Mon. Not. R. Astron. Soc., 371, 852-866 (2006/September-2)
A search for distant radio galaxies from SUMSS and NVSS - III. Radio spectral energy distributions and the z-α correlation.
KLAMER I.J., EKERS R.D., BRYANT J.J., HUNSTEAD R.W., SADLER E.M. and DE BREUCK C.
Abstract (from CDS):
This is the third in a series of papers that present observations and results for a sample of 76 ultrasteep-spectrum radio sources designed to find galaxies at high redshift. Here we present multifrequency radio observations, from the Australia Telescope Compact Array, for a subset of 37 galaxies from the sample. Matched resolution observations at 2.3, 4.8 and 6.2 GHz are presented for all galaxies, with the z < 2 galaxies additionally observed at 8.6 and 18 GHz. New angular size constraints are reported for 19 sources based on high-resolution 4.8- and 6.2-GHz observations. Functional forms for the rest-frame spectral energy distributions are derived: 89 per cent of the sample is well characterized by a single power law, whilst the remaining 11 per cent show some flattening towards higher frequencies: not one source shows any evidence for high-frequency steepening. We discuss the implications of this result in light of the empirical correlation between redshift and spectral index seen in flux-limited samples of radio galaxies. Finally, a new physical mechanism to explain the redshift - spectral index correlation is posited: extremely steep-spectrum radio galaxies in the local universe usually reside at the centres of rich galaxy clusters. We argue that if a higher fractions of radio galaxies, as a function of redshift, are located in environments with densities similar to nearby rich clusters, then this could be a natural interpretation for the correlation. We briefly outline our plans to pursue this line of investigation.
2006 The Authors. Journal compilation © 2006 RAS
surveys - galaxies: active - radio continuum: galaxies - radio continuum: general
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