We construct a catalog of radio sources detected by GB6 ( 6 cm ) , FIRST and NVSS ( 20 cm ) , and WENSS ( 92 cm ) radio surveys , and the SDSS optical survey .
The 2.7 million entries in the publicly-available master catalog are comprised of the closest three FIRST to NVSS matches ( within 30″ ) and vice-versa , and unmatched sources from each survey .
Entries are supplemented by data from the other radio and optical surveys , where available .
All objects with even a small probability of physical association are included , such that catalog users can easily implement their own selection criteria for data analysis .
We perform data analysis in the \sim 3000 \deg ^ { 2 } region of sky where the surveys overlap , which contains 140,000 NVSS-FIRST sources , of which 64,000 are detected by WENSS and 12,000 by GB6 .
About one third of each sample is detected by SDSS .
An automated classification method based on 20 cm fluxes defines three radio morphology classes : complex , resolved , and compact .
Radio color-magnitude-morphology diagrams for these classes show structure suggestive of strong underlying physical correlations .
Complex and resolved sources tend to have a steep spectral slope ( \alpha \sim - 0.8 ) that is nearly constant from 6 to 92 cm , while the compact class ( unresolved on \sim 5 \arcsec scale by FIRST ) contains a significant number of flat-spectrum ( \alpha \sim 0 ) sources .
In the optically-detected sample , quasars dominate the flat-spectrum compact sources while steep-spectrum and resolved objects contain substantial numbers of both quasars and galaxies .
Differential radio counts of quasars and galaxies are similar at bright flux levels ( > 100 mJy at 20 cm ) , while at fainter levels the quasar counts are significantly reduced below galaxy counts .
The optically-undetected sample is strongly biased toward steep-spectrum sources .
In samples of quasars and galaxies with SDSS spectra ( 2,885 and 1,288 respectively ) , we find that radio properties such as spectral slope , morphology , and radio loudness are correlated with optical color and luminosity .