dependent selection biases can mimic RSD signals.

Abstract

We measure the tidal alignment of the major axes of Luminous Red Galaxies (LRGs) from the Legacy Imaging Survey and use it to infer the artificial redshift-space distortion signature that will arise from an orientation-dependent, surface-brightness selection in the Dark Energy Spectroscopic Instrument (DESI) survey. Using photometric redshifts to down-weight the shape-density correlations due to weak lensing, we measure the intrinsic tidal alignment of LRGs. Separately, we estimate the net polarization of LRG orientations from DESI's fiber-magnitude target selection to be of order 10^-2 along the line of sight. Using these measurements and a linear tidal model, we forecast a 0.5% fractional decrease on the quadrupole of the 2-point correlation function for projected separations of 40-80 Mpc/h. We also use a halo catalog from the Abacus Summit cosmological simulation suite to reproduce this false quadrupole.