天体物理学报(英文版)
天體物理學報(英文版)
천체물이학보(영문판)
ACTA ASTROPHYSICA SINICA
2003年
4期
347-358
,共12页
radio continuum: general - methods: statistical - cosmology
We present new statistical results on the spectral index-flux densityrelation for large samples of radio sources using archival data of the most sensitivesurveys, such as 6C, Miyun, WENSS, B3, NVSS, GB87. Instrumental selectioneffects and the completeness of the catalogs are discussed. Based on the spec-tral indices calculated for about 200 000 sources from the WENSS (327 MHz) andNVSS (1.4 GHz) catalogs, we obtained (1) The median spectral index increases fromαmed ~ -0.9 to αmed ~ -0.8 (Sν∝να), while S327 flux densities decrease from0.1 Jy down to 25 mJy. The median spectral indices nearly show no variation withinthe error bars when the flux density is larger than 0.1 Jy. (2) A dependence of thefraction of ultra-steep spectrum sources (USS, -1.5 ≤α<-1.0), steep spectrumsources (SSS, -1.0 ≤α< -0.5) and flat spectrum sources (FSS, -0.5 ≤α≤ 0.0) ispartly responsible for the spectral flattening. Another contribution to the spectralflattening comes from the variation of αmed of steep spectrum sources (α<-0.5)themselves which increases with decreasing flux densities. (3) The spectral flatteningfor faint sources (down to S327 ~ 20 mJy) with steep spectra (α<-0.5) suggeststhat αmed is correlated with luminosity rather than redshift according to the Con-don' model. (4) A strong spectral selection effect occurs when spectral indices arecalculated from samples with a large frequency separation.