8.9. Emission-Line Diagnostics of Reddening and Density
Very little is currently known about the emission-line properties of the GPS and CSS galaxies. The existing anecdotal evidence is that there is evidence for large reddening and high densities in the NLR of at least some objects (e.g., 2021+614, Bartel et al. 1984a; 1934-638, Fosbury et al. 1987 and Fosbury 1990).
Baker & Hunstead (1995, 1996) have compared composite spectra of different subclasses of quasars from the Molonglo quasar sample. The CSS quasar's steep power-law continuum, the high Balmer decrement, low ratio of Ly / C IV, and weak 3000 ä bump are all consistent with dust absorption being stronger in the CSS quasars than in the other types. Baker & Hunstead also find that absorption-line systems associated with the quasar are common in CSS sources. In addition, the low ionization narrow lines have large equivalent widths consistent with an additional source of ionization for the gas, e.g., shocks (cf. Bicknell et al. 1997).
On the other hand, Morganti et al. (1997) present optical spectra of seven CSS sources (five with only NLRs and two with broad-line regions, or BLRs) from a southern subset of the Wall & Peacock (1985) sample. Morganti et al. note that two of the objects have large Balmer decrements, but in general they find that the emission-line properties are generally not distinguishable from those of large-scale radio sources in terms of their [O II] 3727 luminosity or their location on diagnostic line-ratio diagrams. They suggest that the emission-line properties do not show any evidence that these are "frustrated" sources.