We present a new investigation of the intergalactic medium near reionization using dark gaps in the Ly
 NSFPAR ID:
 10486102
 Author(s) / Creator(s):
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 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 The Astrophysical Journal
 Volume:
 932
 Issue:
 2
 ISSN:
 0004637X
 Format(s):
 Medium: X Size: Article No. 76
 Size(s):
 ["Article No. 76"]
 Sponsoring Org:
 National Science Foundation
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Abstract We present a new investigation of the intergalactic medium (IGM) near the end of reionization using “dark gaps” in the Ly α forest. Using spectra of 55 QSOs at z em > 5.5, including new data from the XQR30 VLT Large Programme, we identify gaps in the Ly α forest where the transmission averaged over 1 comoving h −1 Mpc bins falls below 5%. Nine ultralong ( L > 80 h −1 Mpc) dark gaps are identified at z < 6. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than 30 h −1 Mpc, F 30 , as a function of redshift. We measure F 30 ≃ 0.9, 0.6, and 0.15 at z = 6.0, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to z ≃5.3. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain largescale fluctuations in the ionizing UV background at z ≲6 are also potentially consistent with the data. Overall, our results suggest that signatures of reionization in the form of islands of neutral hydrogen and/or largescale fluctuations in the ionizing background remain present in the IGM until at least z ≃ 5.3.more » « less

Abstract Cosmic reionization was the last major phase transition of hydrogen from neutral to highly ionized in the intergalactic medium (IGM). Current observations show that the IGM is significantly neutral at
z > 7 and largely ionized byz ∼ 5.5. However, most methods to measure the IGM neutral fraction are highly model dependent and are limited to when the volumeaveraged neutral fraction of the IGM is either relatively low ( ) or close to unity ( ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}\lesssim {10}^{3}$ ). In particular, the neutral fraction evolution of the IGM at the critical redshift range of ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}\sim 1$z = 6–7 is poorly constrained. We present new constraints on at ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}$z ∼ 5.1–6.8 by analyzing deep optical spectra of 53 quasars at 5.73 <z < 7.09. We derive modelindependent upper limits on the neutral hydrogen fraction based on the fraction of “dark” pixels identified in the Lyα and Lyβ forests, without any assumptions on the IGM model or the intrinsic shape of the quasar continuum. They are the first modelindependent constraints on the IGM neutral hydrogen fraction atz ∼ 6.2–6.8 using quasar absorption measurements. Our results give upper limits of (1 ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}(z=6.3)<0.79\pm 0.04$σ ), (1 ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}(z=6.5)<0.87\pm 0.03$σ ), and (1 ${\overline{x}}_{\mathrm{H}\phantom{\rule{0.25em}{0ex}}\mathrm{I}}(z=6.7)<{0.94}_{0.09}^{+0.06}$σ ). The dark pixel fractions atz > 6.1 are consistent with the redshift evolution of the neutral fraction of the IGM derived from Planck 2018. 
ABSTRACT The presence of excess scatter in the Lyα forest at z ∼ 5.5, together with the existence of sporadic extended opaque GunnPeterson troughs, has started to provide robust evidence for a late end of hydrogen reionization. However, low data quality and systematic uncertainties complicate the use of Lyα transmission as a precision probe of reionization’s end stages. In this paper, we assemble a sample of 67 quasar sightlines at z > 5.5 with high signaltonoise ratios of >10 per ≤15 km s−1 spectral pixel, relying largely on the new XQR30 quasar sample. XQR30 is a large program on VLT/XShooter which obtained deep (SNR > 20 per pixel) spectra of 30 quasars at z > 5.7. We carefully account for systematics in continuum reconstruction, instrumentation, and contamination by damped Lyα systems. We present improved measurements of the mean Lyα transmission over 4.9 < z < 6.1. Using all known systematics in a forward modelling analysis, we find excellent agreement between the observed Lyα transmission distributions and the homogeneousUVB simulations Sherwood and Nyx up to z ≤ 5.2 (<1σ), and mild tension (∼2.5σ) at z = 5.3. Homogeneous UVB models are ruled out by excess Lyα transmission scatter at z ≥ 5.4 with high confidence (>3.5σ). Our results indicate that reionizationrelated fluctuations, whether in the UVB, residual neutral hydrogen fraction, and/or IGM temperature, persist in the intergalactic medium until at least z = 5.3 (t = 1.1 Gyr after the big bang). This is further evidence for a late end to reionization.more » « less

