SAMPLES OF ULTRA-STEEP SPECTRUM RADIO-SOURCES

Radio sources with ultra-steep spectra (USS) have been found to be exc ellent tracers of galaxies at redshifts z greater than or similar to 2 . In order to obtain a large sample of z greater than or similar to 2 galaxies, we have defined several new flux-Limited samples of USS radi o sources. These samples are selected at a range of frequencies from 3 8 MHz to 408 MHz and are fainter by a factor of three than the previou sly well-studied 4C samples of USS sources. We find that complete samp les of radio sources with angular diameters less than or similar to 1 arcmin selected at 38 MHz contain relatively fewer USS sources than sa mples of small radio sources selected at higher (greater than or simil ar to 150 MHz) frequencies. This is interpreted as due to flattening o f the spectra of distant USS sources at frequencies < 100 MHz. As a pr eliminary to optical imaging and spectroscopy, snapshot observations o f sources from these samples have been made with the VLA at 1.5 arcsec resolution. We present the positions, flux densities and radio struct ures for a total number of 605 sources derived from these observations . Our VLA images indicate that samples of USS sources selected at 38 M Hz contain a smaller proportion of small sources and a larger proporti on of diffuse sources than USS sources selected at higher (greater tha n or similar to 150 MHz) frequencies. This is consistent with the samp les of 38 MHz - selected USS sources containing a larger fraction of r elatively nearby radio sources, perhaps in clusters of galaxies.