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

TL;DR

This paper investigates the steep UV continuum of quasar Ton 34, proposing that absorption by crystalline dust, possibly in a high-velocity outflow, explains its spectral features better than current accretion disk models.

Contribution

It provides the first detailed analysis of crystalline dust absorption as an explanation for the UV break in Ton 34, challenging existing models and suggesting dust in high-velocity outflows.

Findings

Crystalline dust extinction fits the UV trough well

The steep UV rise is not explained by dust alone

High-velocity outflow absorption lines support dust hypothesis

Abstract

Luminous quasars are known to display a sharp steepening of the continuum near 1100A. 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. Ton 34 (z=1.928) exhibits the steepest far-UV decline (F_nu prop nu^{-5.3}) 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. We reconstruct the UV spectral energy distribution of Ton 34 by combining HST, IUE and Palomar spectra. 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. 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 km/s), which is observed in absorption in the lines of CIV, OVI, NV and Ly_alpha.