Nanodiamond Dust and the Far‐Ultraviolet Quasar Break

We explore the possibility that the steepening observed shortward of 1000A inthe energy distribution of quasars may result from absorption by dust, beingeither intrinsic to the quasar environment or intergalactic. We find that adust extinction curve consisting of nanodiamonds, composed of terrestrial cubicdiamonds or with surface impurities as found in carbonaceous chondritemeteorites, such as Allende, is successful in reproducing the sharp breakobserved. The intergalactic dust model is partially successful in explainingthe shape of the composite energy distribution, but must be discarded in theend, as the amount of crystalline dust required is unreasonable and would implyan improbable fine tuning among the dust formation processes. The alternativeintrinsic dust model requires a mixture of both cubic diamonds and Allendenanodiamonds and provide a better fit of the UV break. The gas column densitiesimplied are of the order 10^{20} cm^{-2} assuming solar metallicity for carbonand full depletion of carbon into dust. The absorption only occurs in theultraviolet and is totally negligible in the visible. The minimum dust massrequired is of the order ~ 0.003 r_{pc}^{2}M_o, where r_{pc} is the distance inparsec between the dust screen and the continuum source. The intrinsic dustmodel reproduces the flux {\it rise} observed around 660A in key quasar spectraquite well. We present indirect evidence of a shallow continuum break near 670A(18.5 eV), which would be intrinsic to the quasar continuum.Comment: 13 COLOR figures, 6 BW figures, ACCEPTED in ApJ (June 2005