# The Interstellar Medium: The Key Component in Galactic Evolution and Modern Cosmology

## TL;DR

This paper discusses how the FAST radio telescope will significantly advance understanding of the interstellar medium's complex phases, magnetic fields, and molecular gas, through high-resolution surveys and targeted observations.

## Contribution

It introduces planned FAST projects that will provide unprecedented insights into the ISM, including high-resolution surveys and simultaneous multi-data stream observations.

## Key findings

- FAST will enable detailed HI and OH mapping of the northern sky.
- The telescope's commensal mode allows simultaneous HI and pulsar data collection.
- Expected to significantly improve understanding of ISM's complex structure and magnetic fields.

## Abstract

The gases of the interstellar medium (ISM) possess orders of magnitude more mass than those of all the stars combined and are thus the prime component of the baryonic universe. With L-band surface sensitivity even better than the planned phase one Square-Kilometer-Array (SKA1), the Five-hundred-meter Aperture Spherical radio Telescope (FAST) promises unprecedented insights into two of the primary components of ISM, namely, the atomic hydrogen (HI) and the hydroxyl molecule (OH). We discuss here the evolving landscape of our understanding of ISM, particularly, its complex phases, the magnetic fields within, the so-called dark molecular gas (DMG), high velocity clouds, and the connection between local and distant ISM. We lay out, in broad strokes, several expected FAST projects, including an all northern-sky high-resolution HI survey (22,000 deg$^2$, 3\arcmin\ FWHM beam, 0.2 km/s), targeted OH mapping, searching for absorption or masing signals, and etc. Currently under commissioning, the commensal observing mode of FAST will be capable of simultaneously obtaining HI and pulsar data streams, making large-scale surveys in both science areas more efficient.