ABSTRACT We measure the mean free path ($\lambda _{\rm mfp,H\, \small {I}}$), photoionization rate ($\langle \Gamma _{\rm H\, \small {I}} \rangle$), and neutral fraction ($\langle f_{\rm H\, \small {I}} \rangle$) of hydrogen in 12 redshift bins at 4.85 < z < 6.05 from a large sample of moderate resolution XShooter and ESI QSO absorption spectra. The fluctuations in ionizing radiation field are modelled by postprocessing simulations from the Sherwood suite using our new code ‘EXtended reionization based on the Code for Ionization and Temperature Evolution’ (excite). excite uses efficient Octree summation for computing intergalactic medium attenuation and can generate large number of high resolution $\Gamma _{\rm H\, \small {I}}$ fluctuation models. Our simulation with excite shows remarkable agreement with simulations performed with the radiative transfer code Aton and can recover the simulated parameters within 1σ uncertainty. We measure the three parameters by forwardmodelling the Lyα forest and comparing the effective optical depth ($\tau _{\rm eff, H\, \small {I}}$) distribution in simulations and observations. The final uncertainties in our measured parameters account for the uncertainties due to thermal parameters, modelling parameters, observational systematics, and cosmic variance. Our bestfitting parameters show significant evolution with redshift such that $\lambda _{\rm mfp,H\, \small {I}}$ and $\langle f_{\rm H\, \small {I}} \rangle$ decreases and increases by a factor ∼6 and ∼104, respectively from z ∼ 5 to z ∼ 6. By comparing our $\lambda _{\rm mfp,H\, \small {I}}$, $\langle \Gamma _{\rm H\, \small {I}} \rangle$ and $\langle f_{\rm H\, \small {I}} \rangle$ evolution with that in stateoftheart Aton radiative transfer simulations and the Thesan and CoDaIII simulations, we find that our bestfitting parameter evolution is consistent with a model in which reionization completes by z ∼ 5.2. Our bestfitting model that matches the $\tau _{\rm eff, H\, \small {I}}$ distribution also reproduces the dark gap length distribution and transmission spike height distribution suggesting robustness and accuracy of our measured parameters.

Abstract The mean free path of ionizing photons,
λ _{mfp}, is a critical parameter for modeling the intergalactic medium (IGM) both during and after reionization. We present direct measurements ofλ _{mfp}from QSO spectra over the redshift range 5 <z < 6, including the first measurements atz ≃ 5.3 and 5.6. Our sample includes data from the XQR30 VLT large program, as well as new Keck/ESI observations of QSOs nearz ∼ 5.5, for which we also acquire new [Cii ] 158μ m redshifts with ALMA. By measuring the Lyman continuum transmission profile in stacked QSO spectra, we find , ${\lambda}_{\mathrm{mfp}}={9.33}_{1.80}^{+2.06}$ , ${5.40}_{1.40}^{+1.47}$ , and ${3.31}_{1.34}^{+2.74}$ pMpc at ${0.81}_{0.48}^{+0.73}$z = 5.08, 5.31, 5.65, and 5.93, respectively. Our results demonstrate thatλ _{mfp}increases steadily and rapidly with time over 5 <z < 6. Notably, we find thatλ _{mfp}deviates significantly from predictions based on a fully ionized and relaxed IGM as late asz = 5.3. By comparing our results to model predictions and indirectλ _{mfp}constraints based on IGM Lyα opacity, we find that the evolution ofλ _{mfp}is consistent with scenarios wherein the IGM is still undergoing reionization and/or retains large fluctuations in the ionizing UV background well below redshift 6.