This strongly suggests that a fraction of the absorbing structures span ~ 1 upcoming Mpc separation between paired lines of sight. A simple linear fit to the redshift evolution of the lognormal parameters at z < 5.4 reproduces the observed mean Lyα and Lyβ transmitted fluxes over 1.6 < z < 6.2.
Overview
This work aims to shed light on several aspects of the evolution of the high-redshift IGM using several unique observations of quasars at z >4. The evolution of the optical depths is then used to explain the disappearance of transmitted flux atz >6.
Scientific Background
The opacity of the forest can be used to set limits on the baryon density (Rauch et al. 1997). In principle, the intensity of the UV background can be determined from the mean level of the emitted flux in the Lyαforest.
Thesis Outline
We further show that our value for the Hi ionization rate is consistent with the lack of evolution in the UV background above 2. The cross-correlation of the transmitted flux in the Lyα forest shows a 5–7σ peak with zero velocity delay for both pairs.
Introduction
The remainder of the paper is organized as follows: In §2.2 we present our observations together with an overview of the data. In §2.4 we analyze the effects of photon noise and instrumental resolution on the correlation functions.
The Data
We calculate the flux correlation functions in the Lyα forest in §2.3 and statistically examine the similarity between lines of sight as a function of flux. We limit our analysis of the Lyα forest to the wavelength region between each quasar's Lyα and Ovi emission lines.
Comparison of Sightlines
We then identify the pixels in the accompanying spectrum with matching wavelengths and calculate their flux distribution. Each panel shows the normalized distribution of Q1424 or Q1439B pixels in the indicated subsample, along with the distribution of all Lyα forest pixels in that spectrum.
Effects of Photon Noise and Instrumental Resolution
As the spectral resolution decreases, the peak of the autocorrelation also includes more of the surrounding noise (random undulations in the correlation function at ∆v ≫ 0). The solid line shows the autocorrelation calculated from our original ESI spectrum of Q1422 (FWHM = 55 km s−1, S/N = 150 per resolution element).
Metal Systems
Bottom three panels: continuum-normalized ESI spectrum of Q1424 (histogram) showing C iv and H i absorption for the system measured here at z = 3.0843. The Mg ii systems appear along only one line of sight, one towards A at z = 2.266 and the other towards B at z = 1.399. Q1439B is a BAL quasar showing high levels of coupled absorption in C ivand Nv.
Conclusions
Increasing the resolution FWHM increases the velocity width of the autocorrelation and decreases its amplitude as expected. Cosmological tests that make use of the correlation length of the Lyα forest should account for this effect.
Introduction
In §3.4, we demonstrate a significant abundance of Oi systems to the highest redshift object, SDSS J1148+5251 (zQSO = 6.42), and compare the abundance to the number density of lower-redshift Oi systems and other absorbers with high Hi column densities . In §3.6 we discuss the significance of these excess systems for the enrichment and ionization state of the IGM at z ~ 6.
The Data
Overdense regions of the IGM should be the first to become enriched, due to the presence of star-forming sources, and yet the last to remain ionized, due to short recombination times (Oh 2002). Blueward of the Lyα emission line, the continuum for these objects is strongly obscured due to strong absorption by the Lyα forest.
O i Search
The Si ii λ1260 and C ii lines for this system occur in transparent holes in regions of mild telluric absorption. The fitted z and b values for O i correspond to those for Si ii within the fitting errors. Both Si ii λ1260 and Oi fit well to a single line, while the blue edge of the C ii shows a possible additional component.
Overabundance of O i systems towards SDSS J1148+5251
However, it does mean that the relative numbers of expected O detections for SDSS J1148+5251 and the remaining eight lines of sight should be. For reasonable values of α, the ratio of ∆Xeff for SDSS J1148+5251 and the remaining eight lines of sight is essentially constant. The probability of simultaneously detecting at least four systems towards SDSS J1148+5251, while detecting at most two systems along the remaining eight lines of sight.
Metal Abundances
The continuum uncertainty will include the error in the power law amplitude at the wavelength of the damped system. In this system, the NHI is limited by the transferred flow in the wings of the Lyα profile. In this system, the NHI is limited by the transferred flux in the red wing of the Lyβ profile.
Discussion
The significance of the excess Oitowards SDSS J1148+5251 is complicated by the presence of Lyα and Lyβ emission along that line of sight. If the line of sight to SDSS J1148+5251 runs along a filament at z ~ 6, then we can expect to cross more than the average number of absorbers. Three of the four systems toward SDSS J1148+5251 have single components with Doppler widths b≈5−8 km s−1.
