# The Intrinsic Alignment of Red Galaxies in DES Y1 redMaPPer Galaxy Clusters

## TL;DR

This study measures the intrinsic alignment of red galaxies in DES Y1 clusters, revealing non-zero radial alignments and their dependence on cluster properties, which impacts weak lensing cosmology.

## Contribution

It provides the first direct measurement of galaxy intrinsic alignment within clusters using DES data, including correction methods and modeling of the alignment amplitude.

## Key findings

- Detected non-zero radial alignment of galaxies in clusters
- Found stronger alignment of central galaxies with cluster halos at low redshift
- Measured intrinsic alignment amplitude as $A_{IA}=0.15\, ext{±}\,0.04$

## Abstract

Clusters of galaxies are sensitive to the most nonlinear peaks in the cosmic density field. The weak gravitational lensing of background galaxies by clusters can allow us to infer their masses. However, galaxies associated with the local environment of the cluster can also be intrinsically aligned due to the local tidal gradient, contaminating any cosmology derived from the lensing signal. We measure this intrinsic alignment in Dark Energy Survey (DES) Year 1 redMaPPer clusters. We find evidence of a non-zero mean radial alignment of galaxies within clusters between redshift 0.1-0.7. We find a significant systematic in the measured ellipticities of cluster satellite galaxies that we attribute to the central galaxy flux and other intracluster light. We attempt to correct this signal, and fit a simple model for intrinsic alignment amplitude ($A_{\textrm{IA}}$) to the measurement, finding $A_{\textrm{IA}}=0.15\pm 0.04$, when excluding data near the edge of the cluster. We find a significantly stronger alignment of the central galaxy with the cluster dark matter halo at low redshift and with higher richness and central galaxy absolute magnitude (proxies for cluster mass). This is an important demonstration of the ability of large photometric data sets like DES to provide direct constraints on the intrinsic alignment of galaxies within clusters. These measurements can inform improvements to small-scale modeling and simulation of the intrinsic alignment of galaxies to help improve the separation of the intrinsic alignment signal in weak lensing studies.