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We model the OH megamaser emission from the luminous infrared galaxy IIIZw35as arising from a narrow rotating starburst ring of radius 22 pc enclosing amass of 7 x 10^6 Msun. We show how both the compact and apparently diffusemaser emission from this ring can arise from a single phase of unsaturatedmaser clouds amplifying background radio continuum. The masing clouds areestimated to have a diameter of <0.7 pc and internal velocity dispersion ofabout 20 km/s. We find that the clouds are neither self-gravitating norpressure confined but are freely expanding. Their dispersal lifetimes may setthe vertical thickness of the ring. For an estimated internal density of 3 x10^3 cm^-3, cloud masses are of order 24 Msun. The observed spectral featuresand velocity gradients indicate that the clouds must be outflowing and escapingthe nucleus. The cloud mass outflow rate is estimated to be 0.8 Msun/yr, whilethe star formation rate is about 19 Msun/yr. Associated ionised gas, possiblygenerated from dissipated clouds, provides free-free absorption along thesource axis, explaining the observed East-West asymmetries. We show that theclumpiness of a maser medium can have a dramatic effect on what is observedeven in a relatively low gain OH megamaser. Specifically, in IIIZw35 our clumpymaser model naturally explains the large line to continuum ratios, the large1667MHz:1665MHz line ratios and the wide velocity dispersions seen in thecompact maser spots. Other astrophysical masers showing both compact andapparently diffuse emission might be explained by similar clumpy structures.Comment: 14 pages, 6 figures, Accepted for publication in Astronomy and  Astrophysic

A compact starburst ring traced by clumpy OH megamaser emission