Summary
Alternatively, the line of sight may be strongly ionized, with a large-scale density enhancement producing strong line mixing in the Lyα and Lyβ forests. We assume a uniform UV background and an isothermal IGM for the MHR00 model, as has been done in previous works where emitted flux statistics have been used to derive an abrupt change in the IGM atz∼6. If the evolution of the lognormal distribution atz <5 reflects a slowly evolving density field, temperature and UV background, then no sudden change in the IGM atz∼6 occurs due to late reionization.
Introduction
Yet there is much debate about the significance of the disappearance of the emitted flux at z∼6 (Songaila & Cowie 2002; Songaila 2004; Lidz et al. 2006b). Atz∼6, any emitted flux will originate from rare cavities, which lie in the tail of the optical depth distribution. In §4.4 we simply fit the evolution of the lognormal distribution and use it to predict the average emitted flux atz >5.7.
The Data
Determining a quasar continuum is generally a subjective process whose accuracy will depend strongly on how much of the continuum has been absorbed (see Lidz et al. 2004b for a discussion). Atz-3, much of the spectrum will still not be absorbed and errors in the continuum fit will depend on signal-to-noise of the data and the personal bias of the individual performing the fit. By z~5.5, very few transmission regions remain and the continuum must be derived from the slope of the spectrum down from the Lyα emission line.
Flux Probability Distribution Functions
About half of the Lyα regions at z < 3 are better fit by the MHR00 PDF. In Figure 4.9, we show examples of the best-fitting continua superimposed on the corresponding regions of the Lyα forest. Much of the discriminating power of the flux PDF occurs at very shallow optical depths.
Redshift Evolution of Optical Depth
An important feature of the lognormal distribution is that it narrows with redshift faster than the MHR00 distribution. We can use the redshift evolution of the lognormal distribution to predict the evolution of transmitted flux atz&6. The solid line shows τeffβ predicted solely from the lognormal distribution of Lyα optical depths.
An inverse temperature-density relation?
Conclusions
The solid lines show the best fits when the line and zero point are allowed to vary. QSO hzabsia Nbinb Continuity and zero point fixed Continuity and zero point allowed to vary Γ−12c χ2r Γ−12c Cont.d zero point χ2r. QSO hzabsia Nbinb Continuity and zero point fixed Continuity and zero point are allowed to vary.
Introduction
Accurate QSO redshifts are essential, as errors in redshifts measured from high-ionization emission lines (up to a few thousand km s−1 ) can be of the same order of magnitude as the proximity region. A remaining challenge for the proximity effect is to demonstrate that the size of the proximity region scales correctly with the QSO luminosity. However, it has not yet been shown that the size of the proximity region depends on the luminosity asR∝L1/2, which is an implicit assumption in measurements of the proximity effect.
The Data
The vertical dotted lines indicate where the peak would occur based on QSO redshifts measured since the beginning of the Lyα forest. The intrinsic redshift of a QSO can also, in principle, be determined from the onset of the Lα forest. There is compromise in using Mgiior starting Lyαforest for internal redshift QSO.
Proximity Effect Model
To make the most of the optical depth information encoded in the Lyα forest, we will examine how the distribution of transmitted fluxes changes as a function of the distance from a QSO. For the ESI data, we repeated the analysis of Becker et al. 2006a ) after smoothing the HIRES data in that work to ESI resolution. 5.12) Examples of expected flux PDFs in the vicinity region are shown in Figure 5.6. The PDFs are smoothed to reflect a small amount of noise in the spectra (r.m.s= 3% of the continuum).
Results
The average sizes of the proximity areas scale correctly with the QSO brightness, but with a large scatter in the individual measurements. For the three brightness subsets we also overlay the expected profile for the median ΓQ10−12 of the total sample. In the three bottom panels, the expected profile for the median ΓQ10−12 of the entire sample is shown with a dotted line.
Comparison with Previous Results
This total may be lower if, for example, the escape fraction of ionizing photons from QSOs on the faint end of the luminosity function is less than 1. However, as mentioned above, there are large uncertainties in the Lyα forest values related to cosmology, i.e. IGM temperature and radiative transfer effects. On the other hand, as we note above, our measurement of Γbg may be too high due to incorrect modeling of the expected flux distribution in the near region of faint QSOs.
Conclusions
Values of Γbg−12 measured from the mean emitted flux in the Lyα forest are shown from Bolton et al. Their value, when adjusted for cosmology, is consistent with atz~3 measurements based on the average opacity of the Lyα forest (Bolton et al. 2005) and the estimated total contribution to Γbg of star-forming galaxies (Shapley et al. 2006 ) and the associations of Hunt et al. We have used different methods to investigate the ionization state of the high-redshift IGM.
