On the origin of intrinsic alignment in cosmic shear measurements: an analytic argument

TL;DR

This paper develops a simple theoretical model to investigate the origin of galaxy intrinsic alignment in weak-lensing, finding that instantaneous tidal effects are too weak to explain observed alignments, supporting formation-based theories.

Contribution

It provides an analytical model for intrinsic alignment based on the tidal field, offering a theoretical perspective that supports formation and accretion processes as the main cause.

Findings

Intrinsic alignment depends on the tidal field as expected.

The model predicts a weaker signal than observed.

Supports formation and accretion as primary causes.

Abstract

Galaxy intrinsic alignment can be a severe source of error in weak-lensing studies. The problem has been widely studied by numerical simulations and with heuristic models, but without a clear theoretical justification of its origin and amplitude. In particular, it is still unclear whether intrinsic alignment of galaxies is dominated by formation and accretion processes or by the effects of the instantaneous tidal field acting upon them. We investigate this question by developing a simple model of intrinsic alignment for elliptical galaxies, based on the instantaneous tidal field. Making use of the galaxy stellar distribution function, we estimate the intrinsic alignment signal and find that although it has the expected dependence on the tidal field, it is too weak to account for the observed signal. This is an indirect validation of the standard view that intrinsic alignment is caused by formation and/or accretion processes.