# Far to near UV spectroscopy of the interstellar medium at very high resolution and very high signal-to-noise ratio

## TL;DR

This paper discusses the need for a high-resolution, high signal-to-noise UV spectrometer capable of observing all phases of the interstellar medium simultaneously, to better understand their composition, boundaries, and physical processes.

## Contribution

It highlights the design requirements for a spectrometer with R>200,000 and SNR>500 to observe all ISM phases at high spectral resolution in the UV domain.

## Key findings

- Current instruments cannot observe all ISM phases at high resolution.
- A spectrometer with R>200,000 and SNR>500 is needed for comprehensive UV ISM studies.
- High-resolution UV spectroscopy can resolve velocity components separated by less than 2 km/s.

## Abstract

A comprehensive study of interstellar medium phases and the nature of their boundaries or connections requires comparing abundances and velocity profiles from tracers of the different phases. In the UV, studies of a wealth of absorption features appearing in the spectra of hot stars yield fundamental insights into the composition and physical characteristics of all phases of the ISM along with the processes that influence them. They also inform us on the nature of boundaries between them. However no single instrument has as yet given access to species in all ISM phases at the same high spectral resolution: from the molecular bands of CO and H2 in the far-UV, to the cold neutral medium tracers C I and S I and the warm medium tracers like C II, NI, OI, Mg II, Fe II, Si II etc, and to the high ions of the hot ionized medium C IV, Si IV in the UV, as well as O VI in the far UV. We have yet to design the spectrometer that will enable observing the full UV domain at resolving power R>200 000 and signal-to-noise ratio SNR>500. The line FWHM being governed by turbulence, temperature, and species mass, such a resolution is necessary to resolve lines from both the cold molecular hydrogen and the warm metal ions with a turbulence of ~1 km/s, and to differentiate distinct velocity components, typically separated by less than 2 km/s.