General Requirements
Regardless of the masing molecule, there are a few requirements that must be met for a strong maser source to exist. One requirement is a radio continuum background source to provide the radiation amplified by the maser, as all maser transitions take place at radio wavelengths. The masing molecule must have a pumping mechanism to create the population inversion, and sufficient density and path length for significant amplification to take place. These combine to constrain when and where megamaser emission for a given molecule will take place. The specific conditions for each molecule known to produce megamasers are different, as exemplified by the fact that there is no known galaxy that hosts both of the two most common megamaser species, hydroxyl and water. As such, the different molecules with known megamasers will be addressed individually.
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