Astron. Astrophys. 325, L21-L24 (1997)

1. Introduction

One of the major outcomes of the IRAS mission was the finding of a class of galaxies which emit the bulk of their energy in the far infrared (Soifer et al. 1987). On optical images most of the luminous ones show a peculiar morphology indicating interactions (e.g. Klaas and Elsässer 1993, and for a review Sanders and Mirabel 1996). Sanders et al. (1988) defined the class of ultra-luminous IR galaxies with 10 and proposed that these objects represent the evolutionary link between starburst galaxies and quasars.

The evolution of the infrared luminous galaxies and the nature of their heating sources are one of the key issues of the coordinated Central Programme of the Infrared Space Observatory (ISO, Kessler et al. 1996; ISOPHOT, Lemke et al. 1996). In this letter we present detailed 3 to 200 µm photometry obtained with ISOPHOT for three classical interacting systems showing increasing interaction strength:

• 1) Arp 244 (= NGC4038/39), "The Antennae" being the prototype of an interacting system (e.g. Whitmore and Schweizer 1995),
• 2) NGC 6240, an interacting system with the two nuclei separated by only 2 (e.g. Fried and Schulz 1983),
• 3) Arp 220 (= IC 4553), an ultra-luminous merger (e.g. Graham et al. 1990).

© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

helpdesk.link@springer.de