The nature of the infrared luminous galaxies Arp 220 and NGC 6240

An 8–13 μm spectrum of NGC 6240 and an 8–13 plus 17–22 μm spectrum of Arp 220 are presented. Both indicate starburst activity by the presence of the characteristic dust emission features at 7.7, 8.6 and 11.25 μm. Strong silicate absorption is also present in both galaxies. The observed spectrum of NGC 6240 is consistent with a starburst as the major source of luminosity in this galaxy. In Arp 220, however, it is found that starburst activity alone is not sufficient to explain the large far-infrared luminosity. It is supposed that this galaxy combines an obscured Seyfert nucleus with surrounding regions of vigorous star formation.