# Astro2020 Science White Paper: ISM and CGM in external galaxies

## TL;DR

This white paper emphasizes the importance of far-UV space telescopes for studying the ISM and CGM in external galaxies, highlighting the potential for new insights into galaxy evolution and the need for advanced instrumentation.

## Contribution

It advocates for a versatile far-UV space mission capable of detailed ISM and CGM studies in external galaxies up to hundreds of Mpc, addressing current observational limitations.

## Key findings

- Far-UV observations reveal detailed ISM properties in the Milky Way.
- Extending these observations to external galaxies can elucidate galaxy evolution.
- Proposed mission would enable probing multiple quasar lines of sight at various redshifts.

## Abstract

The wavelength range 912-2000A (hereafter far-UV) provides access to absorption lines of the interstellar medium (ISM), circumgalactic medium (CGM), and intergalactic medium (IGM) in phases spanning a wide range of ionization, density, temperature, and molecular gas fraction. Far-UV space telescopes have enabled detailed studies of the ISM in the Milky Way, paving the way to understand in particular depletion of elements by condensation onto dust grains, molecular gas formation pathways, ionization processes, and the dynamics of shocks. An interesting prospect is to transpose the same level of details to the ISM in external galaxies, in particular in metal-poor galaxies, where the ISM chemical composition, physical conditions, and topology (arrangement of various phases) change dramatically, with significant consequences on the star-formation properties and on the overall galaxy evolution. To circumvent current systematic biases in column density determinations and to examine the ISM enrichment as a function of the environment, we advocate for a versatile far-UV space mission able to observe individual O/B stars or stellar clusters in galaxies up to few 100s Mpc away with a spectral resolution power R~10^{4-5}. Such requirements would also make it possible to probe multiple quasar lines of sight intersecting the CGM of galaxies at various redshifts, making it possible to reconcile the various physical scales and phases and to comprehend the role of gas exchanges and flows for galaxy evolution.