AAS 207th Meeting, 8-12 January 2006
Session 21 Starbursts, LIRGS and ULIRGS
Poster, Monday, 9:20am-7:00pm, January 9, 2006, Exhibit Hall

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[21.03] Explaining OH Megamasers

P. Lockett (Centre College), M. Elitzur (University of Kentucky)

OH megamasers differ from typical OH maser sources due to their immense power, large linewidth, and relative line strengths of the ground state maser lines. These masers have luminosities a million times larger than the usual OH maser and their linewidths may be more than 100 km/s. All four ground state transitions have been observed in emission with the main lines much stronger than the satellite lines. The 1667/1665 MHz main line ratio is always greater than one. This ratio is opposite that of galactic star forming regions but similar to that of late type stars. OH megamasers are only found in the most luminous and warm infrared galaxies. The emission consists of a diffuse source having optical depth of order one and there may be compact sources that are much more intense at 1667 MHz. The compact sources have sizes of about a parsec and linewidths that are still tens of km/s. It has been suggested that the compact and diffuse sources need different source conditions, but both may be due to the chance alignment of maser clumps. Each clump would have an opacity of about one, size of about 1 parsec and linewidth of about 20 km/s. We have performed a detailed study of these masers including the effects of line overlap. We find that the masers must be radiatively pumped and that a local linewidth greater than about 10 km/s is needed to explain the maser line ratios. Our calculations are in agreement with the clumpy maser model and the physical conditions believed to exist in the maser region.

This research was supported in part by NASA through the American Astronomical Society's Small Research Grant Program.

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