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The thermal history and structure of the intergalactic medium (IGM) at |$z \ge 4$| is an important boundary condition for reionization, and a key input for studies using the Ly  |$\alpha$| forest to constrain the masses of alternative dark matter candidates. Most such inferences rely on simulations that lack the spatial resolution to fully resolve the hydrodynamic response of IGM filaments and minihaloes to H  i reionization heating. In this letter, we use high-resolution hydrodynamic + radiative transfer simulations to study how these affect the IGM thermal structure. We find that the adiabatic heating and cooling driven by the expansion of initially cold gas filaments and minihaloes sources significant small-scale temperature fluctuations. These likely persist in much of the IGM until |$z \le 4$|⁠. Capturing this effect requires resolving the clumping scale of cold, pre-ionized gas, demanding spatial resolutions of |${\le} 2$|   |$h^{-1}$| kpc. Pre-heating of the IGM by X-rays can slightly reduce the effect. Our preliminary estimate of the effect on the Ly  |$\alpha$| forest finds that, at |$\log (k /[{\rm km^{-1} s}]) = -1.0$|⁠ , the Ly  |$\alpha$| forest flux power (at fixed mean flux) can increase |${\approx} 10~{{\ \rm per\ cent}}$| going from 8 and 2  |$h^{-1}$| kpc resolution at |$z = 4{\!-\!}5$| for gas ionized at |$z \ \lt\ 7$|⁠. These findings motivate more careful analyses of how the effects studied here affect the Ly  |$\alpha$| forest. [ABSTRACT FROM AUTHOR]

Line-intensity mapping (IM) experiments seek to perform statistical measurements of large-scale structure with spectral lines such as 21 cm, CO, and Ly α. A challenge in these observations is to ensure that astrophysical foregrounds, such as galactic synchrotron emission in 21 cm measurements, are properly removed. One method that has the potential to reduce foreground contamination is to cross correlate with a galaxy survey that overlaps with the IM volume. However, telescopes sensitive to high-redshift galaxies typically have small field of views compared to IM surveys. Thus, a galaxy survey for cross correlation would necessarily consist of pencil beams that sparsely fill the IM volume. In this paper, we develop the formalism to forecast the sensitivity of cross correlations between IM experiments and pencil-beam galaxy surveys. We find that a random distribution of pencil beams leads to very similar overall sensitivity as a lattice spaced across the IM survey and derive a simple formula for random configurations that agrees with the Fisher matrix formalism. We explore examples of combining high-redshift James Webb Space Telescope (JWST) observations with both an SPHEREx-like Ly α IM survey and a 21 cm experiment based on the Hydrogen Epoch of Reionization Array (HERA). We find that the JWST-SPHEREx case is promising, leading to a total signal-to-noise ratio of ∼5 after 100 total hours of JWST (at z = 7). We find that HERA is not well-suited for this approach owing to its drift-scan strategy, but that a similar experiment that can integrate down on one field could be. [ABSTRACT FROM AUTHOR]

A recent measurement of the Lyman-limit mean free path at z  = 6 suggests it may have been very short, motivating a better understanding of the role that ionizing photon sinks played in reionization. Accurately modelling the sinks in reionization simulations is challenging because of the large dynamic range required if ∼104−108M⊙ gas structures contributed significant opacity. Thus, there is no consensus on how important the sinks were in shaping reionization's morphology. We address this question with a recently developed radiative transfer code that includes a dynamical sub-grid model for the sinks based on radiative hydrodynamics simulations. Compared to assuming a fully pressure-smoothed intergalactic medium, our dynamical treatment reduces ionized bubble sizes by |$10-20~{{\ \rm per\ cent}}$| under typical assumptions about reionization's sources. Near reionization's midpoint, the 21 cm power at k ∼ 0.1 h Mpc−1 is similarly reduced. These effects are more modest than the |$30-60~{{\ \rm per\ cent}}$| suppression resulting from the higher recombination rate if pressure smoothing is neglected entirely. Whether the sinks played a significant role in reionization's morphology depends on the nature of its sources. For example, if reionization was driven by bright (M UV < −17) galaxies, the sinks reduce the large-scale 21 cm power by at most 20  per cent, even if pressure smoothing is neglected. Conveniently, when bright sources contribute significantly, the morphology in our dynamical treatment can be reproduced accurately with a uniform sub-grid clumping factor that yields the same ionizing photon budget. By contrast, if M UV ∼ −13 galaxies drove reionization, the uniform clumping model can err by up to 40  per cent. [ABSTRACT FROM AUTHOR]

