# On the measurements of assembly bias and splashback radius using optically selected galaxy clusters

## TL;DR

This paper critically evaluates methods used to detect assembly bias and splashback radius in galaxy clusters, demonstrating that contamination and projection effects can lead to false signals and misinterpretations.

## Contribution

It shows that previous observational claims of assembly bias detection are unreliable due to contamination and projection effects, and provides a methodology to identify such biases.

## Key findings

- Contamination from interlopers can mimic assembly bias signals.
- Projection effects bias splashback radius measurements.
- Previous detection methods are unreliable due to these biases.

## Abstract

We critically examine the methodology behind the claimed observational detection of halo assembly bias using optically selected galaxy clusters by Miyatake et al. (2016) and More et al. (2016). We mimic the optical cluster detection algorithm and apply it to two different mock catalogs generated from the Millennium simulation galaxy catalog, one in which halo assembly bias signal is present, while the other in which the assembly bias signal has been expressly erased. We split each of these cluster samples into two using the average cluster-centric distance of the member galaxies to measure the difference in the clustering strength of the subsamples with respect to each other. We observe that the subsamples split by cluster-centric radii show differences in clustering strength, even in the catalog where the true assembly bias signal was erased. We show that this is a result of contamination of the member galaxy sample from interlopers along the line-of-sight. This undoubtedly shows that the particular methodology adopted in the previous studies cannot be used to claim a detection of the assembly bias signal. We figure out the tell-tale signatures of such contamination, and show that the observational data also shows similar signatures. Furthermore, we also show that projection effects in optical galaxy clusters can bias the inference of the 3-dimensional edges of galaxy clusters (splashback radius), so appropriate care should be taken while interpreting the splashback radius of optical clusters.