# Astro2020: The Cycling of Matter from the Interstellar Medium to Stars and back

## TL;DR

This paper emphasizes the importance of spectrally resolved line observations, especially atomic carbon and mid-J CO lines, to understand the matter cycle in galaxies, advocating for large-scale surveys and combined modeling efforts.

## Contribution

It highlights the need for comprehensive surveys of various galactic environments using specific spectral lines, which are currently lacking, to advance understanding of interstellar matter processes.

## Key findings

- Spectrally resolved lines are crucial for studying the interstellar matter cycle.
- Large-scale surveys of different galactic environments are needed.
- Combining observations with chemical modeling is essential for interpretation.

## Abstract

Understanding the matter cycle in the interstellar medium of galaxies from the assembly of clouds to star formation and stellar feedback remains an important and exciting field in comtemporary astrophysics. Many open questions regarding cloud and structure formation, the role of turbulence, and the relative importance of the various feedback processes can only be addressed with observations of spectrally resolved lines. We here stress the importance of two specific sets of lines: the finestructure lines of atomic carbon as a tracer of the dark molecular gas and mid-J CO lines as tracers of the warm, active molecular gas in regions of turbulence dissipation and feedback. The observations must cover a wide range of environments (i.e., physical conditions), which will be achieved by large scale surveys of Galactic molecular clouds, the Galactic Center, the Magellanic clouds, and nearby galaxies. To date, such surveys are completely missing and thus constitute an important science opportunity for the next decade and beyond. For the successful interpretation of the observations, it will be essential to combine them with results from (chemical) modelling and simulations of the interstellar medium.