Infrared photometry of z~1 3C quasars

Abstract

We present JHKL&#39; photometry of a complete sample of steep-spectrum radio-loud quasars from the revised 3CR catalogue in the redshift range 0.65 < z < 1.20. After correcting for contributions from emission lines and the host galaxies, we investigate their spectral energy distributions (SEDs) around 1 micron. About 75% of the quasars are tightly grouped in the plane of optical spectral index, alpha_lo, versus near-infrared spectral index, alpha_hi, with the median value of alpha_lo close to the canonical value, and the median alpha_hi slightly flatter. We conclude that the fraction of moderately-obscured, red quasars decreases with increasing radio power, in accordance with the `receding torus&#39; model which can also explain the relatively flat median near-infrared spectra of the 3CR quasars. Two of the red quasars have inverted infrared spectral indices, and we suggest that their unusual SEDs might result from a combination of dust-scattered and transmitted quasar light.