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Register a new userThe Predicament of Absorption-Dominated Reionization: Increased Demands on Ionizing Sources
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The reionization epoch concludes when ionizing photons reach every corner of the Universe. Reionization has generally been assumed to be limited primarily by the rate at which galaxies produce ionizing photons, but the recent measurement of a surprisingly short ionizing photon mean free path of $0.75^{+0.65}_{-0.45}$ proper Mpc at $z = 6$ by Becker et al. (2021) suggests that absorption by residual neutral hydrogen in the otherwise ionized intergalactic medium may play a much larger role than previously expected. Here we show that consistency between this short mean free path and the coeval dark pixel fraction in the Ly$alpha$ forest requires a cumulative output of $6.1^{+11}_{-2.4}$ ionizing photons per baryon by reionizations end. This represents a dramatic increase in the ionizing photon budget over previous estimates, greatly exacerbating the tension with measurements of the ionizing output from galaxies at later times. Translating this constraint into the instantaneous ionizing production from galaxies in our model, we find $log_{10}f_{rm esc}xi_{rm ion}/text{(erg/Hz)}^{-1} =25.02_{-0.21}^{+0.45}$ at $zsim6$. Even with optimistic assumptions about the ionizing production efficiency of early stellar populations, and assuming the galaxy luminosity function extends to extremely faint sources ($M_{text{UV}}leq-11$), complete reionization requires the escape fraction of ionizing photons to exceed $20%$ across the galaxy population. This is far larger than observed in any galaxy population at lower redshifts, requiring rapid evolution in galaxy properties after the first billion years of cosmic time. This tension cannot be completely relieved within existing observational constraints on the hydrogen neutral fraction and mean free path.
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We forecast the reionization history constraints, inferred from Lyman-alpha damping wing absorption features, for a future sample of $sim 20$ $z geq 6$ gamma-ray burst (GRB) afterglows. We describe each afterglow spectrum by a three-parameter model. First, L characterizes the size of the ionized region (the bubble size) around a GRB host halo. Second, $langle{x_{rm HI}rangle}$ is the volume-averaged neutral fraction outside of the ionized bubble around the GRB, which is approximated as spatially uniform. Finally, $N_{mathrm{HI}}$ denotes the column-density of a local damped Lyman-alpha absorber (DLA) associated with the GRB host galaxy. The size distribution of ionized regions is extracted from a numerical simulation of reionization, and evolves strongly across the Epoch of Reionization (EoR). The model DLA column densities follow the empirical distribution determined from current GRB afterglow spectra. We use a Fisher matrix formalism to forecast the $langle{x_{rm HI}(z)rangle}$ constraints that can be obtained from follow-up spectroscopy of afterglows with SNR = 20 per R=3,000 resolution element at the continuum. We find that the neutral fraction may be determined to better than 10-15% (1-$sigma$) accuracy from this data across multiple independent redshift bins at $z sim 6-10$, spanning much of the EoR, although the precision degrades somewhat near the end of reionization. A more futuristic survey with $80$ GRB afterglows at $z geq 6$ can improve the precision here by a factor of $2$ and extend measurements out to $z sim 14$. We further discuss how these constraints may be combined with estimates of the escape fraction of ionizing photons, derived from the DLA column density distribution towards GRBs extracted at slightly lower redshift. This combination will help in testing whether we have an accurate census of the sources that reionized the universe.
Absorption signatures in the spectra of QSOs are one of our most powerful tools for studying galactic and intergalactic environments at high redshifts. With the discovery of QSOs out to z > 7, QSO absorption lines are now tracing the end stages of reionization on multiple fronts using the hydrogen Lyman-$alpha$ forest and heavy element absorbers. Next-generation QSO absorption line studies with large optical/IR telescopes will reveal in detail how the first galaxies emerged form the cosmic web, transformed their circum- and inter-galactic environments, and completed the last major phase transition of the Universe. These efforts will complement other upcoming studies of reionization, such as those with JWST, ALMA, and redshifted 21cm experiments.
We present a theoretical study of intergalactic metal absorption lines imprinted in the spectra of distant quasars during and after the Epoch of Reionization (EoR). We use high resolution hydrodynamical simulations at high redshift ($4 <z<8$), assuming a uniform UV background Haardt--Madau 12, post-processing with CLOUDY photoionization models and Voigt profile fitting to accurately calculate column densities of the ions CII, CIV, SiII, SiIV and OI in the intergalactic medium (IGM). In addition, we generate mock observations of neutral Hydrogen (HI) at $z<6$. Our simulations successfully reproduce the evolution of the cosmological mass density ($Omega$) of CII and CIV, with $Omega_{CII}$ exceeding $Omega_{CIV}$ at $z >6$, consistent with the current picture of the tail of the EoR. The simulated CII exhibits a bimodal distribution with large absorptions in and around galaxies, and some traces in the lower density IGM. We find some discrepancies between the observed and simulated column density relationships among different ionic species at $z=6$, probably due to uncertainties in the assumed UV background. Finally, our simulations are in good agreement with observations of the HI column density distribution function at $z = 4$ and the HI cosmological mass density $Omega_{HI}$ at $4 < z < 6$.
The mean free path of ionizing photons, $lambda_{rm mfp}$, is a key factor in the photoionization of the intergalactic medium (IGM). At $z gtrsim 5$, however, $lambda_{rm mfp}$ may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of $lambda_{rm mfp}$ that address this bias and extend up to $z sim 6$ for the first time. Our measurements at $z sim 5$ are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At $z sim 6$ we use QSO spectra from Keck ESI and VLT X-Shooter. We measure $lambda_{rm mfp} = 9.09^{+1.62}_{-1.28}$ proper Mpc and $0.75^{+0.65}_{-0.45}$ proper Mpc (68% confidence) at $z = 5.1$ and 6.0, respectively. The results at $z = 5.1$ are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At $z = 6.0$, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at $z = 6.0$ falls well below extrapolations from lower redshifts, indicating rapid evolution in $lambda_{rm mfp}$ over $5 < z < 6$. This evolution disfavors models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by $z = 6$, but is qualitatively consistent with models wherein reionization completed at $z = 6$ or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20% neutral at $z=6$, although our measurement at $z = 6.0$ is even lower than these models prefer.
Heating of neutral gas by energetic sources is crucial for the prediction of the 21 cm signal during the epoch of reionization (EoR). To investigate differences induced on statistics of the 21 cm signal by various source types, we use five radiative transfer simulations which have the same stellar UV emission model and varying combinations of more energetic sources, such as X-ray binaries (XRBs), accreting nuclear black holes (BHs) and hot interstellar medium emission (ISM). We find that the efficient heating from the ISM increases the average global 21~cm signal, while reducing its fluctuations and thus power spectrum. A clear impact is also observed in the bispectrum in terms of scale and timing of the transition between a positive and a negative value. The impact of XRBs is similar to that of the ISM, although it is delayed in time and reduced in intensity because of the less efficient heating. Due to the paucity of nuclear BHs, the behaviour of the 21~cm statistics in their presence is very similar to that of a case when only stars are considered, with the exception of the latest stages of reionization, when the effect of BHs is clearly visible. We find that differences between the source scenarios investigated here are larger than the instrumental noise of SKA1-low at $z gtrsim 7-8$, suggesting that in the future it might be possible to constrain the spectral energy distribution of the sources contributing to the reionization process.
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