Stars are born in the densest regions of molecular clouds (MCs), but the process appears to be very inefficient, with MCs converting only a few percent of their gas budget into stars per dynamical time. The underlying physical processes that regulate the star formation rate in the interstellar medium (ISM) are still unknown and hotly debated, with candidates ranging from turbulence and magnetic fields to stellar feedback. Further debate stems from the observational evidence that while the “dense” regions in MCs in the solar neighbourhood appear to explain the observed galactic star formation relations, the same approach fails to explain the SFR towards the central molecular zone (CMZ) of the Milky Way. The primary reason for this debate is that we still do not understand how MCs are assembled and destroyed — the two processes that ultimately set the timescale over which a cloud can form stars. The problem is that carbon monoxide (CO), the main tracer of MC structure and dynamics, is only sensitive to the cold interiors of MCs and not their envelopes, and models show that CO may form relatively late in the assembly process. Therefore, alternative tracers that can probe gas in the absence of CO are needed to make further progress in understanding MC formation and destruction.
There is a growing consensus within the international community that fine structure line (FSL) tracers -- specifically ionised and atomic carbon ([CII] and [CI]) and atomic oxygen ([OI]) -- are the key probes of the earliest stages of cloud assembly, as they are chemically abundant in regions surrounding MCs where CO is not. As such, FSLs are able to trace the low-density transition from atomic to molecular gas that marks the boundary from the warm ISM to the cold reservoirs in which stars form. They also constitute the main coolants of ISM during this transition, thus providing a way to measure the energetics of the ISM. It is only within the last few years that we have gained the technological capabilities to map FSL emission in and around Galactic star formation sites. First with the Herschel Space Observatory and now with the Stratospheric Observatory for Infrared Astronomy (SOFIA), [CII] and [OI] mapping are possible. The Atacama Pathfinder Experiment (APEX) telescope can also map [CI] emission. With these facilities, it is now possible to measure the mass and dynamics of the “CO-dark” molecular and cold atomic gas that surround and permeate MCs, and combine this with our extensive knowledge of CO-traced cloud and the star formation inventory within.
The main objective of this project was to use FSL tracers alongside more traditional MC tracers to gain a broader understanding of the connection between MCs and their environment. We did this by examining the morphology and dynamics of a sample of clouds and comparing those results to numerical simulations, which helped us to identify the important physical mechanisms in the cloud formation.
