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Abstract Observations of quasar absorption spectra provide strong evidence that reionization extended belowz= 6. The relationship between Lyαforest opacity and local galaxy density (the opacity-density relation) is a key observational test of this scenario. Using narrow-band surveys ofz≈ 5.7 Lyαemitters (LAEs) centered on quasar sight lines, ref. [1] showed that two of the most transmissive Lyαforest segments at this redshift intersect under-densities in the galaxy distribution. This result is in tension with models of a strongly fluctuating ionizing background, including some models of late reionization, which predict that the vast majority of these segments should intersect over-densities where the ionizing intensity is strongest. In this paper, we use radiative transfer simulations to explore in more detail the opacity-density relation predicted by late reionization models. We find that fields like the one toward quasar PSO J359-06 — the more under-dense of the two transmissive sight lines in ref. [1] — are typically associated with recently reionized gas inside of cosmic voids where the hotter temperatures and rarefied densities enhance Lyαtransmission. The opacity-density relation's transmissive end is sensitive to the amount of neutral gas in the voids, as well as its morphology, set by the clustering of reionization sources. These effects are, however, largely degenerate with each other. We demonstrate that models with very different source clustering can nonetheless yield nearly identical opacity-density relations when their reionization histories are calibrated to match Lyα forest mean flux measurements atz< 6. In models with fixed source clustering, a lower neutral fraction increases the likelihood of intersecting hot, recently reionized gas in the voids, increasing the likelihood of observing fields like PSO J359-06. For instance, the probability of observing this field is 15% in a model with neutral fractionx_HI= 5% atz= 5.7, three times more likely than in a model withx_HI= 15%. The opacity-density relation may thus provide a complementary probe of reionization's tail end. 

Abstract During reionization, intergalactic ionization fronts (I-fronts) are sources of Lyαline radiation produced by collisional excitation of hydrogen atoms within the fronts. In principle, detecting this emission could provide direct evidence for a reionizing intergalactic medium (IGM). In this paper, we use a suite of high-resolution one-dimensional radiative transfer simulations run on cosmological density fields to quantify the parameter space of I-front Lyαemission. We find that the Lyαproduction efficiency — the ratio of emitted Lyαflux to incident ionizing flux driving the front — depends mainly on the I-front speed and the spectral index of the ionizing radiation. IGM density fluctuations on scales smaller than the typical I-front width produce scatter in the efficiency, but they do not significantly boost its mean value. The Lyαflux emitted by an I-front is largest if 3 conditions are met simultaneously: (1) the incident ionizing flux is large; (2) the incident spectrum is hard, consisting of more energetic photons; (3) the I-front is traveling through a cosmological over-density, which causes it to propagate more slowly. We present a convenient parameterization of the efficiency in terms of I-front speed and incident spectral index. We make these results publicly available as an interpolation table and we provide a simple fitting function for a representative ionizing background spectrum. Our results can be applied as a sub-grid model for I-front Lyα emissions in reionization simulations with spatial and/or temporal resolutions too coarse to resolve I-front structure. In a companion paper, we use our results to explore the possibility of directly imaging Lyαemission around neutral islands during the last phases of reionization. 

Abstract Long troughs observed in thez> 5.5 Lyαand Lyβforests are thought to be caused by the last remaining neutral patches during the end phases of reionization — termed neutral islands. If this is true, then the longest troughs mark locations where we are most likely to observe the reionizing intergalactic medium (IGM). A key feature of the neutral islands is that they are bounded by ionization fronts (I-fronts) which emit Lyman series lines. In this paper, we explore the possibility of directly imaging the outline of neutral islands with a narrowband survey targeting Lyα. In a companion paper, we quantified the intensity of I-front Lyαemissions during reionization and its dependence on the spectrum of incident ionizing radiation and I-front speed. Here we apply those results to reionization simulations to model the emissions from neutral islands. We find that neutral islands would appear as diffuse structures that are tens of comoving Mpc across, with surface brightnesses in the range ≈ 1 - 5× 10^-21erg s^-1cm^-2arcsec^-2. The islands are brighter if the spectrum of ionizing radiation driving the I-fronts is harder, and/or if the I-fronts are moving faster. We develop mock observations for current and futuristic observatories and find that, while extremely challenging, detecting neutral islands is potentially within reach of an ambitious observing program with wide-field narrowband imaging. Our results demonstrate the potentially high impact of low-surface brightness observations for studying reionization. 

