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

Abstract

We present results from observations obtained with ISOPHOT, on board the ISO satellite, of a representative sample of seventeen CSS/GPS radio galaxies and of a control sample of sixteen extended radio galaxies spanning similar ranges in redshift (0.2 <= z <= 0.8) and radio luminosity (P_{2.7 GHz} >= 10²⁶ 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 sources are associated with very gas rich galaxies, as required by the scenario 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 60mu m is similar to that obtained previously with IRAS but better than that at 90mu m. Seven of the CSS/GPS sources have detections >= 3sigma at one or more wavelengths, one of which is detected at >= 5sigma . For the comparison sample five objects have detections >= 3sigma one of which is at >= 5sigma . By co-adding the data we have obtained average flux densities at the four wavelengths. We found no evidence that the FIR luminosities of the CSS/GPS sources are significantly different from those of the extended objects and therefore there 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 sample 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 attribute it to thermal dust emission. The average FIR luminosity is >= 6 x 10¹¹ L_sun. Over the observed frequency range the infrared spectrum can be described by a power law with spectral index alpha =~ 1.0 +/- 0.2. Assuming the emission to be due to dust, a range of temperatures is required, from >= 80 K to ~ 25 K. The dust masses required to explain the FIR emission range from 5 x 10⁵ M_sun for the hotter component up to 2 x 10⁸ M_sun for the colder one. We present also observations on four nearby (z <= 0.1) GPS radio galaxies, two of which are detected at all four wavelengths. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA.