Hunting for the signatures of molecular cloud formation

TL;DR

This paper explores how the [CII] line can effectively trace CO-dark H2 gas in molecular cloud formation, addressing observational challenges and leveraging simulations to identify new detection methods.

Contribution

It demonstrates through simulations that the [CII] line is a promising tracer for CO-dark molecular gas in the process of cloud formation.

Findings

[CII] line effectively traces CO-dark H2 gas.

Simulations show [CII] detection is feasible with current instruments.

Provides a new approach to study early molecular cloud formation stages.

Abstract

In order to understand how molecular clouds form in the Galactic interstellar medium, we would like to be able to map the structure and kinematics of the gas flows responsible for forming them. However, doing so is observationally challenging. CO, the workhorse molecule for studies of molecular clouds, traces only relatively dense gas and hence only allows us to study those portions of the clouds that have already assembled. Numerical simulations suggest that the inflowing gas that forms these clouds is largely composed of CO-dark H2. These same simulations allow us to explore the usefulness of different tracers of this CO-dark molecular material, and we use them here to show that the [CII] fine structure line is potentially a very powerful tracer of this gas and should be readily detectable using modern instrumentation.