Where the cosmic baryons lie in and around galactic dark matter haloes is only weakly constrained. We develop a method to quickly paint on models for their distribution. Our approach uses the statistical advantages of N -body simulations, while painting on the profile of gas around individual haloes in ways that can be motivated by semi-analytic models or zoom-in hydrodynamic simulations of galaxies. Possible applications of the algorithm include extragalactic dispersion measures to fast radio bursts (FRBs), the Sunyaev–Zeldovich effect, baryonic effects on weak lensing, and cosmic metal enrichment. As an initial application, we use this tool to investigate how the baryonic profile of foreground galactic-mass haloes affects the statistics of the dispersion measure (DM) towards cosmological FRBs. We show that the distribution of DM is sensitive to the distribution of baryons in galactic haloes, with viable gas profile models having significantly different probability distributions for DM to a given redshift. We also investigate the requirements to statistically measure the circumgalactic electron profile for FRB analyses that stack DM with impact parameter to foreground galaxies, quantifying the size of the contaminating 'two-halo' term from correlated systems and the number of FRBs for a high significance detection. Publicly available python modules implement our CGMBrush algorithm. [ABSTRACT FROM AUTHOR]

The distribution of gas in the circumgalactic medium (CGM) of galaxies of all types is poorly constrained. Foreground CGMs contribute an extra amount to the dispersion measure (DM) of fast radio bursts (FRBs). We measure this DM excess for the CGMs of 1011–1013 M ⊙ halos using the CHIME/FRB first data release, a halo mass range that is challenging to probe in any other way. Because of the uncertainty in the FRBs' angular coordinates, only for nearby galaxies is the localization sufficient to confidently associate them with intersecting any foreground halo. Thus we stack on galaxies within 80 Mpc, optimizing the stacking scheme to approximately minimize the stack's variance and marginalize over uncertainties in FRB locations. The sample has 20–30 FRBs intersecting halos with masses of 1011–1012 M ⊙ and also of 1012–1013 M ⊙, and these intersections allow a marginal 1 σ –2 σ detection of the DM excess in both mass bins. The bin of 1011–1012 M ⊙ halos also shows a DM excess at 1–2 virial radii. By comparing data with different models for the CGM gas profile, we find that all models are favored by the data up to 2 σ level compared to the null hypothesis of no DM excess. With 3000 more bursts from a future CHIME data release, we project a 4 σ detection of the CGM. Distinguishing between viable CGM models by stacking FRBs with CHIME-like localization would require tens of thousands of bursts. [ABSTRACT FROM AUTHOR]

Motivated by integral field units (IFUs) on large ground telescopes and proposals for ultraviolet-sensitive space telescopes to probe circumgalactic medium (CGM) emission, we survey the most promising emission lines and how such observations can inform our understanding of the CGM and its relation to galaxy formation. We tie our emission estimates to both HST /COS absorption measurements of ions around z ≈ 0.2 Milky Way mass haloes and models for the density and temperature of gas. We also provide formulas that simplify extending our estimates to other samples and physical scenarios. We find that O  iii  5007 Å and N  ii  6583 Å, which at fixed ionic column density are primarily sensitive to the thermal pressure of the gas they inhabit, may be detectable with KCWI and especially IFUs on 30 m telescopes out to half a virial radius. O  v  630 Å and O  vi 1032,1038 Å are perhaps the most promising ultraviolet lines, with models predicting intensities >100 γ cm−2 s−1 sr−1 in the inner 100 kpc of Milky Way-like systems. A detection of O  vi would confirm the collisionally ionized picture and constrain the density profile of the CGM. Other ultraviolet metal lines constrain the amount of gas that is actively cooling and mixing. We find that C  iii  978 Å and C  iv  1548 Å may be detectable if an appreciable fraction of the observed O  vi column is associated with mixing or cooling gas. H α emission within |$100\,$|  kpc of Milky Way-like galaxies is within reach of current IFUs even for the minimum signal from ionizing background fluorescence, while hydrogen n > 2 Ly-series lines are too weak to be detectable. [ABSTRACT FROM AUTHOR]

