Global VLBI observations of the compact OH megamaser emission from III Zw 35 and IRAS 17208-0014

We have performed global VLBI observations of the OH megamaser galaxies III Zw 35 and IRAS 17208-0014 We detect strong and compact 1667 MHz OH emission from each galaxy, but, apart from one weak detection of a single 1665 MHz OH feature in III Zw 35, no 1665, 1720 MHz, or continuum emission are detected, which indicates that they occur on size scales greater than 0 ".1. We surmise that the OH emission consists of two components. One is diffuse, has unsaturated masers pumped by far-infrared radiation, and is readily explained by the classical OH megamaser model. The second is compact, consists of saturated masers, is responsible for up to 65% of the 1667 MHz flux density, is not associated with any compact continuum emission, and may be pumped by a combination of collisional and radiative processes. One component in III Zw 35 exhibits a well-defined velocity gradient of 32 +/- 4 km s(-1) pc(-1) along a 5.5 pc line of masers. This indicates strong rotation of a molecular cloud with an enclosed mass density of similar to 6 x 10(4) M-. pc(-3).