Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHST/STIS observations of the HeII Gunn-Peterson effect towards HE 2347-4342
103
0
0.0
(
0
)
Authors
Alain Smette
Ask ChatGPT about the research
No Arabic abstract
We present an HST STIS spectrum of the HeII Gunn-Peterson effect towards HE2347-4342. Compared to the previous HST GHRS data obtained by Reimers et al. (1997), the STIS spectrum has a much improved resolution. The 2-D detector also allows us to better characterize the sky and dark background. We confirm the presence of two spectral ranges of much reduced opacity, the opacity gaps, and provide improved lower limits on the HeII G-P opacity in the high opacity regions. We use the STIS spectrum together with a Keck--HIRES spectrum covering the corresponding HI Lya forest to calculate a 1-D map of the softness S of the ionization radiation along the line of sight towards HE 2347-4342, where S is the ratio of the HI to HeII photoionization rates. We find that S is generally large but presents important variations, from S ~ 30 in the opacity gaps to a 1 sigma lower limit of 2300 at z~2.86, in a region which shows an extremely low HI opacity over a 6.5 A range. We note that a large S naturally accounts for most of the large SiIV to CIV ratios seen in other quasar absorption line spectra. We present a simple model that reproduces the shape of the opacity gaps in absence of large individual absorption lines. We extend the model described in Heap et al. (2000) to account for the presence of sources close to the line of sight of the background quasar. As an alternative to the delayed reionization model suggested by Reimers et al. (1997), we propose that the large softness observed at z~2.86 is due to the presence of bright soft sources close to the line of sight, i.e. for which the ratio between the number of HI to HeII ionizing photons reaching the IGM is large. We discuss these two models and suggest ways to discriminate between them.
rate research
Read More
The ultraviolet spectrum (1145--1720A) of the distant quasar Q 0302--003 (z=3.286) was observed at 1.8A resolution with the Space Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A total integration time of 23,280 s was obtained. The spectrum clearly delineates the Gunn-Peterson HeII absorption trough, produced by HeII Lya, along the line of sight over the redshift range z=2.78-3.28. Its interpretation was facilitated by modeling based on Keck HIRES spectra of the HI Lya forest (provided by A. Songaila and by M. Rauch and W. Sargent). We find that near the quasar HeII Lya absorption is produced by discrete clouds, with no significant diffuse gas; this is attributed to a HeII proximity effect in which the quasar fully ionizes He in the diffuse intergalactic medium, but not the He in denser clouds. By two different methods we calculate that the average HeII Lya opacity at z~3.15 is tau >= 4.8. In the Dobrzycki-Bechtold void in the HI Lya forest near z=3.18, the average HeII opacity tau=4.47^{+0.48}_{-0.33}. Such large opacities require the presence of a diffuse gas component as well as a soft UV background spectrum, whose softness parameter, defined as the ratio of the photo-ionization rate in HI over the one in HeII, S=Gamma^J_HI/Gamma^J_HeII~=800, indicating a significant stellar contribution. At z=3.05, there is a distinct region of high HeII Lya transmission which most likely arises in a region where helium is doubly ionized by a discrete local source, quite possibly an AGN. At redshifts z<2.87, the HeII Lya opacity detected by STIS, tau=1.88, is significantly lower than at z>3. Such a reduction in opacity is consistent with Songailas (1998) report that the hardness of the UV background spectrum increases rapidly from z=3 to z=2.9.
