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A Sample of 669 Ultra Steep Spectrum Radio Sources
to Find High Redshift Radio Galaxies
Carlos De Breuck, Wil van Breugel, Huub J. A. Rottgering, George Miley
Received 1999 November 5; accepted 2000 February 10
Abstract
Since radio sources with Ultra Steep Spectra
(USS; alpha <= -1.30; S = S_o nu^alpha) 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 S_1400 > 10~mJy and
covers virtually the entire sky outside the Galactic plane (|b|>15deg).
For our 2 largest samples, covering delta >-35ddeg, we selected only
sources with angular sizes Theta < 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 143,000 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>0.1~Jy to S_100>2.5~Jy.
The identification fraction of our USS sources on the POSS
(R <= 20) is as low as 15%, independent of spectral index
alpha < -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 alpha < -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.
CATS: We serarch for in tables A.1, A.2 and A.3.
Appendices B, C and D are only available
in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html