Primordial alignment of elliptical galaxies in intermediate redshift clusters

TL;DR

This study investigates the primordial alignment of elliptical galaxies in galaxy clusters at intermediate redshift, finding significant alignment in young clusters but not in relaxed ones, with implications for understanding galaxy formation.

Contribution

It provides the first evidence that primordial galaxy alignment varies with cluster dynamical state and highlights the role of bright galaxies and potential observational biases.

Findings

Primordial alignment detected in 4 young clusters at 3σ significance.

No primordial alignment observed in 4 relaxed clusters.

Radial and halo alignment can mimic primordial alignment, but detected signals are genuine.

Abstract

We measure primordial alignments for the red galaxies in the sample of eight massive galaxy clusters in the southern sky from the CLASH-VLT Large Programme, at a median redshift of 0.375. We find primordial alignment with about $3\sigma$ significance in the four dynamically young clusters, but null detection of primordial alignment in the four highly relaxed clusters. The observed primordial alignment is not dominated by any single one of the four dynamically young clusters, and is primarily due to a population of bright galaxies ($M_r<-20.5\ \rm{m}$) residing in the region 300 to 810 kpc from the cluster centers. For the first time, we point out that the combination of radial alignment and halo alignment can cause fake primordial alignment. Finally, we find that the detected alignment for the dynamically young clusters is real rather than fake primordial alignment.