We report the discovery of 14 quasars in the vicinity of HE2347-4342, one of the two quasars whose intergalactic HeII forest has been resolved with FUSE. By analysing the HI and the HeII opacity variations separately, no transverse proximity effect is detected near three foreground quasars of HE2347-4342: QSOJ23503-4328 (z=2.282, $vartheta=3.59$ arcmin), QSOJ23500-4319 (z=2.302, $vartheta=8.77$ arcmin) and QSOJ23495-4338 (z=2.690, $vartheta=16.28$ arcmin). This is primarily due to line contamination and overdensities probably created by large-scale structure. By comparing the HI absorption and the corresponding HeII absorption, we estimated the fluctuating spectral shape of the extragalactic UV radiation field along this line of sight. We find that the UV spectral shape near HE2347-4342 and in the projected vicinity of the three foreground quasars is statistically harder than expected from UV background models dominated by quasars. In addition, we find three highly ionised metal line systems near the quasars. However, they do not yield further constraints on the shape of the ionising field. We conclude that the foreground quasars show a transverse proximity effect that is detectable as a local hardening of the UV radiation field, although the evidence is strongest for QSOJ23495-4338. Thus, the relative spectral hardness traces the proximity effect also in overdense regions prohibiting the traditional detection in the HI forest. Furthermore, we emphasise that softening of quasar radiation by radiative transfer in the intergalactic medium is important to understand the observed spectral shape variations. From the transverse proximity effect of QSOJ23495-4338 we obtain a lower limit on the quasar lifetime of ~25 Myr.
In order to carry out a systematic and thorough measurement of the HI Gunn-Peterson effect at high redshift, a quantitatively testable and repeatable procedure, in particular, a robust statistical weighting technique, is developed. It is applied to an echelle spectrum of resolution 15 km s$^{-1}$ of PKS 1937-101 with z=3.787. A weighted intensity distribution which is derived overwhelmingly from pixels close to the continuum level in the Ly alpha forest region is constructed by the evaluation of how closely correlated each pixel is with its neighboring pixels. The merit of the distribution is its stronger and narrower peak compared to the unweighted, as well as its smaller dependence on uncertainty of strong absorption lines and noise spikes. By comparison to the weighted intensity distribution of synthetic Ly alpha forest spectrum with various chosen diffuse HI opacities, a chi square fit is performed. In addition to a weak line population with power law N_H distribution $beta =1.7$ extrapolated down to 10$^{12}$ cm$^{-2}$, a best chi square fit requires a GP opacity of $0.115 pm 0.025$ at average z=3.4 with estimation of the contribution from the variance of $beta$. Although no evidence of more than 1-2% error is seen in the continuum extrapolation, the possible systematic overestimation due to the slope can be as high as the level of the $chi^2$ fit, which is investigated by splitting the Lya forest region into subsamples to check the continuum drops dependence on absorber redshift.
Understanding the cosmic re-ionization is one of the key goals of the modern observational cosmology. High redshift QSO spectra can be used as background light sources to measure absorption by intervening neutral hydrogen. We investigate neutral hydrogen absorption in a deep, moderate-resolution Keck/Deimos spectrum of QSO CFHQSJ2329-0301 at z=6.4. This QSO is one of the highest redshift QSOs presently known at z=6.4 but is 2.5 mag fainter than a previously well-studied QSO SDSSJ1148+5251 at z=6.4. Therefore, it has a smaller Stromgren sphere, and allows us to probe the highest redshift hydrogen absorption to date. The average transmitted flux at 5.915<z_abs<6.365 (200 comoving Mpc) is consistent with zero, in Ly_alpha, Ly_beta, and Ly_gamma absorption measurements. This corresponds to the lower limit of optical depth, tau_eff>4.9. These results are consistent with strong evolution of the optical depth at z>5.7.
We present FUSE observations of the HeII Lyman alpha forest in the redshift range 2.3 < z < 2.7 towards HS1700+6416. Between October 2002 and February 2003, the brightness of the QSO increased by a factor 2. Therefore, with an exposure time of 203 ks during orbital night, the quality of the resulting spectrum is comparable to the HE2347-4342 data. This second line of sight with a resolved HeII Lyman alpha forest reveals a similar variation of several orders of magnitude of the column density ratio eta = N(HeII)/N(HI) and confirms the results of previous studies. The well-known metal line spectrum of HS1700+6416 permits to examine the influence of metal line absorption on the HeII column densities.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
