EVN and MERLIN observations of III Zw 35 -: A starburst continuum and an OH maser ring

EVN and MERLIN observations of the 18 cm continuum and OH maser emission in the luminous infrared galaxy III Zw35 are presented. The continuum consists of two components. The first component is made up of a number of compact sources, resembling the types of objects observed in Arp 220 and Mrk 273. In III Zw35 the origin of these compact components is unclear, and we discuss the possibility they are individual young radio supernovae. Alternatively they could be clusters or clumps of young supernova remnants. The second continuum component is diffuse and has an angular size of similar to 460 mas (230 pc) and T-b similar to 10(5) K. We argue that this component is also powered by star formation. From the FIR flux we estimate a star formation rate of 19 M-. yr(-1), which in turn implies a supernova rate of 0.8 yr(-1), sufficient to power the diffuse radio emission. Turning to the OH maser emission we find, in agreement with earlier VLBI observations of III Zw35, that the bright OH maser emission is concentrated in two compact clumps. However, in addition we observe diffuse maser emission which lies in a clear ring structure of radius r similar to 22 pc. The two compact maser features occur at the tangent points of the ring. This geometry and their similar 1667/1665 MHz line ratios suggests that the compact and diffuse masers appear different because of path length effects and not pumping or other physical conditions. A simple model with many clouds within a ring seems able to fit the main properties of both the diffuse and compact maser emission. We find a velocity gradient across the diffuse maser emitting region which confirms rotation. From this gradient we estimate a dynamical mass of 7 x 10(6) M-. within r similar to 22 pc. OH absorption is also present just outside the ring of maser emission. Both the OH maser emission and the continuum is weaker on the Eastern side of the source. We argue that this could be due to free-free absorption effects, which implies gas with an emission measure of 7 x 10(6) cm(-6) pc.