Abstract We report that the neutral hydrogen (Hi) mass density of the Universe (ρ_Hi) increases with cosmic time sincez ∼ 5, peaks atz ∼ 3, and then decreases towardz ∼ 0. This is the first result of Qz5, our spectroscopic survey of 63 quasars atz ≳ 5 with VLT/X-SHOOTER and Keck/ESI aimed at characterizing intervening Higas absorbers atz ∼ 5. The main feature of Qz5 is the high resolution (R ∼ 7000–9000) of the spectra, which allows us to (1) accurately detect high column density Higas absorbers in an increasingly neutral intergalactic medium atz ∼ 5 and (2) determine the reliability of previousρ_Himeasurements derived with lower resolution spectroscopy. We find five intervening damped Lyαabsorbers (DLAs) atz > 4.5, which corresponds to the lowest DLA incidence rate ( $0.034_{0.02}^{0.05}$) atz ≳ 2. We also measure the lowestρ_Hiatz ≳ 2 from our sample of DLAs and subDLAs, corresponding toρ_Hi $=0.56_{0.31}^{0.82}\times 10^8{M}_{\odot }$Mpc⁻³atz ∼ 5. Taking into account our measurements atz ∼ 5 and systematic biases in the DLA detection rate at lower spectral resolutions, we conclude thatρ_Hidoubles fromz ∼ 5 toz ∼ 3. From these results emerges a qualitative agreement between how the cosmic densities of Higas mass, molecular gas mass, and star formation rate build up with cosmic time. 

ABSTRACT Recent measurements of the ionizing photon mean free path (MFP) based on composite quasar spectra may point to reionization ending at z < 6. These measurements are challenging because they rely on assumptions about the proximity zones of the quasars. For example, some quasars might have been close to neutral patches where reionization was still ongoing (‘neutral islands’), and it is unclear how they would affect the measurements. We address this question with mock MFP measurements from radiative transfer simulations. We find that, even in the presence of neutral islands, our mock MFP measurements agree to within $$30~{{\ \rm per\ cent}}$$ with the true spatially averaged MFP in our simulations, which includes opacity from both the ionized medium and the islands. The inferred MFP is sensitive at the $$\lt ~50~{{\ \rm per\ cent}}$$ level to assumptions about quasar environments and lifetimes for realistic models. We demonstrate that future analyses with improved data may require explicitly modelling the effects of neutral islands on the composite spectra, and we outline a method for doing this. Lastly, we quantify the effects of neutral islands on Lyman-series transmission, which has been modelled with optically thin simulations in previous MFP analyses. Neutral islands can suppress transmission at λrest < 912 Å significantly, up to a factor of 2 for zqso = 6 in a plausible reionization scenario, owing to absorption by many closely spaced lines as quasar light redshifts into resonance. However, the suppression is almost entirely degenerate with the spectrum normalization and thus does not significantly bias the inferred MFP. 

Aims.This study explores the metal enrichment signatures attributed to the first generation of stars (Pop III) in the Universe, focusing on the E-XQR-30 sample – a collection of 42 high signal-to-noise ratio spectra of quasi-stellar objects (QSOs) with emission redshifts ranging from 5.8 to 6.6. We aim to identify traces of Pop III metal enrichment by analyzing neutral gas in the interstellar medium of primordial galaxies and their satellite clumps, detected in absorption. Methods.To chase the chemical signature of Pop III stars, we studied metal absorption systems in the E-XQR-30 sample, selected through the detection of the neutral oxygen absorption line at 1302 Å. The O Iline is a reliable tracer of neutral hydrogen and allowed us to overcome the challenges posed by the Lyman-αforest’s increasing saturation at redshifts above ∼5 to identify damped Lyman-αsystems (DLAs). We detected and analyzed 29 O Isystems atz ≥ 5.4, differentiating between proximate DLAs (PDLAs) and intervening DLAs. Voigt function fits were applied to obtain ionic column densities, and relative chemical abundances were determined for 28 systems. These were then compared with the predictions of theoretical models. Results.Our findings expand the study of O Isystems atz ≥ 5.4 fourfold. No systematic differences were observed in the average chemical abundances between PDLAs and intervening DLAs. The chemical abundances in our sample align with literature systems atz >  4.5, suggesting a similar enrichment pattern for this class of absorption systems. A comparison between these DLA-analogs at 4.5 <  z <  6.5 with a sample of very metal-poor DLAs at 2 <  z <  4.5 shows in general similar average values for the relative abundances, with the exception of [C/O], [Si/Fe] and [Si/O] which are significantly larger for the high-zsample. Furthermore, the dispersion of the measurements significantly increases in the high-redshift bin. This increase is predicted by the theoretical models and indicates a potential retention of Pop III signatures in the probed gas. Conclusions.This work represents a significant advancement in the study of the chemical properties of highly neutral gas atz ≥ 5.4, shedding light on its potential association with the metal enrichment from Pop III stars. Future advancements in observational capabilities, specifically high-resolution spectrographs, are crucial for refining measurements and addressing current limitations in the study of these distant absorption systems. 

