The Intrinsic Alignment of Galaxies and its Impact on Weak Gravitational Lensing in an Era of Precision Cosmology

TL;DR

The intrinsic alignment of galaxies is identified as a key systematic that can bias weak lensing measurements used for precision cosmology. The paper surveys theoretical IA models (linear, nonlinear, halo-based, and semi-analytic), analytic expressions for IA power spectra and bispectra, and insights from simulations, alongside extensive measurements from surveys. It then details mitigation strategies—marginalization, tomography, nulling, self-calibration, and cross-correlation with complementary data—and discusses their impact on preserving cosmological information. The work emphasizes that while IA remains challenging, a combination of modeling, empirical constraints, and cross-probe calibration enables robust weak-lensing in upcoming Stage III/IV surveys.

Abstract

The wealth of incoming and future cosmological observations will allow us to map out the structure and evolution of the observable universe to an unprecedented level of precision. Among these observations is the weak gravitational lensing of galaxies, e.g., cosmic shear that measures the minute distortions of background galaxy images by intervening cosmic structure. Weak lensing and cosmic shear promise to be a powerful probe of astrophysics and cosmology, constraining models of dark energy, measuring the evolution of structure in the universe, and testing theories of gravity on cosmic scales. However, the intrinsic alignment of galaxies -- their shape and orientation before being lensed -- may pose a great challenge to the use of weak gravitational lensing as an accurate cosmological probe, and has been identified as one of the primary physical systematic biases in cosmic shear studies. Correlations between this intrinsic alignment and the lensing signal can persist even for large physical separations, and isolating the effect of intrinsic alignment from weak lensing is not trivial. A great deal of work in the last two decades has been devoted to understanding and characterizing this intrinsic alignment, which is also a direct and complementary probe of structure formation and evolution in its own right. In this review, we report in a systematic way the state of our understanding of the intrinsic alignment of galaxies, with a particular emphasis on its large-scale impact on weak lensing measurements and methods for its isolation or mitigation. (Abridged)

Figures (23)

• Figure 1: The geometry of the lens equation. The observer, lens, source, and image positions are shown. The angle between the lens and image is given by $\bm{\theta}$, the angle between the lens and source by $\bm{\beta}$, and the deflection angle by $\bm{\hat{\alpha}}$. The angular diameter distance from the observer to the lens is $D_l$, from the observer to the source is $D_s$, and from the lens to the source is $D_{ls}$. For small angles, the relationship $\bm{\theta} D_{s}= \bm{\beta} D_{s}+\bm{\hat{\alpha}} D_{ls}$ holds. The impact parameter $\xi$ is also shown and can be approximated as a straight-line distance for small $\bm{\theta}$.
• Figure 2: The convergence auto- and cross-power spectra in Eq. (\ref{['eq:pstomo']}) are shown for the base $\Lambda$CDM model with a cosmological constant, $\Omega_m=0.27$, $\Omega_{\Lambda}=0.73$, $n_s=0.96$, and $\sigma_8=0.84$. The redshift distribution given in Eq. (\ref{['eq:fz']}) is split into two bins with boundary $z=0.8$, the median of the distribution. The auto-spectra of the lower and higher redshift bins are labeled '11' and '22', respectively, while the cross-spectrum is labeled '12'.
• Figure 3: The convergence auto- and cross-bispectra in Eq. (\ref{['eq:bspec2']}) are shown for the base $\Lambda$CDM model with a cosmological constant, $\Omega_m=0.27$, $\Omega_{\Lambda}=0.73$, $n_s=0.96$, and $\sigma_8=0.84$. The redshift distribution given in Eq. (\ref{['eq:fz']}) is split into two bins with boundary $z=0.8$, the median of the distribution. The auto-bispectra of the lower and higher redshift bins are labeled '111' and '222', respectively, while the cross-bispectra are labeled '112' and '122'.
• Figure 4: The convergence power spectrum in Eq. (\ref{['eq:spec']}) is shown for several CDM models. The base $\Lambda$CDM model is taken to have a cosmological constant $\Lambda$, $\Omega_m=0.27$, $\Omega_{\Lambda}=0.73$, $n_s=0.96$, and $\sigma_8=0.84$. The wCDM model has dark energy equation of state $w=w_0+w_a(1-a)$. The galaxy distribution is given by Eq. (\ref{['eq:fz']}).
• Figure 5: The convergence bispectrum in Eq. (\ref{['eq:bspec']}) is shown for several CDM models. The base $\Lambda$CDM model is taken to have a cosmological constant $\Lambda$, $\Omega_m=0.27$, $\Omega_{\Lambda}=0.73$, $n_s=0.96$, and $\sigma_8=0.84$. The wCDM model has dark energy equation of state $w=w_0+w_a(1-a)$. The galaxy distribution is given by Eq. (\ref{['eq:fz']}).