The first generation (Pop-III) stars can ionize 1–10 per cent of the universe by z  = 15, when the metal-enriched (Pop-II) stars may contribute negligibly to the ionization. This low ionization tail might leave detectable imprints on the large-scale CMB E-mode polarization. However, we show that physical models for reionization are unlikely to be sufficiently extended to detect any parameter beyond the total optical depth through reionization. This result is driven in part by the total optical depth inferred by Planck , indicating a reionization midpoint around z  = 8, which in combination with the requirement that reionization completes by z ≈ 5.5 limits the amplitude of an extended tail. To demonstrate this, we perform semi-analytic calculations of reionization including Pop-III star formation in minihalos with Lyman-Werner feedback. We find that standard Pop-III models need to produce very extended reionization at z > 15 to be distinguishable at 2-σ from Pop-II-only models, assuming a cosmic variance-limited measurement of the low-ℓ EE power spectrum. However, we show that unless there is a late-time quenching mechanism such as from strong X-ray feedback or some other extreme Pop-III scenario, structure formation makes it quite challenging to produce high enough Thomson scattering optical depth from z > 15, τ(z > 15), and still be consistent with other observational constraints on reionization. [ABSTRACT FROM AUTHOR]

The intergalactic medium is expected to clump on scales down to M⊙ before the onset of reionization. The impact of these small-scale structures on reionization is poorly understood despite the modern understanding that gas clumpiness limits the growth of regions. We use a suite of radiation-hydrodynamics simulations that capture the Jeans mass of unheated gas to study density fluctuations during reionization. Our simulations track the complex ionization and hydrodynamical response of gas in the wake of ionization fronts. The clumping factor of ionized gas (proportional to the recombination rate) rises to a peak value of 5–20 approximately Δt = 10 Myr after ionization front passage, depending on the incident intensity, redshift, and degree to which the gas had been preheated by the first X-ray sources. The clumping factor reaches its relaxed value of ≈3 by Δt = 300 Myr. The mean free path of Lyman-limit photons evolves in unison, being up to several times shorter in unrelaxed, recently reionized regions compared to those that were reionized much earlier. Assessing the impact of this response on the global reionization process, we find that unrelaxed gaseous structures boost the total number of recombinations by ≈50% and lead to spatial fluctuations in the mean free path that persist appreciably for several hundred million years after the completion of reionization. [ABSTRACT FROM AUTHOR]

We examine the impact of baryon-dark matter relative velocities on intergalactic small-scale structure and the 21 cm signal during reionization. Streaming velocities reduced clumping in the intergalactic medium on mass scales of ∼104–108M⊙. This effect produced a distinct baryon acoustic oscillation (BAO) feature in the 21 cm power spectrum at wavenumbers k ∼ 0.1 h/Mpc, near which forthcoming surveys will be most sensitive. In contrast to the highly uncertain impact of streaming velocities on star formation, the effect on clumping is better constrained because it is set mainly by cosmology and straightforward gas dynamics. We quantify the latter using coupled radiation-hydrodynamic simulations that capture the Jeans scale of pre-reionization gas. The clumping factor of ionized gas is reduced by 5%–10% in regions with rms streaming velocities. The suppression peaks ≈5 Myr after a region is reionized, but disappears within 200 Myr due to pressure smoothing. We model the corresponding impact on the 21 cm signal and find that the BAO feature is most likely to appear at ≈10% ionization. During this phase, the feature may appear at the 1% (5%) level at k ∼ 0.1 (0.06) h/Mpc with an amplitude that varies by a factor of <10 across a range of reionization histories. We also provide a model for the signal originating from streaming velocity's impact on ionizing sources, which can vary by 4 orders of magnitude depending on highly uncertain source properties. We find that the clumping signal probably dominates the source one unless Population III star formation in 106–108M⊙ halos contributed significantly to the first 10% of reionization. [ABSTRACT FROM AUTHOR]

Reionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the |$z$| ∼ 5 Ly α forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring |${\sim}20\text{--}60{{\ \rm per\ cent}}$| extra power at k ∼ 0.002 s km−1, with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales (k ≳ 0.1 s km−1), we find that temperature fluctuations suppress power by |${\lesssim}10{{\ \rm per\ cent}}$|⁠. We find that the shape of the power spectrum is mostly sensitive to the reionization mid-point rather than temperature fluctuations from reionization's patchiness. However, for all of our models with reionization mid-points of |$z$| ≤ 8 (⁠|$z$| ≤ 12), the shape differences are |${\lesssim}20{{\ \rm per\ cent}}$| (⁠|${\lesssim}40{{\ \rm per\ cent}}$|⁠) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at k ≳ 0.01 s km−1. Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization mid-points of |$z$| < 8 is strongly preferred over another and that all of our models with mid-points of |$z$| ≥ 8 are excluded at 2.5σ. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower k or, alternatively, if the error bars on the high- k power can be reduced by a factor of 1.5. [ABSTRACT FROM AUTHOR]

Spatial fluctuations in ultraviolet backgrounds can subtly modulate the distribution of extragalactic sources, a potential signal and systematic for large-scale structure surveys. While this modulation has been shown to be significant for 3D Ly α forest surveys, its relevance for other large-scale structure probes has been hardly explored, despite being the only astrophysical process that likely can affect clustering measurements on the scales of ≳Mpc. We estimate that the background fluctuations, modulating the amount of H  i, have a fractional effect of (0.03–0.3) × (k /[10−2 Mpc−1])−1 on the power spectrum of 21 cm intensity maps at z  = 1–3. We find a smaller effect for H α and Ly α intensity mapping surveys of (0.001–0.1) × (k /[10−2 Mpc−1])−1 and even smaller effect for more traditional surveys that correlate the positions of individual H α or Ly α emitters. We also estimate the effect of backgrounds on low-redshift galaxy surveys in general based on a simple model in which background fluctuations modulate the rate halo gas cools, modulating star formation: We estimate a maximum fractional effect on the power of ∼0.01 (k /[10−2 Mpc−1])−1 at z  = 1. We compare sizes of these imprints to cosmological parameter benchmarks for the next generation of redshift surveys: We find that ionizing backgrounds could result in a bias on the squeezed triangle non-Gaussianity parameter f NL that can be larger than unity for power spectrum measurements with a SPHEREx-like galaxy survey, and typical values of intensity bias. Marginalizing over a shape of the form k −1 PL, where PL is the linear matter power spectrum, removes much of this bias at the cost of |${\approx } 40{{\ \rm per\ cent}}$| larger statistical errors. [ABSTRACT FROM AUTHOR]

We present a formalism for computing time-dependent fluctuations in the cosmological photoionizing radiation background, extending background fluctuation models beyond the steady-state approximation. We apply this formalism to estimate fluctuations in the H  |$\rm \scriptstyle I$| Ly α flux redshift-space power spectrum and its spatial correlation function at redshifts 2 < z < 4, assuming the photoionization background is dominated by quasi-stellar objects (QSOs) and/or galaxies. We show that the shot noise in the power spectrum due to discrete sources is strongly suppressed relative to the steady-state value at low wavenumbers by a factor proportional to the lifetime of the sources, and that this suppression may be used to constrain QSO lifetimes. The total H  |$\rm \scriptstyle I$| Ly α power spectrum including shot noise is affected at tens of per cent on short scales and by as much as an order of magnitude or more on scales exceeding the mean free path. The spatial correlation function is similarly found to be sensitive to the shot noise, although moderately insensitive to the effects of time dependence on the non-shot-noise contribution. Photoionization rate fluctuations substantially modify the shape of the baryonic acoustic oscillation peak in the correlation function, including a small increase in its position that must be accounted for to avoid biasing estimates of cosmological parameters based on the peak position. We briefly investigate solving the full frequency-dependent equation, finding that it agrees with the frequency-independent to better than per cent accuracy. Simple formulas are provided for the power spectrum of fluctuations in the photoionization rate that approximate the full computations. [ABSTRACT FROM AUTHOR]

