Resolved structure in the nuclear region of the ultraluminous infrared galaxy Markarian 273

We have studied the core morphology of the ultraluminous infrared galaxy Mrk 273 by combining a high- resolution adaptive optics near-infrared image with an optical image from the Hubble Space Telescope and interferometric radio continuum data, all at spatial resolutions of 150 mas or better. The near-infrared image reveals that the nucleus has two main components, both of which have radio counterparts. The strongest component (N) shows very similar extended structure in the radio and near-infrared. It has a hat radio spectrum and is resolved into a double-lobed structure (Ne; Nw), with a separation of 90 +/- 5 mas (70 pc). A similar structure is detected in the near-infrared. We identify this component as the location of the active nucleus. The second component (SW), strong in the near-infrared but relatively weak in the radio, is located similar to 1 '' to the southwest. We interpret this as an obscured starburst region associated with the merger. The radio continuum images show a third, strong, component (SE) that has previously been interpreted as a second nucleus. However it shows no associated optical or near-infrared emission, which suggests that it is in fact a background source.