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Register a new userHow to Search for Islands of Neutral Hydrogen in the $z sim 5.5$ IGM
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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 $< x_{text{HI}} >gtrsim 0.2$.
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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.
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.
We present a large spectroscopic campaign with Keck/MOSFIRE targeting Lyman-alpha emission (Ly$alpha$) from intrinsically faint Lyman-break Galaxies (LBGs) behind 12 efficient galaxy cluster lenses. Gravitational lensing allows us to probe the more abundant faint galaxy population to sensitive Ly$alpha$ equivalent width limits. During the campaign we targeted 70 LBG candidates with MOSFIRE Y-band, selected photometrically to cover Ly$alpha$ over the range $7<z<8.2$. We detect $S/N>5$ emission lines in 2 of these galaxies and find that they are likely Ly$alpha$ at $z=7.148pm0.001$ and $z=7.161pm0.001$. We present new lens models for 4 of the galaxy clusters, using our previously published lens models for the remaining clusters to determine the magnification factors for the source galaxies. Using a Bayesian framework that employs large scale reionization simulations of the intergalactic medium (IGM) as well as realistic properties of the interstellar medium and circumgalactic medium, we infer the volume-averaged neutral hydrogen fraction, $overline{x}_{mathrm{HI}}$, in the IGM during reionization to be $overline{x}_{mathrm{HI}}=0.88^{+0.05}_{-0.10}$ at $z=7.6pm0.6$. Our result is consistent with a late and rapid reionization scenario inferred by Planck.
We present the results of HI spectral stacking analysis of Giant Metrewave Radio Telescope (GMRT) observations targeting the COSMOS field. The GMRT data cube contains 474 field galaxies with redshifts known from the zCOSMOS-bright 10k catalogue. Spectra for the galaxies are co-added and the stacked spectrum allows us to make a $sim 3sigma$ measurement of the average HI mass. Using this average HI mass along with the integral optical $B$-band luminosity of the galaxies and the luminosity density of the COSMOS field, a volume normalisation is applied to obtain the cosmic HI mass density ($Omega_{rm HI}$). We find a cosmic HI mass density of $Omega_{rm HI}$ = (0.42 $pm$ 0.16) $times$ 10$^{-3}$ at $z sim 0.37$, which is the highest-redshift measurement of $Omega_{rm HI}$ ever made using HI spectral stacking. The value we obtained for $Omega_{rm HI}$ at $z sim 0.37$ is consistent with that measured from large blind 21-cm surveys at $z = 0$ as well as measurements from other HI stacking experiments at lower redshifts. Our measurement in conjunction with earlier measurements indicates that there has been no significant evolution of HI gas abundance over the last 4 Gyr. A weighted mean of $Omega_{rm HI}$ from all 21-cm measurements at redshifts $z lesssim 0.4$ gives $Omega_{rm HI}$ = (0.35 $pm$ 0.01) $times$ 10$^{-3}$. The $Omega_{rm HI}$ measured (from HI 21-cm emission measurements) at $z lesssim 0.4$ is however approximately half that measured from Damped Lyman-$alpha$ Absorption (DLA) systems at $z gtrsim 2$. Deeper surveys with existing and upcoming instruments will be critical to understand the evolution of $Omega_{rm HI}$ in the redshift range intermediate between $z sim 0.4$ and the range probed by DLA observations.
We use a large N-body simulation to examine the detectability of HI in emission at redshift z ~ 1, and the constraints imposed by current observations on the neutral hydrogen mass function of galaxies at this epoch. We consider three different models for populating dark matter halos with HI, designed to encompass uncertainties at this redshift. These models are consistent with recent observations of the detection of HI in emission at z ~ 0.8. Whilst detection of 21 cm emission from individual halos requires extremely long integrations with existing radio interferometers, such as the Giant Meter Radio Telescope (GMRT), we show that the stacked 21 cm signal from a large number of halos can be easily detected. However, the stacking procedure requires accurate redshifts of galaxies. We show that radio observations of the field of the DEEP2 spectroscopic galaxy redshift survey should allow detection of the HI mass function at the 5-12 sigma level in the mass range 10^(11.4) M_sun/h < M_halo < 10^(12.5)M_sun/h, with a moderate amount of observation time. Assuming a larger noise level that corresponds to an upper bound for the expected noise for the GMRT, the detection significance for the HI mass function is still at the 1.7-3 sigma level. We find that optically undetected satellite galaxies enhance the HI emission profile of the parent halo, leading to broader wings as well as a higher peak signal in the stacked profile of a large number of halos. We show that it is in principle possible to discern the contribution of undetected satellites to the total HI signal, even though cosmic variance limitation make this challenging for some of our models.
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