ISO observations of a sample of CSS/GPS radio galaxies

C. Fanti^1,2, F. Pozzi^3,2, R. Fanti^1,2, S.A. Baum⁴, C.P. O'Dea⁴. M. Bremer⁵, D. Dallacasa^3,2, H. Falcke⁶, T. de Graauw⁷, A. Marecki⁸, G. Miley⁹, H. Rottgering⁹, R.T. Schilizzi¹⁰, I. Snellen¹¹, R.E Spencer¹², C. Stanghellini¹³

Astronomy & Astrophysics, in press

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 micron. 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 60 micron is similar to that obtained previously with IRAS but better than that at 90 micron. Seven of the CSS/GPS sources have detections >= 3 sigma at one or more wavelengths, one of which is detected at >= 5 sigma. For the comparison sample five objects have detections >= 3 sigma one of which is at >= 5 sigma. 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. In our opinion this argues against 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 * 10¹¹ L_o. 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 required dust masses range from 5 * 10^5 M_o for the hotter component up to 2 * 10^8 M_o for the colder one. We present also observations on four nearby (z <= 0.1) GPSs, two of which are detected at all four wavelengths.

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