Intrinsic Alignment of Cluster Galaxies: the Redshift Evolution

TL;DR

This study measures galaxy alignments in clusters from SDSS data, revealing redshift and magnitude dependencies for BCG alignments and highlighting potential measurement biases affecting satellite alignment detection.

Contribution

It provides new insights into the redshift evolution of galaxy alignments and discusses the impact of measurement methods on observed satellite alignments.

Findings

BCG alignment strengthens with decreasing redshift and BCG magnitude.

Satellite alignment is only detected with isophotal PAs, possibly due to contamination.

No passband dependence observed in alignment measurements.

Abstract

We present measurements of two types of cluster galaxy alignments based on a volume limited and highly pure ($\ge$ 90%) sample of clusters from the GMBCG catalog derived from SDSS DR7. We detect a clear BCG alignment (the alignment of major axis of the BCG toward the distribution of cluster satellite galaxies). We find that the BCG alignment signal becomes stronger as the redshift and BCG absolute magnitude decrease, and becomes weaker as BCG stellar mass decreases. No dependence of the BCG alignment on cluster richness is found. We can detect a statistically significant ($\ge$ 3 sigma) satellite alignment (the alignment of the major axes of the cluster satellite galaxies toward the BCG) only when we use the isophotal fit position angles (PAs, hereafter), and the satellite alignment depends on the apparent magnitudes rather than the absolute magnitudes of the BCGs. This suggests the detected satellite alignment based on isophotoal PAs from the SDSS pipeline is possibly due to the contamination from the diffuse light of nearby BCGs. We caution that this should not be simply interpreted as non-existence of the satellite alignment, but rather that we cannot detect them with our current photometric SDSS data. We perform our measurements on both SDSS $r$ band and $i$ band data, but did not observe a passband dependence of the alignments.