Symmetric parsec-scale OH megamaser structures in ARP 220

The parsec-scale OH megamaser emission in the luminous IR galaxy Arp 220 has been imaged in detail using a global VLBI array. Four major emission regions are revealed in the 1667 MHz line, each with complex spatial and velocity structure showing intriguing symmetries. These emission regions have no associated continuum emission to stringent limits, and the brighter components have a maser amplification ratio exceeding 800. No compact emission is detected in the 1665 MHz line. The compact maser emission, with high amplification and unmeasurably small 1665/1667 line ratio, appears to be the result of saturated masers in physically compact masing clouds. The diffuse emission, on the other hand, appears to fit the traditional OH megamaser model of a low-gain masing screen on scales of hundreds of parsecs. Infrared pumping is indicated for the diffuse emission, but collisional pumping is probably important for the compact components. The compact components may trace shock fronts in the dense nuclear environment and may be related to active galactic nucleus activity.