On the optical-to-far-infrared spectral energy distributions of radio-quiet quasars

Spectral energy distributions of 36 radio-quiet quasars covering observed wavelengths of 0.3-100 microns are presented using data from the published literature. All have IRAS detections or interesting upper limits. Most of the objects are low-luminosity quasars, but several are moderate to high in luminosity. The continua show a variety of shapes which can be explained as a combination of three components. These components are (1) a flat power- law (α ~ -0.2;fnu_ is proportional to νalpha^) optical continuum, (2) a convex infrared continuum produced by warm dust heated directly by the quasar UV, and (3) a host galaxy component consisting of starlight and cool dust emission. The third component should only affect the continua of low- to moderate-luminosity objects. Detailed model fits to representative sources show that the overall continuum shapes can be accounted for by these components alone. No nonthermal infrared source is necessary in radio-quiet quasars. Evidence for contaminating emission from host galaxies is given in plots of optical and near-infrared spectral indices versus quasar luminosity. Low-luminosity objects tend to have flatter near- infrared and steeper optical spectral indices relative to higher luminosity objects, consistent with the addition of starlight components (1) and (2) above. The amount of starlight required is equal to the mean starlight luminosity of known quasar host galaxies. The dust model for the infrared explains both the short- and long-wavelength cutoffs of the infrared spectrum as being a direct result of the maximum and minimum temperatures that grains can attain. The long-wavelength cutoff in the submillimeter region corresponds to the lowest possible grain temperatures (~ 3-10 K). The short-wavelength cutoff, which produces the universally seen inflection in the continuum at 1 micron, is a result of grain evaporation at high temperatures (~ 1500 K). In the optical, the mean value for the spectral index in high-luminosity objects (i.e., those uncontaminated by host galaxies) is -0.2. This is very close to the spectral index of simple optically thin free-free emission, and it is suggested that this mechanism may be responsible for the optical continuum.