Timing Of Reionization Research Articles

Distinguishing the impact and signature of black holes from different origins in early cosmic history

ABSTRACT We semi-analytically model the effects of primordial black hole (PBH) accretion on the cosmic radiation background during the epoch of reionization (z ≳ 6). PBHs in the intergalactic medium (IGM) and haloes, where star formation can occur, are considered. For stars with a mass $\gtrsim 25 \rm \ {\rm M}_{\odot }$, formed in suitable host haloes, we assume they quickly burn out and form stellar-remnant black holes (SRBHs). These SRBHs, like PBHs, also accrete material, and are considered to have similar radiation feedback in the halo environment. To improve the background radiation estimation, we consider the impact of PBHs on structure formation, allowing a modified halo mass function. We consider the radiation feedback from a broad suite of black holes: PBHs, SRBHs, high-mass X-ray binaries, and supermassive black holes. The result shows that at z ≳ 30, the radiation background energy density is generated by PBHs accreting in the IGM, whereas at lower redshifts, halo accretion dominates. We also dissect the total power density by modelling the accretion spectral energy distribution across different wavebands. In the UV band, we find that for fPBH ≲ 10−3, the H-ionizing and Lyman-α fluxes from PBH accretion feedback do not violate existing constraints on the timing of reionization, and on the effective Wouthuysen–Field coupling of the 21-cm spin temperature of neutral hydrogen to the IGM kinetic temperature. However, in the X-ray band, with the same abundance, PBHs contribute significantly and could explain the unresolved part of the cosmic X-ray background.

Ghostly Galaxies: Accretion-dominated Stellar Systems in Low-mass Dark Matter Halos

Wide-area deep imaging surveys have discovered large numbers of extremely low surface brightness (LSB) dwarf galaxies, which challenge galaxy formation theory and, potentially, offer new constraints on the nature of dark matter. Here we discuss one as-yet-unexplored formation mechanism that may account for a fraction of LSB dwarfs. We call this the “ghost galaxy” scenario. In this scenario, inefficient radiative cooling prevents star formation in the “main branch” of the merger tree of a low-mass dark matter halo, such that almost all its stellar mass is acquired through mergers with less massive (but nevertheless star-forming) progenitors. Present-day systems formed in this way would be “ghostly” isolated stellar halos with no central galaxy. We use merger trees based on the extended Press–Schechter formalism and the Copernicus Complexio cosmological N-body simulation to demonstrate that mass assembly histories of this kind can occur for low-mass halos in ΛCDM, but they are rare. They are most probable in isolated halos of present-day mass ∼4 × 109 M ⊙, occurring for ∼5% of all halos of that mass under standard assumptions about the timing and effect of cosmic reionization. The stellar masses of star-forming progenitors in these systems are highly uncertain; abundance-matching arguments imply a bimodal present-day mass function having a brighter population (median M ⋆ ∼ 3 × 106 M ⊙) consistent with the tail of the observed luminosity function of ultradiffuse galaxies. This suggests that observable analogs of these systems may await discovery. We find that a stronger ionizing background (globally or locally) produces brighter and more extended ghost galaxies.

Convergence of small scale Ly α structure at high-z under different reionization scenarios

ABSTRACT The Ly α forest (LAF) at z > 5 probes the thermal and reionization history of the intergalactic medium (IGM) and the nature of dark matter, but its interpretation requires comparison to cosmological hydrodynamical simulations. At high-z, convergence of these simulations is more exacting since transmission is dominated by underdense voids that are challenging to resolve. With evidence mounting for a late end to reionization, small structures down to the sub-kpc level may survive to later times than conventionally thought due to the reduced time for pressure smoothing to impact the gas, further tightening simulation resolution requirements. We perform a suite of simulations using the Eulerian cosmological hydrodynamics code Nyx, spanning domain sizes of 1.25 − 10 h−1 Mpc and 5 − 80 h−1 kpc cells, and explore the interaction of these variables with the timing of reionization on the properties of the matter distribution and the simulated LAF at z = 5.5. In observable Ly α power, convergence within 10 per cent is achieved for k < 0.1 s km–1, but larger k shows deviation of up to 20 per cent. While a later reionization retains more small structure in the density field, because of the greater thermal broadening there is little difference in the convergence of LAF power between early (z = 9) and later (z = 6) reionizations. We conclude that at z ∼ 5.5, resolutions of 10 kpc are necessary for convergence of LAF power at k < 0.1 s km–1, while higher-k modes require higher resolution, and that the timing of reionization does not significantly impact convergence given realistic photoheating.

Probing helium reionization with kinetic Sunyaev-Zel’dovich tomography

Reionization of helium is expected to occur at redshifts $z\sim3$ and have important consequences for quasar populations, galaxy formation, and the morphology of the intergalactic medium, but there is little known empirically about the process. Here we show that kinetic Sunyaev-Zeldovich (kSZ) tomography, based on the combination of CMB measurements and galaxy surveys, can be used to infer the primordial helium abundance as well as the time and duration of helium reionization. We find a high-significance detection at ${\sim10\sigma}$ can be expected from Vera Rubin Observatory and CMB-S4 in the near future. A more robust characterization of helium reionization will require next-generation experiments like MegaMapper (a proposed successor to DESI) and CMB-HD.

