ROETTGERING H.J.A., LACY M., MILEY G.K., CHAMBERS K.C. and SAUNDERS R.
Abstract (from CDS):
Radio sources with ultra-steep spectra (USS) have been found to be excellent tracers of galaxies at redshifts z≳2. In order to obtain a large sample of z≳2 galaxies, we have defined several new flux-limited samples of USS radio sources. These samples are selected at a range of frequencies from 38MHz to 408MHz and are fainter by a factor of three than the previously well-studied 4C samples of USS sources. We find that complete samples of radio sources with angular diameters ≲1arcmin selected at 38MHz contain relatively fewer USS sources than samples of small radio sources selected at higher (≳150MHz) frequencies. This is interpreted as due to flattening of the spectra of distant USS sources at frequencies <100MHz. As a preliminary to optical imaging and spectroscopy, snapshot observations of sources from these samples have been made with the VLA at 1.5arcsec resolution. We present the positions, flux densities and radio structures for a total number of 605 sources derived from these observations. Our VLA images indicate that samples of USS sources selected at 38MHz contain a smaller proportion of small sources and a larger proportion of diffuse sources than USS sources selected at higher (≳150MHz) frequencies. This is consistent with the samples of 38MHz - selected USS sources containing a larger fraction of relatively nearby radio sources, perhaps in clusters of galaxies.
astronomical data bases: surveys - galaxies: active - radio continuum: galaxies