Recent observations have shown that the scatter in opacities amongst coeval segments of the Ly α forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ≈ 5.5 requires a short spatially averaged mean free path of <λ912 mfp > ≲ 15 h-1 comoving Mpc, a factor of ≳3 shorter than direct measurements at z = 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the global HI photoionization rate, which stay approximately constant over the interval z ≈ 4.8-5.5. However, we also show that measurements of the mean free path at z > 5 are likely biased towards higher values by the quasar proximity effect. This bias can reconcile the short values of <λ912 mfp > that are required to explain the large scatter in opacities.We discuss the implications of this scenario for cosmological reionization. Finally, we investigate whether other statistics applied to the z > 5 Lyα forest can shed light on the origin of the scatter. Compared to a model with a uniform ionizing background, models that successfully account for the scatter lead to enhanced power in the line-of-sight flux power spectrum on scales k ≲0.1 h Mpc-1.We find tentative evidence for this enhancement in observations of the high-redshift Ly α forest. [ABSTRACT FROM AUTHOR]

Motivated by the claimed detection of a large population of faint active galactic nuclei (AGNs at high redshift, recent studies have proposed models in which AGNs contribute significantly to the z > 4 HI ionizing background. In some models, AGNs are even the chief sources of reionization. If proved true, these models would make necessary a complete revision to the standard view that galaxies dominated the high-redshift ionizing background. It has been suggested that AGN-dominated models can better account for two recent observations that appear to be in conflict with the standard view: (1) large opacity variations in the z ∼ 5.5 HI Ly α forest, and (2) slow evolution in the mean opacity of the He II Ly α forest. Large spatial fluctuations in the ionizing background from the brightness and rarity of AGNs may account for the former, while the earlier onset of He II reionization in these models may account for the latter. Here we show that models in which AGN emissions source ≳50 per cent of the ionizing background generally provide a better fit to the observed HI Ly α forest opacity variations compared to standard galaxy-dominated models. However, we argue that these AGN-dominated models are in tension with constraints on the thermal history of the intergalactic medium (IGM). Under standard assumptions about the spectra of AGNs, we show that the earlier onset of He II reionization heats up the IGM well above recent temperature measurements.We further argue that the slower evolution of the mean opacity of the He II Ly α forest relative to simulations may reflect deficiencies in current simulations rather than favour AGN-dominated models as has been suggested. [ABSTRACT FROM AUTHOR]

The bulk of cosmic matter resides in a dilute reservoir that fills the space between galaxies, the intergalactic medium (IGM). The history of this reservoir is intimately tied to the cosmic histories of structure formation, star formation, and supermassive black hole accretion. Our models for the IGM at intermediate redshifts (2≲ z≲5) are a tremendous success, quantitatively explaining the statistics of Lyα absorption of intergalactic hydrogen. However, at both lower and higher redshifts (and around galaxies) much is still unknown about the IGM. We review the theoretical models and measurements that form the basis for the modern understanding of the IGM, and we discuss unsolved puzzles (ranging from the largely unconstrained process of reionization at high z to the missing baryon problem at low z), highlighting the efforts that have the potential to solve them. [ABSTRACT FROM AUTHOR]

