# The Formation of Ultra-Diffuse Galaxies in Clusters

**Authors:** Laura V. Sales, Julio F. Navarro, Louis Penafiel, Eric W. Peng, Sungsoon Lim, Lars Hernquist  
**arXiv:** [1909.01347](https://arxiv.org/abs/1909.01347)  
**Date:** 2019-09-03  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/1909.01347v1/

## Key points

- Laura V. Sales, Julio F. Navarro, Louis Penafiel, Eric W. Peng, Sungsoon Lim, and Lars Hernquist used the IllustrisTNG cosmological hydrodynamical simulation to investigate the formation of ultra-diffuse galaxies (UDGs) in galaxy clusters.
- The authors supplemented the simulations with a realistic mass-size relation for galaxies at the time of infall into the cluster and an analytical model that describes the tidally-induced evolution of their stellar mass, velocity dispersion and size.
- Contrary to recent claims, their model assumes "cuspy" NFW halos and successfully reproduces the observed number of UDGs in clusters.
- Their results suggest that UDG population is composed of two distinct populations: "normal" low surface brightness galaxies found in the field ("born" UDGs or B-UDGs), as well as those whose large size and low surface brightness are due to cluster tides ("tidal" or T-UDGs).
- The simulations indicate that T-UDGs entered the cluster earlier than B-UDGs and should be more common near its center.
- Additionally, T-UDGs should have lower velocity dispersion, higher metallicities and lower dark matter content than B-UDGs at given stellar mass.
- The authors' findings suggest that systems like DF-44 are consistent with having been born as UDGs while others such as DF2, DF4 and VLSB-D are possibly extreme examples of T-UDG formation.
- This study provides valuable insights into how UDGs form in galaxy clusters and emphasizes the importance of considering both internal properties (such as stellar mass) and external factors (such as tidal forces) when studying these enigmatic objects.

## Summary

In their paper "The Formation of Ultra-Diffuse Galaxies in Clusters," Laura V. Sales, Julio F. Navarro, Louis Penafiel, Eric W. Peng, Sungsoon Lim, and Lars Hernquist use the IllustrisTNG cosmological hydrodynamical simulation to investigate the formation of ultra-diffuse galaxies (UDGs) in galaxy clusters. The authors supplement the simulations with a realistic mass-size relation for galaxies at the time of infall into the cluster and an analytical model that describes the tidally-induced evolution of their stellar mass, velocity dispersion and size. Contrary to recent claims, their model assumes "cuspy" NFW halos and successfully reproduces the observed number of UDGs in clusters. Their results suggest that UDG population is composed of two distinct populations: "normal" low surface brightness galaxies found in the field ("born" UDGs or B-UDGs), as well as those whose large size and low surface brightness are due to cluster tides ("tidal" or T-UDGs). The simulations indicate that T-UDGs entered the cluster earlier than B-UDGs and should be more common near its center. Additionally, T-UDGs should have lower velocity dispersion, higher metallicities and lower dark matter content than B-UDGs at given stellar mass. The authors' findings suggest that systems like DF-44 are consistent with having been born as UDGs while others such as DF2, DF4 and VLSB-D are possibly extreme examples of T-UDG formation. This study provides valuable insights into how UDGs form in galaxy clusters and emphasizes the importance of considering both internal properties (such as stellar mass) and external factors (such as tidal forces) when studying these enigmatic objects.