2000A&AS..143..303D

Since radio sources with Ultra Steep Spectra (USS; α≲-1.30; S ∝ν^α) are efficient tracers of high redshift radio galaxies (HzRGs), we have defined three samples of such USS sources using the recently completed WENSS, TEXAS, MRC, NVSS and PMN radio-surveys. Our combined sample contains 669 sources with S₁₄₀₀ >10mJy and covers virtually the entire sky outside the Galactic plane (|b|>15°). For our 2 largest samples, covering δ>-35°, we selected only sources with angular sizes Θ<1'. For 410 sources, we present radio-maps with 0.3" to ∼5" resolution from VLA and ATCA observations or from the FIRST survey, which allows the optical identification of these radio sources. Comparison with spectrally unbiased samples at similar flux density levels, shows that our spectral index, flux density, and angular size selections do not affect the angular size distribution of the sample, but do avoid significant contributions by faint foreground spiral galaxies. We find that the spectral index distribution of 143000 sources from the WENSS and NVSS consists of a steep spectrum galaxy and a flat spectrum quasar population, with the relative contribution of flat spectrum sources doubling from S₁₄₀₀>0.1Jy to S₁₄₀₀>2.5Jy. The identification fraction of our USS sources on the POSS (R≲20) is as low as 15%, independent of spectral index α←1.30. We further show that 85% of the USS sources that can be identified with an X-ray source are probably contained in galaxy clusters, and that α←1.6 sources are excellent Galactic pulsar candidates, because the percentage of these sources is four times higher in the Galactic plane. Our sample has been constructed to start an intensive campaign to obtain a large sample of high redshift objects (z>3) that is selected in a way that does not suffer from dust extinction or any other optical bias.