# The formation pathways of compact elliptical galaxies

**Authors:** Simon Deeley, Michael Drinkwater, Sarah Sweet, Kenji Bekki, Warrick Couch, Duncan Forbes  
**arXiv:** [2308.00305](https://arxiv.org/abs/2308.00305)  
**Date:** 2023-08-01  
**Summary page:** https://www.summarizepaper.com/en/paper/2308.00305v1/

## Key points

- Compact elliptical (cE) galaxies are challenging to study
- Isolated cEs and host-associated cEs have different formation pathways
- Link between these pathways not established, contributions in cosmological context unknown
- Study combines data from SAMI galaxy survey and IllustrisTNG cosmological simulation
- Spatially-resolved observational sample of cEs analyzed to understand their formation
- Observed differences between isolated and host-associated cEs: redder, smaller, older for those near a host galaxy
- Two main formation pathways identified: stripping of a spiral galaxy by larger host (32%), gradual build-up of stellar mass in isolated environments (68%)
- All isolated cEs formed via in-situ formation, 77% of host-associated cEs formed through tidal stripping
- Differences between isolated and host-associated cEs can be explained by dominance of different formation pathways in each environment
- Study provides insights into complex processes involved in the creation of compact elliptical galaxies.

## Summary

Compact elliptical (cE) galaxies have long been a challenging class of galaxies to study. Recent observations have indicated that isolated cEs and host-associated cEs follow different formation pathways, and simulations have also suggested multiple pathways for the formation of cE galaxies. However, a definitive link between these pathways has not yet been established, and the relative contributions of each pathway in a cosmological context remain unknown. To address this gap in knowledge, the authors of this study combined data from the SAMI galaxy survey with a matched sample of galaxies from the IllustrisTNG cosmological simulation. By analyzing a spatially-resolved observational sample of cEs, they aimed to gain insights into how these galaxies form. The results revealed distinct differences between isolated and host-associated cEs. The observed cEs located near a host galaxy appeared redder, smaller, and older compared to isolated cEs. This finding supports previous evidence suggesting that there are multiple formation pathways for cE galaxies. Tracing the simulated cEs back through time, the researchers identified two main formation pathways. Approximately 32 ± 5 percent of cEs formed through the stripping of a spiral galaxy by a larger host galaxy, while 68 ± 4 percent formed through a gradual build-up of stellar mass in isolated environments. Importantly, this study confirms that cEs in different environments do indeed form via different pathways. All isolated cEs in the sample were found to have formed via in-situ formation (i.e., none were ejected from a previous host), while 77 ± 6 percent of host-associated cEs formed through tidal stripping. By separating the cEs based on their formation pathway, the researchers were able to reproduce the observed differences between isolated and host-associated cEs. This suggests that these differences can be fully explained by the dominance of different formation pathways in each environment. Overall, this study provides valuable insights into understanding how compact elliptical galaxies form by combining observational data with simulations to shed light on their complex processes involved in their creation.