# Cool and gusty, with a chance of rain: Dynamics of multiphase CGM around massive galaxies in the Romulus simulations

**Authors:** Vida Saeedzadeh, S. Lyla Jung, Douglas Rennehan, Arif Babul, Michael Tremmel, Thomas R. Quinn, Zhiwei Shao, Prateek Sharma, Lucio Mayer, E. OSullivan, S. Ilani Loubser  
**arXiv:** [2304.03798](https://arxiv.org/abs/2304.03798)  
**Date:** 2023-04-07  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.03798v1/

## Key points

- The paper investigates the origin and evolution of the circumgalactic medium (CGM) around massive central galaxies in group-scale halos.
- The CGM is found to be multiphase and highly dynamic, with seven distinct patterns of evolution identified across various conditions.
- Two pathways by which gas cools are identified: filamentary cooling inflows and condensations forming from rapidly cooling density perturbations.
- Condensations can form even when the median $t_\mathrm{cool} / t_\mathrm{ff}$ of X-ray emitting gas is above the canonical threshold of 10 or 20.
- Strong amplitude perturbations can provoke runaway cooling regardless of the state of the background gas, while weak perturbations may not condense despite local $t_\mathrm{cool} / t_\mathrm{ff}$ ratios dropping below the threshold.
- Using a simple threshold argument to infer CGM evolution for structures forming hierarchically is challenging, and median hot gas properties are suboptimal determinants of the CGM's state and dynamics.
- Realistic CGM models must factor in the effects and after-effects of mergers and orbiting satellites, along with its heating and cooling cycles.

## Summary

In their paper titled "Cool and gusty, with a chance of rain: Dynamics of multiphase CGM around massive galaxies in the Romulus simulations," Vida Saeedzadeh and colleagues use high-resolution Romulus simulations to investigate the origin and evolution of the circumgalactic medium (CGM) in the zone 0.1 $\leq \mathrm{R}/\mathrm{R}_\mathrm{500} \leq$ 1 around massive central galaxies in group-scale halos. The authors find that the CGM is both multiphase and highly dynamic, with seven distinct patterns of evolution identified across various conditions. The authors also identify two pathways by which gas cools: filamentary cooling inflows and condensations forming from rapidly cooling density perturbations. In their cosmological simulations, these perturbations are mainly seeded by orbiting substructures. Interestingly, the authors find that condensations can form even when the median $t_\mathrm{cool} / t_\mathrm{ff}$ (the ratio of cooling time to free-fall time) of X-ray emitting gas is above the canonical threshold of 10 or 20. Strong amplitude perturbations can provoke runaway cooling regardless of the state of the background gas. Additionally, weak perturbations are found that do not condense despite local $t_\mathrm{cool} / t_\mathrm{ff}$ ratios dropping below the threshold. These weak perturbations are temporarily swept up in satellite wakes and carried to larger radii; their $t_\mathrm{cool} / t_\mathrm{ff}$ ratios decrease because the denominator ($t_\mathrm{ff}$) is increasing rather than because the numerator ($t_\mathrm{cool}$) is decreasing. The study highlights that using a simple threshold argument to infer CGM evolution for structures forming hierarchically is challenging. Moreover, it emphasizes that median hot gas properties are suboptimal determinants of the CGM's state and dynamics. Realistic CGM models must factor in the effects and after-effects of mergers and orbiting satellites, along with its heating and cooling cycles. Overall, this study provides valuable insights into complex dynamics of CGM around massive galaxies in group-scale halos, shedding light on its multiphase nature as well as identifying distinct patterns of evolution.