What molecule acts as a tracer for invisible molecular hydrogen in cold Giant Molecular Clouds?
Carbon monoxide (CO)
Carbon monoxide ($ ext{CO}$) serves a vital function in identifying the existence and location of the cold, dense Giant Molecular Clouds (GMCs) where stars begin to form. While GMCs are predominantly composed of molecular hydrogen ($ ext{H}_2$), molecular hydrogen itself is extremely difficult to observe directly because it does not radiate efficiently at the extremely low temperatures found within these nurseries (typically below 100 Kelvin). $ ext{CO}$, however, is abundant enough within these cold gas complexes and possesses emission lines detectable in the radio and microwave regions of the electromagnetic spectrum. Astronomers map the emissions from $ ext{CO}$, using its distribution as a proxy or 'tracer' to map out where the vast reservoirs of invisible molecular hydrogen are concentrated, thereby confirming the necessary starting conditions for gravitational collapse.
