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Register a new userLarge Lyman-alpha opacity fluctuations and low CMB tau in models of late reionization with large islands of neutral hydrogen extending to z<5.5
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High-redshift QSO spectra show large spatial fluctuations in the Ly-alpha opacity of the intergalactic medium on surprisingly large scales at z>~5.5. We present a radiative transfer simulation of cosmic reionization driven by galaxies that reproduces this large scatter and the rapid evolution of the Ly-alpha opacity distribution at 5<z<6. The simulation also reproduces the low Thomson scattering optical depth reported by the latest CMB measurement and is consistent with the observed short near-zones and strong red damping wings in the highest-redshift QSOs. It also matches the rapid disappearance of observed Ly-alpha emission by galaxies at z>~6. Reionization is complete at z=5.3 in our model, and 50% of the volume of the Universe is ionized at z=7. Agreement with the Ly-alpha forest data in such a late reionization model requires a rapid evolution of the ionizing emissivity of galaxies that peaks at z~6.8. The late end of reionization results in a large scatter in the photoionisation rate and the neutral hydrogen fraction at redshifts as low as z<~5.5 with large residual neutral islands that can produce very long Gunn-Peterson troughs resembling those seen in the data.
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Observations of the Lyman-alpha (Ly-$alpha$) forest may allow reionization to complete as late as $z sim 5.5$, provided the ionization state of the intergalactic medium (IGM) is sufficiently inhomogeneous at these redshifts. In this case, significantly neutral islands may remain amongst highly ionized gas with the ionized regions allowing some transmission through the Ly-$alpha$ forest. This possibility has the important virtue that it is eminently testable with existing Ly-$alpha$ forest data. In particular, we describe three observable signatures of significantly neutral gas in the $z sim 5.5$ IGM. We use mock quasar spectra produced from numerical simulations of reionization to develop these tests. First, we quantify how the abundance and length of absorbed regions in the forest increase with the volume-averaged neutral fraction in our reionization model. Second, we consider stacking the transmission profile around highly absorbed regions in the forest. If and only if there is significantly neutral gas in the IGM, absorption in the damping wing of the Ly-$alpha$ line will cause the transmission to recover slowly as one moves from absorbed to transmitted portions of the spectrum. Third, the deuterium Ly-$beta$ line should imprint a small but distinctive absorption feature slightly blueward of absorbed neutral regions in the Ly-$beta$ forest. We show that these tests can be carried out with existing Keck HIRES spectra at $z sim 5.5$, with the damping wing being observable for $< x_{text{HI}} >gtrsim 0.05$ and the deuterium feature observable with additional high-resolution spectra for $< x_{text{HI}} >gtrsim 0.2$.
Recent observations of the Lyman-alpha forest show large-scale spatial variations in the intergalactic Lyman-alpha opacity that grow rapidly with redshift at z>5, far in excess of expectations from empirically motivated models. Previous studies have attempted to explain this excess with spatial fluctuations in the ionizing background, but found that this required either extremely rare sources or problematically low values for the mean free path of ionizing photons. Here we report that much -- or potentially all -- of the observed excess likely arises from residual spatial variations in temperature that are an inevitable byproduct of a patchy and extended reionization process. The amplitude of opacity fluctuations generated in this way depends on the timing and duration of reionization. If the entire excess is due to temperature variations alone, the observed fluctuation amplitude favors a late-ending but extended reionization process that was roughly half complete by z~9 and that ended at z~6. In this scenario, the highest opacities occur in regions that reionized earliest, since they have had the most time to cool, while the lowest opacities occur in the warmer regions that reionized most recently. This correspondence potentially opens a new observational window into patchy reionization.
A long (110 cMpc/$h$) and deep absorption trough in the Ly$alpha$ forest has been observed extending down to redshift 5.5 in the spectrum of ULAS J0148+0600. Although no Ly$alpha$ transmission is detected, Ly$beta$ spikes are present which has led to claims that the gas along this trough must be ionized. Using high resolution cosmological radiative transfer simulations in large volumes, we show that in a scenario where reionization ends late ($z sim 5.2$), our simulations can reproduce troughs as long as observed. In this model, we find that the troughs are caused by islands of neutral hydrogen. Small ionized holes within the neutral islands allow for the transmission of Ly$beta$. We have also modelled the Ly$alpha$ emitter population around the simulated troughs, and show that there is a deficit of Ly$alpha$ emitters close to the trough as is observed.
The first generation of redshifted 21 cm detection experiments, carried out with arrays like LOFAR, MWA and GMRT, will have a very low signal-to-noise ratio per resolution element (sim 0.2). In addition, whereas the variance of the cosmological signal decreases on scales larger than the typical size of ionization bubbles, the variance of the formidable galactic foregrounds increases, making it hard to disentangle the two on such large scales. The poor sensitivity on small scales on the one hand, and the foregrounds effect on large scales on the other hand, make direct imaging of the Epoch of Reionization of the Universe very difficult, and detection of the signal therefore is expected to be statistical.Despite these hurdles, in this paper we argue that for many reionization scenarios low resolution images could be obtained from the expected data. This is because at the later stages of the process one still finds very large pockets of neutral regions in the IGM, reflecting the clustering of the large-scale structure, which stays strong up to scales of sim 120 comoving Mpc/h (sim 1 degree). The coherence of the emission on those scales allows us to reach sufficient S/N (sim 3) so as to obtain reionization 21 cm images. Such images will be extremely valuable for answering many cosmological questions but above all they will be a very powerful tool to test our control of the systematics in the data. The existence of this typical scale (sim 120 comoving Mpc/h) also argues for designing future EoR experiments, e.g., with SKA, with a field of view of at least 4 degree.
We combine high resolution hydrodynamical simulations with an intermediate resolution, dark matter only simulation and an analytical model for the growth of ionized regions to estimate the large scale distribution and redshift evolution of the visibility of Lyman-alpha emission in 6<=z<=8 galaxies. The inhomogeneous distribution of neutral hydrogen during the reionization process results in significant fluctuations in the Lyman-alpha transmissivity on large scales. The transmissivity depends not only on the ionized fraction of the intergalactic medium by volume and the amplitude of the local ionizing background, but is also rather sensitive to the evolution of the relative velocity shift of the Lyman-alpha emission line due to resonant scattering. We reproduce a decline in the space density of Lyman-alpha emitting galaxies as rapid as observed with a rather rapidly evolving neutral fraction between z=6-8, and a typical Lyman-alpha line velocity offset of 100 km/s redward of systemic at z=6 which decreases toward higher redshift. The new (02/2015) Planck results indicate such a recent end to reionization is no longer disfavoured by constraints from the cosmic microwave background.
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