8.6. Near-IR Colors
Anecdotal evidence suggests that at least some GPS sources have reddened nuclei that become bright in the near-IR (O'Dea et al. 1992; Stickel et al. 1996a, 1996b; Akujor et al. 1996; Bremer & Snellen 1996; Snellen et al. 1996c). Stickel et al. (1996a) report that some GPS "galaxies" are variable in the near-IR, suggesting that these are in fact members of the class of extremely red objects known as "red quasars" (Rieke, Lebofsky, & Kinman 1979; Rieke, Wisniewski, & Lebofsky 1982) or perhaps "optically quiet quasars" (Cotton, Owen, & Mahoney 1989; Kollgaard et al. 1995).
de Vries et al. (1998b, 1998c) present the results of a study of the spectral energy distribution across the R, J, H, and K bands of samples of GPS, CSS, and extended 3CR radio galaxies. They find that GPS, CSS, and 3CR sources have similar broadband spectra in the near-IR, consistent with them having similar stellar populations and AGNs (r - K colors presented by Snellen et al. 1996a are also consistent with this). The distribution of the ratio of nuclear to extended light at K band in all three samples is comparable - consistent with only a few GPS or CSS or 3CR galaxies being "red quasars." The sources that are red quasars are likely to be obscured by material in the host galaxy rather than an intervening galaxy.
de Vries et al. find the R - K spectral energy distribution (SED) is best-fitted with a metallicity that is consistent with solar and fairly old stellar populations (older than 5 Gyr) implying redshift of formation in the range 5-10 depending on the cosmology. de Vries et al. also find an extra near-IR component is needed that can be modeled by emission from dust at a temperature of ~ 1000 K. If this is the correct interpretation, this may be produced by circumnuclear material heated by the AGN, perhaps in the putative obscuring torus.