Recent years have brought more precise temperature measurements of the low-density intergalactic medium (IGM). These new measurements constrain the processes that heated the IGM, such as the reionization of HI and of He II. We present a semi-analytical model for the thermal history of the IGM that follows the photoheating history of primordial gas. Our model adopts a multizone approach that, compared to previous models, more accurately captures the inhomogeneous heating and cooling of the IGM during patchy reionization. We compare our model with recent temperature measurements spanning z = 1.6-4.8, finding that these measurements are consistent with scenarios in which the He II was reionized at z = 3-4 by quasars. Significantly longer duration or higher redshift He II reionization scenarios are ruled out by the measurements. For hydrogen reionization, we find that only low-redshift and high-temperature scenarios are excluded. For example, a model in which the IGM was heated to 30 000K when an ionization front passed, and with hydrogen reionization occurring over 6 < z <9, is ruled out. Finally, we place constraints on how much heating could owe to TeV blazars, cosmic rays, and other non-standard mechanisms. We find that by z = 2, a maximum of 1 eV of additional heat could be injected per baryon over standard photoheating-only models, with this limit becoming ≲0.5 eV at z > 3. [ABSTRACT FROM AUTHOR]

The conditions within the ionization front of a quasar during reionization (T~30 000 K, neutral hydrogen fraction xHI ~ 0.5) are ideal for producing Lyα emission via collisional excitation of hydrogen atoms. Observations of this emission, which could subtend ≳10 arcmin2 on the sky, would definitively demonstrate the presence of a neutral intergalactic medium at the observed epoch, placing valuable constraints on the progress of reionization. We find that the expected Lyα surface brightness is significantly weaker than previously determined and may be impossible to observe with current and near-future instruments. Past work calculated the Lyα emission from a quasar ionization front in a homogeneous medium with a clumping factor approximation to account for inhomogeneities.We find using 1D radiative transfer calculations that this approximation overestimates the emission by a factor of ≳3. Our calculations model the propagation of ionizing photons and compute the Lyα emission from quasar ionization fronts on sightlines from a hydrodynamic cosmological simulation at z = 7.1. To better understand the physical properties of the emission, we also develop an analytic model that accurately describes the results of the full radiative transfer calculation. [ABSTRACT FROM AUTHOR]

Cosmological simulations of the low-density intergalactic medium exhibit a strikingly tight power-law relation between temperature and density that holds over two decades in density. It is found that this relation should roughly apply Δz ~ 1-2 after a reionization event, and this limiting behaviour has motivated the power-law parameterizations used in most analyses of the Ly α forest. This relation has been explained by using equations linearized in the baryonic overdensity (which does not address why a tight power-law relation holds over two decades in density) or by equating the photoheating rate with the cooling rate from cosmological expansion (which we show is incorrect). Previous explanations also did not address why recombination cooling and Compton cooling off of the cosmic microwave background, which are never negligible, do not alter the character of this relation. We provide an understanding for why a tight power-law relation arises for unshocked gas at all densities for which collisional cooling is unimportant. We also use our results to comment on (1) how quickly fluctuations in temperature redshift away after reionization processes, (2) how much shock heating occurs in the low-density intergalactic medium, and (3) how the temperatures of collapsing gas parcels evolve. [ABSTRACT FROM AUTHOR]

We develop and test with cosmological simulations a physically motivated theory for how the interplay between gravity, pressure, cooling, and self-shielding set the redshift-dependent mass scale at which haloes can accrete intergalactic gas. This theory provides a physical explanation for the halo mass scale that can accrete unshocked intergalactic gas, which has been explained with ad hoc criteria tuned to reproduce the results of a few simulations. Furthermore, it provides an intuitive explanation for how this mass scale depends on the reionization redshift, the amplitude of the ionizing background, and the redshift. We show that accretion is inhibited on to more massive haloes than had been thought because previous studies had focused on the gas fraction of haloes rather than the instantaneous mass that can accrete gas. A halo as massive as 1011 M⊙ cannot accrete intergalactic gas at z = 0, even though typically its progenitors were able to accrete gas at higher redshifts. We describe a simple algorithm that can be implemented in semi-analytic models, and we compare the predictions of this algorithm to numerical simulations. [ABSTRACT FROM PUBLISHER]

