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

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.Comment: 7 figures, to appear in A&

Relation between source and temperature fluctuations in photoionized nebulae

The magnitude of the temperature fluctuations (t^2) required to explain the observed inconsistencies between metallicities inferred from recombination lines and from forbidden lines cannot be attained by steady-state equilibrium photoionization models. If on the other hand the nebular ionizing source was variable, the temperature fluctuations t^2 would be significantly larger. We investigate the time-dependent response of the nebular ionization and temperature structure when photoionized by a periodically varying source. We study how the asymptotic mean value, , behaves as a function of the period or amplitude of the source variability. We find that the temperature fluctuations occur only in the outer section of the nebula, close to the ionization front, within a zone corresponding to 8-20% of the ionized layer's thickness. We conclude that the amplitude of the exciting star variations required to achieve a = 0.025 (as in the Orion nebula) is unacceptably large. Source variability is therefore not a viable mechanism to explain the observed values of t^2. We reach a similar conclusion from studies of the temporal variability resulting from intermittent shadows behind opaque condensations. We find that photoionized nebulae are on average less massive but somewhat hotter in the case of cyclicly variable ionizing sources.Comment: 15 pages, 6 figures, submitted to Revista Mexicana de Astronomia y Astrofisica, revised versio

Technique for detecting warm-hot intergalactic gas in quasar UV spectra

The ionizing spectral energy distribution of quasars exhibits a steepening of the distribution shortward of ~ 1200 A. The change of the power-law index from approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being intrinsic to quasars. We consider the possibility that the steepening may result from a tenuous absorption component that is anticorrelated with large mass overdensities. UV sensitive satellites, whose detectors can extend down to 1000 A, can set a useful limit to such an absorption component through the search of a flux increase in the window 1050-1190 A (observer frame) with respect to an extrapolation of the continuum above 1230 A. Since the recent FUSE or HST-STIS data do not show any obvious discontinuity in this region, this effectively rules out the possibility that intergalactic HI absorption is very important, and it is concluded that most if not all of the steepening is intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however, be ruled out yet and would still be consistent with the warm-hot intergalactic component if it amounts to 30% of the baryonic mass, as predicted by some models of large scale structure formation, provided its temperature lies around 10^{5.5} K.Comment: 13 pages, 5 figures and 1 table, accepted for publication in Astrophysical Journal, typos corrected and correction for the faulty scaling of variable n_H^0 with redshif