ISO observations of a sample of Compact Steep Spectrum and GHz Peaked Spectrum radio galaxies

We present results from observations obtained with ISOPHOT, on board the IS O satellite(1), of a representative sample of seventeen CSS/GPS radio galax ies and of a control sample of sixteen extended radio galaxies spanning sim ilar ranges in redshift (0.2 less than or equal to z less than or equal to 0.8) and radio luminosity (P-2.7GHz greater than or equal to 10(26) W/Hz). The observations have been performed at lambda = 60, 90, 174 and 200 mu m. The original purpose of these observations was to check whether CSS/GPS sou rces are associated with very gas rich galaxies, as required by the scenari o in which the growth of the radio source is inhibited by the dense medium of the host galaxy. Unfortunately the resulting performance of ISOPHOT was worse than expected. As a consequence, the detection limit at 60 mu m is similar to that obtain ed previously with IRAS but better than that at 90 mu m. Seven of the CSS/GPS sources have detections greater than or equal to 3 sig ma at one or more wavelengths, one of which is detected at greater than or equal to 5 sigma. For the comparison sample five objects have detections gr eater than or equal to 3 sigma one of which is at greater than or equal to 5 sigma. By co-adding the data we have obtained average flux densities at the four w avelengths. We found no evidence that the FIR luminosities of the CSS/GPS sources are s ignificantly different from those of the extended objects and therefore the re is not any support for CSS/GPS sources being objects "frustrated" by an abnormally dense ambient medium. The two samples were then combined, providing FIR information on a new samp le of radio galaxies at intermediate redshifts. We compare this information with what previously known from IRAS and discuss the average properties of radio galaxies in the redshift range 0.2 - 0.8. The FIR emission cannot be accounted for by extrapolation of the synchrotron radio spectrum and we at tribute it to thermal dust emission. The average FIR luminosity is greater than or equal to 6 x 10(11)L.. Over the observed frequency range the infrar ed spectrum can be described by a power law with spectral index alpha simil ar or equal to 1.0 +/- 0.2. Assuming the emission to be due to dust, a rang e of temperatures is required, from greater than or equal to 80 K to approx imate to 25 K. The dust masses required to explain the FIR emission range f rom 5 x 10(5) M. for the hotter component up to 2 x 10(8) M. for the colder one. We present also observations on four nearby (z less than or equal to 0.1) G PS radio galaxies, two of which are detected at all four wavelengths.