Several recent papers have proposed schemes by which a dispersion measure, and hence electron column, could be obtained from a time-steady, incoherent radio source at a cosmological distance (such as an active galactic nucleus). If correct, this would open a new window on the distribution of intergalactic baryons. These schemes are based on the statistical properties of the received radiation, such as the 2- or 4-point correlation function of the received electric field, and in one case on the quantum nature of the electromagnetic field. We show, on the basis of general principles, that these schemes are not sensitive to dispersion measure (or have an extremely small signal-to-noise ratio), because (i) the classical 2-point correlation function is unaffected by dispersion; (ii) for a source with a large number of incoherently emitting electrons, the central limit theorem obliterates additional information in higher order functions; and (iii) such an emitter produces a radiation density matrix that is equivalent to a statistical distribution of coherent states, which contains no information that is not already in the statistics of the classical waveforms. Why the proposed observables do not depend on dispersion measure (or have extremely tiny dependences) is discussed in detail. [ABSTRACT FROM AUTHOR]

A number of experiments are currently working toward a measurement of the 21 cm signal from the epoch of reionization (EoR). Whether or not these experiments deliver a detection of cosmological emission, their limited sensitivity will prevent them from providing detailed information about the astrophysics of reionization. In this work, we consider what types of measurements will be enabled by the next generation of larger 21 cm EoR telescopes. To calculate the type of constraints that will be possible with such arrays, we use simple models for the instrument, foreground emission, and the reionization history. We focus primarily on an instrument modeled after the ∼0.1 km2 collecting area Hydrogen Epoch of Reionization Array concept design and parameterize the uncertainties with regard to foreground emission by considering different limits to the recently described “wedge” footprint in k space. Uncertainties in the reionization history are accounted for using a series of simulations that vary the ionizing efficiency and minimum virial temperature of the galaxies responsible for reionization, as well as the mean free path of ionizing photons through the intergalactic medium. Given various combinations of models, we consider the significance of the possible power spectrum detections, the ability to trace the power spectrum evolution versus redshift, the detectability of salient power spectrum features, and the achievable level of quantitative constraints on astrophysical parameters. Ultimately, we find that 0.1 km2 of collecting area is enough to ensure a very high significance (≳ 30σ) detection of the reionization power spectrum in even the most pessimistic scenarios. This sensitivity should allow for meaningful constraints on the reionization history and astrophysical parameters, especially if foreground subtraction techniques can be improved and successfully implemented. [ABSTRACT FROM AUTHOR]

ABSTRACT We examine the constraints on soft X-ray photon emissions from the reionization era. It is generally assumed that the Universe was reionized by ultraviolet photons radiated from massive stars. However, it has been argued that X-ray photons associated with the death of these stars would have contributed ∼10 per cent to the total number of ionizations via several channels. The parameter space for a significant component of cosmological reionization to be sourced by X-rays is limited by a few observations. We revisit the unresolved soft X-ray background constraint on high-redshift X-ray production and show that soft X-ray background measurements significantly limit the contribution to reionization from several potential sources: X-rays from X-ray binaries, from Compton scattering off supernovae-accelerated electrons, and from the annihilation of dark matter particles. We discuss the additional limits on high-redshift X-ray photon production from (1) z ∼ 3 measurements of metal absorption lines in quasar spectra, (2) the consensus that helium reionization was ending at z ≈ 3 and (3) measurements of the intergalactic medium's thermal history. We show that observations of z ∼ 3 metal lines allow little room for extra coeval soft X-ray emission from a non-standard X-ray sources. In addition, we show that the late reionization of helium makes it quite difficult to also ionize the hydrogen at z > 6 with a single source population (such as quasars) and that it likely requires the spectrum of ionizing emissions to soften with increasing redshift. However, we find that it is difficult to constrain an X-ray contribution to reionization from the intergalactic temperature history. We show that the intergalactic gas would have been heated to a narrower range of temperatures than is typically assumed at reionization, 2-3 × 104 K, with this temperature depending weakly on the ionizing sources' spectra. [ABSTRACT FROM AUTHOR]

