# The hidden circumgalactic medium

## TL;DR

This paper discusses the importance of the circumgalactic medium in galaxy evolution, highlighting the potential significance of its cold phase and proposing large single-dish telescopes as the key to studying it.

## Contribution

It introduces the concept of a cold, multiphase CGM and emphasizes the need for large single-dish telescopes to observe its large-scale structures.

## Key findings

- Cold CGM may contain significant baryonic mass.
- Interferometers like ALMA cannot detect large-scale cold structures.
- Large single-dish telescopes are essential for probing the cold CGM.

## Abstract

The cycling of baryons in and out of galaxies is what ultimately drives galaxy formation and evolution. The circumgalactic medium (CGM) represents the interface between the interstellar medium and the cosmic web, hence its properties are directly shaped by the baryon cycle. Although traditionally the CGM is thought to consist of warm and hot gas, recent breakthroughs are presenting a new scenario according to which an important fraction of its mass may reside in the cold atomic and molecular phase. This would represent fuel that is readily available for star formation, with crucial implications for feeding and feedback processes in galaxies. However, such cold CGM, especially in local galaxies where its projected size on sky is expected to be of several arcminutes, cannot be imaged by ALMA due to interferometric spatial scale filtering of large-scale structures. We show that the only way to probe the multiphase CGM including its coldest component is through a large (e.g. 50-m) single dish (sub-)mm telescope.