First Evidence of Intrinsic Alignments of Red Galaxies at $z > 1$: Cross-correlation between CFHTLenS and FastSound Samples

TL;DR

This study provides the first evidence of intrinsic alignments of red galaxies at redshift greater than 1, indicating a significant impact on weak lensing measurements and opening new avenues for cosmological research.

Contribution

It presents the first measurement of intrinsic alignments of red galaxies at z>1 using cross-correlation, revealing larger IA amplitudes than expected from lower-redshift models.

Findings

Detected IA amplitude at 2.4-2.6 sigma significance.

Estimated ~20% contamination of weak lensing power spectrum.

Results suggest IA is stronger at high redshift than previously modeled.

Abstract

We report the first evidence for intrinsic alignment (IA) of red galaxies at $z>1$. We measure the gravitational shear-intrinsic ellipticity (GI) cross-correlation function at $z\sim1.3$ using galaxy positions from the FastSound spectroscopic survey and galaxy shapes from Canada-Hawaii-France telescope lensing survey data. Adopting the non-linear alignment model, we obtain a $2.4\sigma$-level detection of the IA amplitude $A^{\rm LA}=27.48_{-11.54}^{+11.53}$ (and $2.6\sigma$ with $A^{\rm LA}=29.43_{-11.49}^{+11.48}$ when weak-lensing contaminations are taken into account), which is larger than the value extrapolated from the constraints obtained at lower redshifts. Our measured IA is translated into a $\sim 20\%$ contamination to the weak lensing power spectrum for the red galaxies. This marginal detection of IA for red galaxies at $z>1$ motivates the continuing investigation of the nature of IA for weak lensing studies. Furthermore, our result provides the first step to utilize IA measurements in future high-$z$ surveys as a cosmological probe, complementary to galaxy clustering and lensing.