ABSTRACT Inhomogeneous reionization acts as a source of arcminute-scale anisotropies in the cosmic microwave background (CMB), the most important of which is the kinetic Sunyaev-Zel'dovich (kSZ) effect. Observational efforts with the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT) are poised to detect this signal for the first time, with projected 1 μ K2-level sensitivity to the dimensionless kSZ power spectrum around a multipole of l= 3000, [Δ l3000]2. Indeed, recent SPT measurements place a bound of [Δ l3000 ]2 < 2.8 μ K2 at 95 per cent confidence level, which degrades to [Δ l3000 ]2 < 6 μ K2 if a significant correlation between the thermal Sunyaev-Zel'dovich (tSZ) effect and the cosmic infrared background (CIB) is allowed. To interpret these and upcoming observations, we compute the kSZ signal from a suite of ≈100 reionization models using the publicly available code 21cmfast. Our physically motivated reionization models are parametrized by the ionizing efficiency of high-redshift galaxies, the minimum virial temperature of haloes capable of hosting stars, and the ionizing photon mean free path - a parametrization motivated by previous theoretical studies of reionization. We predict the contribution of patchy reionization to the l= 3000 kSZ power to be 1.5-3.5 μ K2. Therefore, even when adopting the lowest estimate in the literature for the post-reionization signal of , none of our models are consistent with the aggressive 2σ SPT bound that does not include correlations. This implies the following: (i) the early stages of reionization occurred in a much more homogeneous manner than suggested by the stellar-driven scenarios we explore, such as would be the case if, e.g. very high energy X-rays or exotic particles contributed significantly and/or (ii) that there is a significant correlation between the CIB and the tSZ. The later is perhaps not surprising, as massive haloes should host both hot gas and star-forming galaxies. On the other hand, the conservative SPT bound of [Δ l3000 ]2≲ 6 μ K2 is compatible with all of our models and is on the threshold of constraining physically motivated reionization models. The largest patchy kSZ signals correspond to an extended reionization process, in which the sources of ionizing photons are abundant and there are many recombinations (absorptions in sinks). We point out that insights into the astrophysics of the early Universe are encoded in both the amplitude and shape of the kSZ power spectrum. [ABSTRACT FROM AUTHOR]

We study the properties of self-shielding intergalactic absorption systems and their implications for the ionizing background. We find that cosmological simulations post-processed with detailed radiative transfer calculations generally are able to reproduce the observed abundance of Lyman-limit systems, and we highlight possible discrepancies between the observations and simulations. This comparison tests cosmological simulations at overdensities of ~100. Furthermore, we show that the properties of Lyman-limit systems in these simulations, in simple semianalytic arguments, and as suggested by recent observations indicate that a small change in the ionizing emissivity of the sources would have resulted in a much larger change in the amplitude of the intergalactic H I-ionizing background (with this scaling strengthening with increasing redshift). This strong scaling could explain the rapid evolution in the Lyα forest transmission observed at z. ≈ 6. Our calculations agree with the suggestion of simpler models that the comoving ionizing emissivity was constant or even increasing from z = 3 to 6. Our calculations also provide a more rigorous estimate than in previous studies for the clumping factor of intergalactic gas after reionization, which we estimate was ≈ 2-3 at z. = 6. [ABSTRACT FROM AUTHOR]

ABSTRACT The next frontier of Lyα forest studies is the reconstruction of three-dimensional (3D) correlations from a dense sample of background sources. The measurement of 3D correlations has the potential to improve constraints on fundamental cosmological parameters, ionizing background models and the re-ionization history. This study addresses the sensitivity of spectroscopic surveys to 3D correlations in the Lyα forest. We show that the sensitivity of a survey to this signal can be quantified by just a single number, a noise-weighted number density of sources on the sky. We investigate how the sensitivity of a spectroscopic quasar (or galaxy) survey scales as a function of its depth, area and redshift. We propose a simple method for weighting sightlines with varying signal-to-noise ratio (S/N) levels to estimate the correlation function, and we show that this estimator generally performs nearly as well as the minimum-variance quadratic estimator. In addition, we show that the sensitivity of a quasar survey to the flux correlation function is generally maximized if it observes each field just long enough to achieve S/N ≈ 2 in a 1-Å pixel on an L* quasar while acquiring spectra for all quasars with L > L*: little is gained by integrating longer on the same targets or by including fainter quasars. We quantify how these considerations relate to constraints on the angular diameter distance, the curvature of space-time and the re-ionization history. [ABSTRACT FROM AUTHOR]

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