# Tracing the Total Stellar Mass and Star Formation of High-Redshift Protoclusters

**Authors:** Roxana Popescu, Alexandra Pope, Kyoung-Soo Lee, Stacey Alberts, Yi-Kuan Chiang, Sowon Lee, Mark Brodwin, Jed McKinney, Vandana Ramakrishnan  
**arXiv:** [2308.00745](https://arxiv.org/abs/2308.00745)  
**Date:** 2023-08-01  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.00745v1/

## Key points

- Protoclusters are the progenitors of present-day galaxy clusters
- Understanding how typical galaxies grow within protoclusters is uncertain
- Stacking analysis of 211 protoclusters selected as Planck cold sources at redshifts between 2 and 4 was performed
- WISE and Herschel/SPIRE images were stacked to measure angular size and spectral energy distribution
- Total stellar mass and star formation within protoclusters were estimated, including contributions from faint undetected members
- Fluxes of protoclusters selected as Planck cold sources can be contaminated by line-of-sight interlopers
- On average, protoclusters exhibit twice as much star formation and four times more stellar mass compared to individually-detected galaxies in spectroscopically-confirmed protoclusters
- A significant portion of the total flux observed within protoclusters comes from galaxies with luminosities below the detection limit of SPIRE
- Protoclusters have a half-light radius of 2.8 arcminutes (corresponding to a physical scale of 4.2-5.8 co-moving megaparsecs), consistent with simulations

## Summary

This study focuses on protoclusters, which are the progenitors of present-day galaxy clusters. Protoclusters provide valuable insights into galaxy evolution, but our understanding of how typical galaxies grow within these young structures is still uncertain. Previous observations of protoclusters have been limited to detected constituent galaxies at UV and/or infrared wavelengths. To address this knowledge gap, the authors perform a stacking analysis of 211 protoclusters selected as Planck cold sources at redshifts between 2 and 4. By stacking WISE and Herschel/SPIRE images, they measure the angular size and spectral energy distribution of the integrated light from the protoclusters. This approach allows them to estimate the total stellar mass and star formation within these structures, including contributions from faint undetected members. However, it is important to note that the fluxes of protoclusters selected as Planck cold sources can be contaminated by line-of-sight interlopers. To account for this contamination, the authors use the WebSky simulation to estimate that a single protocluster contributes an average of 33±15% of the flux observed from a Planck cold source after correcting for contamination. The authors find a total star formation rate of (7.3±3.2) × 10^3 solar masses per year and a total stellar mass of (4.9±2.2) × 10^12 solar masses within the studied protoclusters. These results indicate that on average protoclusters exhibit twice as much star formation and four times more stellar mass compared to individually-detected galaxies in spectroscopically-confirmed protoclusters; suggesting that a significant portion of the total flux observed within protoclusters at redshifts between 2 and 4 comes from galaxies with luminosities below the detection limit of SPIRE (with infrared luminosities lower than 3 × 10^12 solar luminosities). Lastly, the authors find that protoclusters have a half-light radius of 2.8 arcminutes (corresponding to a physical scale of 4.2-5.8 co-moving megaparsecs), which is consistent with simulations.