The Effect of 53 μm IR Radiation on 18 cm OH Megamaser Emission

OH megamasers (OHMs) emit primarily in the main lines at 1667 and 1665 MHz, and differ from their Galactic counterparts due to their immense luminosities, large linewidths and 1667/1665 MHz flux ratios, which are always greater than one. We find that these maser properties result from strong 53 μm radiative pumping combined with line overlap effects caused by turbulent linewidths ∼ 20 km s; pumping calculations that do not include line overlap are unreliable. A minimum dust temperature of ∼ 45 K is needed for inversion, and maximum maser efficiency occurs for dust temperatures ∼ 80 – 140 K. We find that warmer dust can support inversion at lower IR luminosities, in agreement with observations. Our results are in good agreement with a clumpy model of OHMs, with clouds sizes . 1 pc and OH column densities ∼ 5×10cm, that is able to explain both the diffuse and compact emission observed for OHMs. We suggest that all OH main line masers may be pumped by far-IR radiation, with the major differences between OHMs and Galactic OH masers caused by differences in linewidth produced by line overlap. Small Galactic maser linewidths tend to produce stronger 1665 MHz emission. The large OHM linewidths lead to inverted ground state transitions having approximately the same excitation temperature, producing 1667/1665 MHz flux ratios greater than one and weak satellite line emission. Finally, the small observed ratio of pumping radiation to dense molecular gas, as traced by HCN and HCO, is a possible reason for the lack of OH megamaser emission in NGC 6240. Subject headings: masers — radiative transfer — galaxies: starburst — infrared: galaxies — radio lines: galaxies Centre College, 600 West Walnut Street, Danville, KY 40422; [email protected] Physics and Astronomy, University of Kentucky, Lexington, KY 40506-0055; [email protected]