# The impact of galaxy selection on the splashback boundaries of galaxy clusters

**Authors:** Stephanie O'Neil, Josh Borrow, Mark Vogelsberger, Benedikt Diemer  
**arXiv:** [2202.05277](https://arxiv.org/abs/2202.05277)  
**Date:** 2022-02-10  
**Summary page:** https://www.summarizepaper.com/en/paper/2202.05277v2/

## Key points

- O'Neil et al. investigate the impact of galaxy selection on splashback boundaries of galaxy clusters using the IllustrisTNG simulation.
- Splashback radius ($R_{\rm sp}$) is measured by analyzing subhalo population density in stacked clusters within specific mass ranges.
- Various selection criteria are applied, including cuts in subhalo mass, galaxy stellar mass, absolute magnitude, and specific star formation rate.
- Galaxies with higher masses and luminosities tend to trace smaller splashback radii compared to dark matter radius.
- Quenched galaxies can be used to reliably reconstruct the dark matter splashback radius.
- Discrepancies between observational predictions of $R_{\rm sp}$ are attributed to variations in selection functions employed by different studies.
- Changes in $R_{\rm sp}$ measured through number counts cannot solely be attributed to a change in galaxy abundance inside and outside of the cluster.
- The study emphasizes the complex nature of splashback boundaries and highlights the importance of considering multiple factors when studying them.

## Summary

In their study, O'Neil et al. investigate the impact of galaxy selection on the splashback boundaries of galaxy clusters using the IllustrisTNG cosmological galaxy formation simulation. The splashback radius ($R_{\rm sp}$) is measured by analyzing the number density of the subhalo population in stacked clusters within specific mass ranges. The authors focus on clusters with halo masses ranging from $10^{13} \leq M_{\rm 200, mean} / {\rm M}_\odot \leq 10^{15}$ and apply various selection criteria including cuts in subhalo mass, galaxy stellar mass, $i$-band absolute magnitude, and specific star formation rate. By examining these different criteria, they aim to understand how the measured splashback radius changes relative to the intrinsic dark matter radius. The results show that galaxies with higher masses and luminosities tend to trace smaller splashback radii compared to the dark matter radius. This suggests that changes in the galaxy population play a role in determining the observed trends. Interestingly, the study also reveals that quenched galaxies can be used to reliably reconstruct the dark matter splashback radius. Furthermore, O'Neil et al. address discrepancies between observational predictions of $R_{\rm sp}$ based on galaxy number counts and dark matter measurements. They attribute these differences to variations in selection functions employed by different studies, such as those using optically- or SZ-selected clusters. By considering these selection effects, they are able to reconcile conflicting predictions. The authors demonstrate that changes in $R_{\rm sp}$ measured through number counts cannot be solely attributed to a simple change in galaxy abundance inside and outside of the cluster. This finding highlights the complex nature of splashback boundaries and emphasizes the importance of considering multiple factors when studying them. Overall, this study provides valuable insights into how galaxy selection affects our understanding of splashback boundaries in galaxy clusters. The findings contribute to ongoing efforts to improve our knowledge of the relationship between galaxies and dark matter structures in the universe.