Infrared to millimetre photometry of ultra-luminous IR galaxies : New evidence favouring a 3-stage dust model

Abstract

Infrared to millimetre spectral energy distributions (SEDs) have been obtained for 41 bright ultra- luminous infrared galaxies (ULIRGs). The observations were carried out with ISOPHOT between 10 and 200 m and supplemented for 16 sources with JCMT/SCUBA at 450 and 850 m and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 m with the 2.2 m telescope on Calar Alto. These new SEDs represent the most complete set of infrared photometric templates obtained so far on ULIRGs in the local universe. The SEDs peak at 60{100 m and show often a quite shallow Rayleigh-Jeans tail. Fits with one single modied blackbody yield a high FIR opacity and small dust emissivity exponent T >30 K) to cold (30 K >T >10 K) temperatures. This provides evidence for two dust stages, the cool starburst dominated one and the cold cirrus-like one. The third stage with several hundred Kelvin warm dust is identied in the AGN dominated ULIRGs, showing up as a NIR-MIR power- law flux increase. While AGNs and SBs appear indistinguishable at FIR and submm wavelengths, they dier in the NIR-MIR. This suggests that the cool FIR emitting dust is not related to the AGN, and that the AGN only powers the warm and hot dust. In comparison with optical and MIR spectroscopy, a criterion based on the SED shapes and the NIR colours is established to reveal AGNs among ULIRGs. Also the possibility of recognising evolutionary trends among the ULIRGs via the relative amounts of cold, cool and warm dust components is investigated.