Resolved OH megamaser emission in the nuclear region of the ultraluminous infrared galaxy Markarian 273

We present high angular resolution MERLIN observations of the 18-cm OH mase r and continuum emission associated with the active core of the ultralumino us infrared galaxy Markarian 273. The continuum emission comes from three d istinct regions in the central arcsecond of the galaxy. The brightest regio n of emission has a double-peaked structure which is spatially coincident w ith similar structures observed at 6 cm and 2.2 mu m. The peak of the OH ma ser emission is spatially coincident with the peak in the continuum. For th e first time the maser emission is spatially resolved, allowing us to measu re the gas motion within the central 100 pc of the galaxy. Maser emission i s found in both the 1665- and 1667-MHz lines, with no systematic offset fou nd in the spatial locations of the two lines. The brighter component of the maser emission shows ordered motion and is aligned along the axis of the d ouble-peaked structure in the brightest continuum region. The gas motion en ables us to estimate the central mass density to be 850 +/- 50 M. pc(-3), w hich corresponds to a total mass of approximate to 1.5x10(8) M..