Astronomy & Astrophysics

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Issue		A&A Volume 404, Number 1, June II 2003
Page(s)		57 - 62
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20030476

A&A 404, 57-62 (2003)
DOI: 10.1051/0004-6361:20030476

New results on the spectral index-flux density relation from the WENSS/NVSS catalogs

X. Zhang^{1, 2}, W. Reich², P. Reich² and R. Wielebinski²

¹ National Astronomical Observatories, CAS, Beijing 100012, PR China
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 30 July 2002 / Accepted 24 March 2003 )

Abstract
We present new statistical results on the spectral index-flux density relation for large samples of radio sources using archival data of the most sensitive surveys, such as WENSS and NVSS. Instrumental selection effects and the completeness of the catalogs used in this study are discussed. Our main results are based on the spectral indices calculated for about 185 800 sources from the WENSS (327 MHz) and the NVSS (1.4 GHz) catalogs and are summarized as follows: (1) The median spectral index increases from $\rm\alpha_{med} \sim -0.9$ to $\rm\alpha_{med} \sim -0.8$ ( $S_\nu \propto \nu^{\alpha}$ ) for S₃₂₇ flux densities decreasing from 0.1 Jy down to 23 mJy. The median spectral indices nearly show no variation within the error bars in the flux density range above 100 mJy up to several Jy. The median spectral index slightly increases again for S₃₂₇ above several Jy. The new results confirm published models of the radio luminosity function (RLF) for sources with S₃₂₇ > 0.1 Jy and give constraints to the models for sources of $0.023~{\rm Jy} < S_ < 0.1~\rm Jy$ , respectively. (2) A dependence of the fractions of ultra-steep-spectrum sources (USS, $\rm -1.5 \le \alpha < -1.0$ ), steep-spectrum sources (SSS, $\rm -1.0 \le \alpha <-0.5$ ) and flat-spectrum sources (FSS, $\rm -0.5 \le \alpha \le 0.0$ ) is partly responsible for the spectral flattening. Another contribution to the spectral flattening comes from the variation of $\rm\alpha_{med}$ of steep-spectrum sources ( $\alpha < -0.5$ ) themselves which increases with decreasing flux densities. (3) The spectral flattening for faint sources (down to $S_ \sim 20\rm ~mJy$ ) with steep spectra ( $\alpha < -0.5$ ) suggests that $\rm\alpha_{med}$ is correlated with luminosity rather than redshift according to the source evolution model of Condon (1984).

Key words: methods: statistical -- radio continuum: galaxies -- galaxies: evolution

Offprint request: X. Zhang, zxz@ns.bao.ac.cn

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