Determine which interstellar-medium phase dominates [C II] emission

Determine the proportion of [C II] emission arising from the dominant interstellar-medium phase, because the [C II] line traces more than one phase and its interpretation is required for reliable high-redshift galaxy-kinematic comparisons.

Background

At redshifts above approximately four, CO observations become difficult, so [C II] is frequently used as an alternative tracer of galaxy kinematics. However, [C II] emission can arise from multiple interstellar-medium phases, including neutral atomic gas, making the physical interpretation of its measured velocity dispersion and rotation uncertain. Establishing the relative contribution of each phase is therefore an unresolved issue for comparing [C II] kinematics with ionised- and molecular-gas measurements.

References

However, this line traces more than one phase of the ISM, and the proportion of the dominant phase is still under study (for example, \citealt{casavecchia_atomic_2025} find that [C\,{\sc ii}] mostly traces neutral atomic gas).

— MUSE-ALMA Haloes XV: Cold molecular, ionised, and stellar kinematics of HI-rich galaxies at z $\sim$ 0.5  (2610.10113 - Barfety et al., 7 Oct 2026) in Section 1, Introduction