15.4.4. Redshift/Spectral-Index Diagrams
A plot of the integral number N(, z | S, ) d d[log(z)] of sources per steradian stronger than S at frequency with spectral indices to + d and redshifts log(z) to log(z) + d[log(z)] shows the correlation of on z (or L) for steep-spectrum sources (Laing and Peacock 1980, and references therein). Such a redshift/spectral-index diagram for sources with S 2 Jy at = 1.4 GHz is plotted in Figure 15.8.
Figure 15.8. Redshift/spectral-index diagram for a sample of 202 sources in 3.86 sr stronger than S = 2 Jy at = 1.4 GHz. Quasar candidates are indicated by crosses at z 1, and empty fields by triangles at z 0.5. Abscissa: spectral index between 1.4 and 5 GHz. Ordinate: log redshift. Contours: 1, 2, 5, 10, 20, 30, 40, 50, 60, and 70 sr-1.