The reionization of He II and the temperature evolution of the intergalactic medium

A number of observations suggest that He II in the intergalactic medium(IGM) was fully ionized at z ～ 3, probably by quasi-stellar objects(QSOs). Here we construct a simple model of a QSO to study the reionization of He II and the corresponding thermal evolution of the IGM. We assume that QSOs are triggered by major mergers of dark matter halos, and the luminosity evolution of individual QSOs is described by an initial accretion stage with a constant Eddington ratio and then a powerlaw decay driven by long term disk evolution or fueling. Once a QSO is triggered, it immediately ionizes its surrounding area as an ionized bubble. The resulting changes in size and volume of the bubble are determined by the luminosity evolution of the central QSO. With the emergence of more and more bubbles, they eventually overlap each other and finally permeate the whole universe. During the He II reionization,the IGM temperature increases due to the photoheating by the ionization processes.Applying the bubble model and considering various heating and cooling mechanisms,we trace the thermal evolution of the IGM and obtain the average IGM temperature as a function of redshift, which is very consistent with observations. The increase in IGM temperature due to the reionization of He II may be determined more accurately in the future, which may put robust constraints on the QSO model and the physics of He II reionization.     

Directional radiation and photodissociation regions in molecular hydrogen clouds

Some astrophysical observations of molecular hydrogen point to a broadening of the velocity distribution for molecules at excited rotational levels. This effect is observed in both Galactic and high-redshift clouds. Analysis of H₂, HD, and CI absorption lines has revealed the broadening effect in the absorption system of QSO 1232+082 (z _abs = 2.33771). We analyze line broadening mechanisms by considering in detail the transfer of ultraviolet radiation (in the resonance lines of the Lyman and Werner H₂ molecular bands) for various velocity distributions at excited rotational levels. The mechanism we suggest includes the saturation of the lines that populate excited rotational levels (radiative pumping) and manifests itself most clearly in the case of directional radiation in the medium. Based on the calculated structure of a molecular hydrogen cloud in rotational level populations, we have considered an additional mechanism that takes into account the presence of a photodissociation region. Note that disregarding the broadening effects we investigated can lead to a significant systematic error when the data are processed.     

An Intrinsic Model for the Polarization Position Angle Swing Observed in QSO 1150＋812

The rapid polarization position angle swing of ~ 180?observed in QSO 1150+812 at 2cm by Kochenov and Gabuzda is quite a regular event. One interesting property of the event is that, during the time of the swing the polarized flux density remained almost constant. We suggest that such an event can be explained in terms of a relativistic thin shock propagating through a uniform helical magnetic field, giving rise to relativistic aberration effects as the transverse field component rotates. The model may also be applicable to other similar events in which variations in polarization are not accompanied by variations in total flux density.     

The spectrum of BAL QSO Q1303+308: intrinsic variability and line-locking stability

A recently taken WHT optical spectrum of the BAL QSO Q1303+308 shows interesting changes in the spectral characteristics of the absorption systems. In particular, the equivalent widths of many of the broad absorption lines have markedly increased with respect to observations taken 15 yr earlier by Foltz et al., with the more blueshifted components showing the greatest increase by a factor of ≈5 in optical depth. Perhaps the most remarkable feature of the data is that, in the rest frame of the quasar, the line-of-sight outflow velocity of the clouds has only increased by ≈55 km s⁻¹ in a rest-frame time interval of 5–6 yr. Various examples of doublet line locking are still visible in the system, and we present a provisional model analysis of the spectrum showing that theories based on small cloud velocity interactions via line locking can successfully explain the general appearance of the spectrum and its variability. Finally we highlight the need for further ongoing monitoring of this fascinating object.     

An astrophysical oscillator strength for the S ii 94.7-nm resonance line and S abundances in DLAs

By using UV spectra for the O star HD 93521 taken with the ORFEUS II echelle spectrograph, we determine an 'astrophysical' f value for the S  ii   λ 94.7-nm line: f =0.00498_−0.00138^+0.00172 , error at 1 σ level. This is almost a factor of 30 smaller than the guessed value found in the Kurucz data base (  f =0.1472) , which was until now the only one available for this transition. We use our 'astrophysical' f to investigate the S abundance in two damped Ly α absorption systems (DLAs) observed with the UV–Visual Echelle Spectrograph (UVES) at the European Southern Observatory's 8.2-m Kueyen telescope. In the case of the absorber at z _abs=3.02486 towards QSO 0347-3819, we find a sulphur column density which is consistent, within errors, with that determined by Centurión et al. by means of the λ 125.9-nm line, thus providing an external check on the accuracy of our f value. For the damped absorber at z _abs=4.4680 towards BR J0307-4945, we determine a high value of the S abundance, which, however, is probably the result of blending with Ly α forest lines.     

ROSAT X射线源1057＋4316被证认为一个z＝0．32的类星体

根据北京天文台２．１６ｍ望远镜的有缝光谱观测，证认出位于１０ｈ５７ｍ４８．５ｓ＋４３°１６′１３″（１９５０．０）的一个ＲＯＳＡＴＸ射线源是一个ｚ＝０．３２的类星体。     

Q2237+0305 source structure and dimensions from light-curve simulation

Assuming a two-component quasar structure model consisting of a central compact source and an extended outer feature, we produce microlensing simulations for a population of compact masses in the lensing galaxy of Q2237+0305. Such a model is a simplified version of that adopted to explain the brightness variations observed in Q0957. The microlensing light curves generated for a range of source parameters were compared to the light curves obtained in the framework of the Optical Gravitational Lensing Experiment program. With a large number of trials, we built, in the domain of the source structure parameters, probability distributions to find 'good' realizations of light curves. The values of the source parameters which provide the maximum of the joint probability distribution calculated for all the image components have been accepted as estimates for the source structure parameters. The results favour the two-component model of the quasar brightness structure over a single compact central source model, and in general the simulations confirm the Schild–Vakulik model that previously described successfully the microlensing and other properties of Q0957. Adopting 3300 km s⁻¹ for the transverse velocity of the source, the effective size of the central source was determined to be about  2 × 10¹⁵ cm  , and  ɛ≈ 2  was obtained for the ratio of the integral luminosity of the outer feature to that of the central source.