Since radio sources with Ultra Steep Spectra (USS; alpha less than or simil
ar to -1.30; S proportional to nu(alpha)) are efficient tracers of high red
shift radio galaxies (HzRGs), we have defined three samples of such USS sou
rces using the recently completed WENSS, TEXAS, MRC, NVSS and PMN radio-sur
veys. Our combined sample contains 669 sources with S(140)0 > 10 mJy and co
vers virtually the entire sky outside the Galactic plane (\b\ > 15 degrees)
. For our 2 largest, samples, covering delta > -35 degrees, we selected onl
y sources with angular sizes Theta < 1', For 410 sources, we present radio-
maps with 0." 3 to similar 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 d
o not affect the angular size distribution of the sample, but do avoid sign
ificant contributions by faint foreground spiral galaxies. We find that the
spectral index distribution of 143000 sources from the WENSS and NVSS cons
ists of a steep spectrum galaxy and a flat spectrum quasar population, with
the relative contribution of flat spectrum sources doubling from S-1400 >
0.1 Jy to S-1400 > 2 5 Jy, The identification fraction of our USS sources o
n the POSS (R less than or similar to 20) is as low as 15%, independent of
spectral index alpha < -1.30. We further show that 85% of the USS sources t
hat can be identified with art X-ray source are probably contained in galax
y clusters, and that alpha < -1.6 sources are excellent Galactic pulsar can
didates, because the percentage of these sources is four times higher in th
e Galactic piano.
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 tha
t does not suffer from dust extinction or any other optical bias(1).