Continuum and spectral line observations of the OH Megamaser galaxy Arp 220

We present MERLIN observations of the continuum (both 1.6 and 5 GHz) and OH maser emission towards Arp 220. The correct spatial configuration of the various components of the galaxy is revealed. In the eastern component the masers are shown to be generally coincident with the larger-scale continuum emission; in the west, the masers and continuum do not generally arise from the same location. A velocity gradient [(0.32 ± 0.03) km s⁻¹ pc⁻¹] is found in the eastern nuclear region on MERLIN scales; this gradient is three times smaller than that seen in H i and implies that the OH gas lies inside the H i. A re-analysis of previously presented global very long baseline interferometry data reveals a very high velocity gradient [(18.67 ± 0.12) km s⁻¹ pc⁻¹] in one component, possibly the site of a heavily obscured active galactic nucleus.