COMPACT OH-MEGAMASER AND PROBABLE QUASAR ACTIVITY IN THE GALAXY ARP-220

ARP 220 is the prototype far-infrared ultraluminous galaxy, and the or igin of its luminosity-a burst of massive star formation or a quasar o bscured by a layer of dense gas and dust-has been the subject of much debate(1,2) It also contains the prototypical OH megamaser(3)-an extre mely luminous version of masers (microwave lasers) commonly found in o ur own galaxy. It has been thought that the dense gas in the inner few hundred parsecs of megamaser galaxies acts as a low-gain masing scree n, pumped by the far-infrared radiation, which amplifies background co ntinuum emission from the nuclear regions(4-6). Here we show, using ne w very-long-baseline interferometry observations, that the OH line pea k in Arp 220 originates in a structure less than or equal to 1 pc acro ss, positionally aligned with a weak continuum feature, and that most of the emission originates on scales of less than or equal to 10 pc. T hese results imply that the maser is physically 10-100 times smaller t han previously thought(5,6), strongly suggesting that much of the far- infrared radiation from Arp 220 arises in a very small region, possibl y a dense molecular torus, surrounding a quasar nucleus.