The dependence of intrinsic alignment of galaxies on wavelength using KiDS and GAMA

TL;DR

This study investigates how galaxy intrinsic alignments vary with wavelength by analyzing shape measurements across different bands, revealing significant differences especially among red satellite galaxies, which impacts weak lensing studies.

Contribution

It provides the first detailed analysis of wavelength dependence of galaxy intrinsic alignments using KiDS and GAMA data, highlighting the role of galaxy type and color.

Findings

Significant alignment differences between g, r, and i bands.

Red satellite galaxies dominate the wavelength dependence.

Differences exceed linear alignment model predictions on small scales.

Abstract

The outer regions of galaxies are more susceptible to the tidal interactions that lead to intrinsic alignments of galaxies. The resulting alignment signal may therefore depend on the passband if the colours of galaxies vary spatially. To quantify this, we measured the shapes of galaxies with spectroscopic redshifts from the GAMA survey using deep gri imaging data from the KiloDegree Survey. The performance of the moment-based shape measurement algorithm DEIMOS was assessed using dedicated image simulations, which showed that the ellipticities could be determined with an accuracy better than 1% in all bands. Additional tests for potential systematic errors did not reveal any issues. We measure a significant difference of the alignment signal between the g,r and i-band observations. This difference exceeds the amplitude of the linear alignment model on scales below 2 Mpc/h. Separating the sample into central/satellite and red/blue galaxies, we find that that the difference is dominated by red satellite galaxies.