The merger Seyfert galaxy Arp 220. Line and continuum absorption and   emission
  The line and continuum spectra of the merger galaxy Arp 220 are analysed with the aim of investigating the ionizing and heating sources. We refer to radio, optical, infrared and X-ray spectra. The results show that in agreement with other merger galaxies, the optical lines are emitted from gas photoionised by the AGN and heated by the shocks in the extended NLR. The infrared lines are better explained by the emission from gas close to the starburst. The starburst dominates the infrared emission. [OI] and [CI] lines in the far-infrared are formed in the internal region of extended clouds and are therefore absorbed, while [CII] lines are emitted from the external edges of outflowing clouds. The O/H relative abundances are about solar and N/H are higher than solar by a factor of 1.5, throughout the starburst region, while in the AGN extended NLR the O/H ratio is half solar. A relatively high dust-to-gas ratio is indicated by modelling the dust reprocessed radiation peak consistently with bremsstrahlung emitted from the clouds. The observed radio emission is thermal bremsstrahlung, while synchrotron radiation created by the Fermi mechanism at the shock front is absorbed. 
M. Contini
astro-ph.GA astro-ph.CO