The unusual UV continuum of quasar Ton 34 and the possibility of crystalline dust absorption

Context. Luminous quasars are known to display a sharp steepening of the continuum near 1100 Å. This spectral feature is not well fitted by current accretion disk models, unless comptonization of the disk emission is invoked. Absorption by carbon crystalline dust has been proposed to account for this feature. Aims. Ton 34 (zq = 1.928) exhibits the steepest far-UV decline (Fν ∝ ν ) among the 183 quasar HST-FOS spectra analyzed by Telfer et al. It is an ideal object to test the crystalline dust hypothesis as well as alternative interpretations of the UV break. Methods. We reconstruct the UV spectral energy distribution of Ton 34 by combining HST, IUE and Palomar spectra. Results. The far-UV continuum shows a very deep continuum trough, which is bounded by a steep far-UV rise. We fit the trough assuming nanodiamond dust grains. Conclusions. Extinction by carbon crystalline dust reproduces the deep absorption trough of Ton 34 reasonably well, but not the observed steep rise in the extreme UV. We also study the possibility of an intrinsic continuum rollover. The dust might be part of a high velocity outflow (≃ 13000 kms), which is observed in absorption in the lines of C ivλ1549, Oviλ1035, Nvλ1240 and Lyα.