EVN and MERLIN observations of III Zw 35

Abstract

EVN and MERLIN observations of the 18 cm continuum and OH maser emission in the luminous infrared galaxy III Zw 35 are presented. The continuum consists of two components. The rst component is made up of a number of compact sources, resembling the types of objects observed in Arp 220 and Mrk 273. In III Zw 35 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 diuse and has an angular size of460 mas (230 pc) and Tb 10 5 K. We argue that this component is also powered by star formation. From the FIRflux we estimate a star formation rate of 19 M yr 1 ,w hich in turn implies a supernova rate of 0: 8y r 1 , sucient to power the diuse radio emission. Turning to the OH maser emission we nd, in agreement with earlier VLBI observations of III Zw 35, that the bright OH maser emission is concentrated in two compact clumps. However, in addition we observe diuse maser emission which lies in a clear ring structure of radius r 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 diuse masers appear dierent because of path length eects and not pumping or other physical conditions. A simple model with many clouds within a ring seems able to t the main properties of both the diuse and compact maser emission. We nd a velocity gradient across the diuse maser emitting region which conrms rotation. From this gradient we estimate a dynamical mass of 7 10 6 M within r 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 eects, which implies gas with an emission measure of 7 10 6 cm 6 pc.