Global VLBI Observations of the Compact OH Megamaser Emission from III Zw 35 and IRAS 17208−0014

Abstract

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 01. 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 ~6×104 M☉ pc-3.