This study introduces novel constraints on the free streaming of thermal relic warm dark matter (WDM) from Lyman- $\alpha$forest flux power spectra. Our analysis utilizes a high resolution, high redshift sample of quasar spectra observed using the HIRES and UVES spectrographs ( $z=4.2–5.0$). We employ a Bayesian inference framework and a simulation-based likelihood that encompasses various parameters including the free streaming of dark matter, cosmological parameters, the thermal history of the intergalactic medium, and inhomogeneous reionization to establish lower limits on the mass of a thermal relic WDM particle of 5.7 keV (at 95% CL). This result surpasses previous limits from the Lyman- $\alpha$forest through reduction of the measured uncertainties due to a larger statistical sample and by measuring clustering to smaller scales ( $k_{\max }=0.2{\mathrm{km}}^{-1}\mathrm{s}$). The approximately two-fold improvement due to the expanded statistical sample suggests that the effectiveness of Lyman- $\alpha$forest constraints on WDM models at high redshifts are limited by the availability of high quality quasar spectra. Restricting the analysis to comparable scales and thermal history priors as in prior studies ( $k_{\max }<0.1{\mathrm{km}}^{-1}\mathrm{s}$) lowers the bound on the WDM mass to 4.1 keV. As the precision of the measurements increases, it becomes crucial to examine the instrumental and modeling systematics. On the modeling front, we argue that the impact of the thermal history uncertainty on the WDM particle mass constraint has diminished due to improved independent observations. At the smallest scales, the primary source of modeling systematic arises from the structure in the peculiar velocity of the intergalactic medium and inhomogeneous reionization. Published by the American Physical Society2024 

ABSTRACT Intervening metal absorbers in quasar spectra at z > 6 can be used as probes to study the chemical enrichment of the Universe during the Epoch of Reionization. This work presents the comoving line densities (dn/dX) of low-ionization absorbers, namely, Mg ii (2796 Å), C ii (1334 Å), and O  i (1302 Å) across 2 < z < 6 using the E-XQR-30 metal absorber catalogue prepared from 42 XSHOOTER quasar spectra at 5.8 < z < 6.6. Here, we analyse 280 Mg ii (1.9 < z < 6.4), 22 C ii (5.2 < z < 6.4), and 10 O i (5.3 < z < 6.4) intervening absorbers, thereby building up on previous studies with improved sensitivity of 50 per cent completeness at an equivalent width of W > 0.03 Å. For the first time, we present the comoving line densities of 131 weak (W < 0.3 Å) intervening Mg ii absorbers at 1.9 < z < 6.4 which exhibit constant evolution with redshift similar to medium (0.3 < W < 1.0 Å) absorbers. However, the cosmic mass density of Mg ii – dominated by strong Mg ii systems – traces the evolution of global star formation history from redshift 1.9 to 5.5. E-XQR-30 also increases the absorption path-length by a factor of 50 per cent for C ii and O i whose line densities show a rising trend towards z > 5, in agreement with previous works. In the context of a decline in the metal enrichment of the Universe at z > 5, the overall evolution in the incidence rates of absorption systems can be explained by a weak – possibly soft fluctuating – ultraviolet background. Our results, thereby, provide evidence for a late reionization continuing to occur in metal-enriched and therefore, biased regions in the Universe. 

Abstract The variations in Lyαforest opacity observed atz> 5.3 between lines of sight to different background quasars are too strong to be caused by fluctuations in the density field alone. The leading hypothesis for the cause of this excess variance is a late, ongoing reionization process at redshifts below six. Another model proposes strong ionizing background fluctuations coupled to a short, spatially varying mean free path of ionizing photons, without explicitly invoking incomplete reionization. With recent observations suggesting a short mean free path atz∼ 6, and a dramatic improvement inz> 5 Lyαforest data quality, we revisit this latter possibility. Here, we apply the likelihood-free inference technique of approximate Bayesian computation (ABC) to jointly constrain the hydrogen photoionization rate Γ_HIand the mean free path of ionizing photonsλ_mfpfrom the effective optical depth distributions atz= 5.0–6.1 from XQR-30. We find that the observations are well-described by fluctuating mean free path models with average mean free paths that are consistent with the steep trend implied by independent measurements atz∼ 5–6, with a concomitant rapid evolution of the photoionization rate. 

Abstract The first stars were born from chemically pristine gas. They were likely massive, and thus they rapidly exploded as supernovae, enriching the surrounding gas with the first heavy elements. In the Local Group, the chemical signatures of the first stellar population were identified among low-mass, long-lived, very metal-poor ([Fe/H] < −2) stars, characterized by high abundances of carbon over iron ([C/Fe] > +0.7): the so-called carbon-enhanced metal-poor stars. Conversely, a similar carbon excess caused by first-star pollution was not found in dense neutral gas traced by absorption systems at different cosmic time. Here we present the detection of 14 very metal-poor, optically thick absorbers at redshift z ∼ 3–4. Among these, 3 are carbon-enhanced and reveal an overabundance with respect to Fe of all the analyzed chemical elements (O, Mg, Al, and Si). Their relative abundances show a distribution with respect to [Fe/H] that is in very good agreement with those observed in nearby very metal-poor stars. All the tests we performed support the idea that these C-rich absorbers preserve the chemical yields of the first stars. Our new findings suggest that the first-star signatures can survive in optically thick but relatively diffuse absorbers, which are not sufficiently dense to sustain star formation and hence are not dominated by the chemical products of normal stars.