The Distribution of Quasars and Galaxies in Radio Color-Color and   Morphology Diagrams
  We positionally match the 6 cm GB6, 20 cm FIRST and NVSS, and 92 cm WENSS radio catalogs and find 16,500 matches in ~3,000 deg2 of sky. Using this unified radio database, we construct radio "color-magnitude-morphology" diagrams and find that they display a clear structure, rather than a random scatter. We propose a simple, yet powerful, method for morphological classification of radio sources based on FIRST and NVSS measurements. For a subset of matched sources, we find optical identifications using the SDSS Data Release 1 catalogs, and separate them into quasars and galaxies. Compact radio sources with flat radio spectra are dominated by quasars, while compact sources with steep spectra, and resolved radio sources, contain substantial numbers of both quasars and galaxies. 
Z. Ivezic, R.J. Siverd, W. Steinhardt, A.S. Jagoda, G.R. Knapp, R.H.   Lupton, D. Schlegel, P.B. Hall, G.T. Richards, J.E. G...