Which molecule acts as a proxy tracer for observationally silent molecular hydrogen in cold clouds?
Answer
Carbon monoxide (CO)
Molecular hydrogen ($ ext{H}_2$), despite being the most abundant constituent of molecular clouds, is extremely difficult to observe directly because it does not emit significant radiation in the visible or easily detectable spectral ranges at such low temperatures. To map the structure and movement of these cold, dense regions, astronomers utilize trace molecules that do radiate. Carbon monoxide ($ ext{CO}$) is the primary choice because it radiates effectively in the millimeter-wave part of the spectrum, allowing scientists to use its emission lines as a reliable observational proxy for tracing the distribution of the much more prevalent, but radiatively silent, molecular hydrogen.
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