Extinction due to amorphous carbon grains in red quasars from the Sloan Digital Sky Survey

Abstract

We construct a quasar extinction curve based on the blue and red composite quasar spectra of Richards et al. prepared from the Sloan Digital Sky Survey. This extinction curve does not show any traces of the 2200-Å feature characteristic of the interstellar medium, and this indicates that graphite grains are likely to be absent close to quasar nuclei. The extinction is best modelled by an AC sample of amorphous carbon grains, assuming a standard distribution of grain sizes (p = 3.5) but slightly larger minimum grain size (amin = 0.016 μm) and lower maximum grain size (amin = 0.12 μm) than the respective canonical values for the interstellar medium. The dust composition is thus similar to that of the dust in carbon reach asymptotic giant branch stars. Since graphite grains form from amorphous carbon exposed to strong ultraviolet irradiation, the results indicate that the dust forms either surprisingly far from the active nucleus or in a wind that leaves the nucleus quickly enough to avoid crystallization into graphite.