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 \mu m radiative pumping combined with line overlap effects caused by turbulent linewidths \sim 20 km s ^ { -1 } ; pumping calculations that do not include line overlap are unreliable .
A minimum dust temperature of \sim 45 K is needed for inversion , and maximum maser efficiency occurs for dust temperatures \sim 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 \lesssim 1 pc and OH column densities \sim 5 \times 10 ^ { 16 } cm ^ { -2 } , 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 .