3.4. Evolution of Radio Sources

At radio wavelengths, Hook, Shaver, & McMahon (1998), building on the work of Shaver et al. (1996), have carried out an important survey that bears on the question of possible obscuration by dust at high redshifts. Their sample contains 442 radio sources with S_2.7GHz 0.25 Jy and stellar identifications. The highest redshift object has z = 4.46. For objects with radio power P_lim > 7.2 × 10²⁶ W Hz^-1 sr^-1, they find an evolution of the space density very similar to WHO, SSG, and the SDSS. This is strong evidence against dust reddening being the main cause of the decline at high redshifts.

At the same time, Webster et al. (1995) and Gregg et al. (2002) have argued that the finding of significant numbers of radio-selected quasars with very red values of B - K in the Parkes and FIRST surveys indicates that up to 80% of the population is being missed in traditional optical surveys because of dust obscuration. This interpretation has been challenged by Benn et al. (1998) and Whiting, Webster, & Francis (2001) on the grounds that the brightness in K can arise from the emission of the host galaxy and/or synchrotron radiation, not from obscuration in the B band by dust. Until this issue can be resolved, the question of dust-obscured quasars remains important to our understanding of quasar evolution, as indicated by the new X-ray results described below. The finding of reddened quasars in the 2MASS survey (Marble et al. 2003, and references therein) is also contributing important information on this subject.