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

Infrared to millimetre spectral energy distributions (SEDs) have been obtai ned for 41 bright ultraluminous infrared galaxies (ULIRGs). The observation s were carried out with ISOPHOT between 10 and 200 mum and supplemented for 16 sources with JCMT/SCUBA at 450 and 850 mum and with SEST at 1.3 mm. In addition, seven sources were observed at 1.2 and 2.2 mum with the 2.2 m tel escope on Calar Alto. These new SEDs represent the most complete set of inf rared photometric templates obtained so far on ULIRGs in the local universe . The SEDs peak at 60-100 mum and show often a quite shallow Rayleigh-Jeans tail. Fits with one single modified blackbody yield a high FIR opacity and small dust emissivity exponent beta < 2. However, this concept leads to co nflicts with several other observational constraints, like the low PAH exti nction or the extended filamentary optical morphology. A more consistent pi cture is obtained using several dust components with <beta> = 2, low to mod erate FIR opacity and cool (50 K > T > 30 K) to cold (30 K > T > 10 K) temp eratures. This provides evidence for two dust stages, the cool starburst do minated one and the cold cirrus-like one. The third stage with several hund red Kelvin warm dust is identified in the AGN dominated ULIRGs, showing up as a NIR-MIR power-law flux increase. While AGNs and SBs appear indistingui shable at FIR and submm wavelengths, they differ in the NIR-MIR. This sugge sts 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 es tablished 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.