An infrared study of quasars

New high quality near infrared data are presented for a significant number of quasars with redshifts out to 3.12. The colours (J–H) and (H–K) are highly redshift dependent, becoming larger to smaller redshifts. When the effects of emission lines in the bandpasses have been taken into account, these colours demand the presence of excess continuum emission in the 1–3 μm region (in the rest frame), over and above the continuum components present at shorter and longer wavelengths. Although there is considerable dispersion in this excess, it appears to be present in almost all quasars for which the relevant data are available. This excess emission in the 1–3 μm region is interpreted in terms of thermal emission from hot dust (T ~ 1200 K), which is shown to exist predominantly in regions where the broad emission lines are formed. The colours of a ‘mean’ zero-redshift quasar have been determined to be (J–H) = 0.95, (H–K) = 1.15. These can be combined with various percentages of typical galaxy colours to form a locus which is populated by Seyfert galaxies.