U. Klaas 1, M. Haas 1, & B. Schulz 2
1 Max-Planck-Institut für Astronomie Heidelberg, Germany
2 ISO Science Operations Centre
3 Astrophysics Division, Space Science Department of ESA Villafranca, Spain
As part of the coordinated ISO Central Programme we have determined with
ISOPHOT the spectral energy distributions from 10 to 240 micron of the 35
brightest (
)
ULIRGs. The sample has considerable overlap
with
the spectroscopic survey performed with ISOPHOT-S, SWS (and LWS) by
Genzel et al. and the imaging survey with ISOCAM by Mirabel et al..
The ISOPHOT photometry in 10 wavelength bands allows to better disentangle the various dust components attributed to different heating sources and to determine more accurately global properties of these galaxies, like the maximum of the infrared energy distribution, bolometric luminosities, dust temperatures and masses. In particular, the 4 photometric points between 120 and 200 allow now to recognize for the first time the peak of the FIR emission and the relative contribution by cold dust. Also in the 12 micron wavelength region the SEDs are much better defined now, as IRAS provided in many cases only upper limits.
We have established photometric templates for the archetype galaxies Arp 220 and NGC 6240 with the densest photometric coverage in 23 wavelength bands from 3.3 to 240 micron. Furthermore, in order to explore the dependence of the SEDs on redshift, we have measured the FIR spectral energy distributions of four ULIRGs in the redshift range z = 0.3 - 0.4 from 120 to 240 micron. This statistical sample allows to look for signatures of evolution in the SEDs.
We are able to fit the measured SEDs by two dust components. The temperature of the warmer component varies between 100 and 150 K, while the colder component is found to be in the range 30 to 55 K. The latter component dominates the total energy output of these objects. There is no correlation found between warm and cold component. There is no indication for a strong excess of very cold dust in these objects. In comparison to other statistical samples of the ISOPHOT Guaranteed Time extragalactic programme (Seyfert and quasar samples) typical differences in the SEDs can be worked out. Correlations with the photometric and spectroscopic properties in the X-ray, optical, IR, and radio are also done.