Multiwavelength energy distributions of ultraluminous IRAS galaxies - I. Submillimetre and X-ray observations

We present photometric observations at 350, 450, 800 and 1100 μm of nine out of 10 ultraluminous IRAS galaxies from the BGS sample (Sanders et al.), carried out with the JCMT telescope, and X-ray observations of six of them, carried out with the ROSAT PSPC. Together with other published and archived data the multiwavelength spectral energy distributions for all the 10 galaxies of the sample, from radio out to the X-rays, are constructed and discussed. Submillimetre observations are consistent with a thermal origin of the energy output of these systems in almost all cases, and in particular are well fitted by the starburst model proposed by Rowan-Robinson & Efstathiou. The weak soft X-ray emission excludes the possibility of the objects being quasars with a modest line-of-sight reddening; however, the absorbing columns expected from CO measurements are enough to obliterate soft X-rays completely. There is evidence for two single-nucleus objects to be a Seyfert 1 (Mrk 231) and a Seyfert 2 (IRAS 05189) galaxy respectively, but even these objects are extremely X-ray-quiet. It is suggested that in most objects starburst activity rather than accretion on to a massive black hole is responsible for the tremendous amounts of energy released from infrared galaxies with luminosities L_FIR<~10^12 L_solar.