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تسجيل مستخدم جديدConstraining the neutral fraction of hydrogen in the IGM at redshift 7.5
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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.
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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, significant
ly 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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