2005A&A...436..457G

We present the results of radio (VLA) and optical (ESO/La Silla) imaging of a sample of 52 radio sources having an ultra-steep radio spectrum with α mostly steeper than -1.1 at decimetre wavelengths (median α=-1.22). Radio-optical overlays are presented to an astrometric accuracy of ∼1''. For 41 of the sources, radio spectral indices are newly determined using unpublished observations made with the 100-m Effelsberg radio telescope. For 14 of the sources identified with relatively brighter optical counterparts, spectroscopic observations were also carried out at La Silla and their redshifts are found to lie in the range 0.4 to 2.6. These observations have revealed three distant clusters of galaxies with redshifts of 0.55, 0.75 and 0.79, and we suggest that, together with an ultra-steep radio spectrum and relaxed radio morphology, the presence of a LINER spectrum in the optical can be used as a powerful indicator of rich clusters of galaxies. Additional candidates of this type in our sample are pointed out. Also, sources exhibiting particularly interesting radio-optical morphological relationships are highlighted. We further note the presence of six sources in our sample for which the optical counterpart (either detected or undetected) is fainter than R∼24 and the radio extent is small (<10''). These ultra-steep spectrum radio sources are good signposts for discovering massive galaxies out to very large redshifts.