LYRA. III. The Smallest Reionization Survivors

The dividing line between galaxies that are quenched by reionization (“relics”) and galaxies that survive reionization (i.e., continue forming stars) is commonly discussed in terms of a halo mass threshold. We probe this threshold in a physically more complete and accurate way than has been possible to date, using five extremely high resolution (M target = 4 M ⊙) cosmological zoom-in simulations of dwarf galaxies within the halo mass range (1–4) × 109 M ⊙. The employed LYRA simulation model features resolved interstellar medium physics and individual, resolved supernova explosions. Interestingly, two out of five of the simulated dwarf galaxies lie close to the threshold mass but are neither full reionization relics nor full reionization survivors. These galaxies initially quench at the time of reionization but merely remain quiescent for ∼500 Myr. At z ∼ 5 they recommence star formation in a synchronous way and remain star-forming until the present day. The parallel timing indicates consistent sound-crossing and cooling times between the halos. While the star formation histories we find are diverse, we show that they are directly related to the ability of a given halo to retain and cool gas. Whereas the latter is most strongly dependent on the mass (or virial temperature) of the host halo at the time of reionization, it also depends on its growth history, the UV background (and its decrease at late times), and the amount of metals retained within the halo.

Signal of cosmic strings in cross-correlation of 21-cm redshift and CMB polarization maps

We study the signal of cosmic string wakes present before the time of reionization in the cross-correlation signal of 21-cm redshift and B-mode CMB polarization maps. The specific non-Gaussian signal of strings in the position space cross-correlation maps can be extracted by means of a matched filtering analysis. Signals of strings with tension somewhat lower than those corresponding to the current upper bound can be identified when embedded in a background of Gaussian fluctuations from a Planck best-fit LCDM model.

Optical depth to reionization from perturbative 21 cm clustering

The optical depth τ is the least well determined parameter in the standard model of cosmology, and one whose precise value is important for both understanding reionization and for inferring fundamental physics from cosmological measurements. We forecast how well future epoch of reionization experiments could constraint τ using a symmetries-based bias expansion that highlights the special role played by anisotropies in the power spectrum on large scales. Given a parametric model for the ionization evolution inspired by the physical behavior of more detailed reionization simulations, we find that future 21 cm experiments could place tight constraints on the timing and duration of reionization and hence constraints on τ that are competitive with proposed, space-based CMB missions provided they can measure k ≈ 0.1 h Mpc-1 with a clean foreground wedge across redshifts spanning the most active periods of reionization, corresponding to ionization fractions 0.2 ≲ x ≲ 0.8. Significantly improving upon existing CMB-based measurements with next-generation 21 cm surveys would require substantially longer observations (∼ 5 years) than standard \U0001d4aa(1000 hour) integration times. Precise measurements of smaller scales will not improve constraints on τ until a better understanding of the astrophysics of reionization is achieved. In the presence of noise and foregrounds even future 21 cm experiments will struggle to constrain τ if the ionization evolution deviates significantly from simple parametric forms.

Cosmic Reionization on Computers: Physical Origin of Long Dark Gaps in Quasar Absorption Spectra

I explore the properties of “dark gaps”—regions in quasar absorption spectra without significant transmission—with several simulations from the Cosmic Reionization On Computers (CROC) project. The CROC simulations in the largest available boxes (120 cMpc) come close to matching both the distribution of mean opacities and the frequency of dark gaps, but alas not in the same model: the run that matches the mean opacities fails to contain enough dark gaps and vice versa. Nevertheless, the run that matches the dark gap distributions serves as a counterexample to claims in the literature that the dark gap statistics requires a late end to reionization—in that run reionization ends at z = 6.7 (likely too early). While multiple factors contribute to the frequency of large dark gaps in the simulations, the primary factor that controls the overall shape of the dark gap distribution is the ionization level in voids—the lowest-density regions produce the highest transmission spikes that terminate long gaps. As the result, the dark gap distribution correlates strongly with the fraction of the spectrum above the gap detection threshold and the observed distribution is matched by the simulation in which this fraction is 2%. Hence, the gap distribution by itself does not constrain the timing of reionization, although it may do so in combination with the distribution of mean opacities.

Deep forest: neural network reconstruction of intergalactic medium temperature

ABSTRACT We explore the use of deep learning to infer the temperature of the intergalactic medium from the transmitted flux in the high-redshift Ly α forest. We train neural networks on sets of simulated spectra from redshift z = 2–3 outputs of cosmological hydrodynamic simulations, including high-temperature regions added in post-processing to approximate bubbles heated by He ii reionization. We evaluate how well the trained networks are able to reconstruct the temperature from the effect of Doppler broadening in the simulated input Ly α forest absorption spectra. We find that for spectra with high resolution (10 $\, {\rm km}\, {\rm s}^{-1}$ pixel) and moderate signal-to-noise ratio (20–50), the neural network is able to reconstruct the intergalactic medium temperature smoothed on scales of $\sim 6 \, h^{-1}\, {\rm Mpc}$ quite well. