A most fascinating and important development in molecular astrophysics is the detection of molecules in external galaxies. The opportunities to compare different types of galaxies and to study galactic structure from without, rather than within, more than compensate for the weaker signals and relatively poor spatial resolution when compared with sources in our own Galaxy, the Milky Way. Maser emission has been detected in three species: OH, H2O and H2CO. The last one is unique in that currently, more extragalactic (four) than Galactic H2CO maser sources have been detected. Most work on extragalactic masers has concentrated on studies of OH and H2O, and these are the species that will be covered here.
KeywordsRadio Source Molecular Cloud Column Density Spiral Galaxy Maser Emission
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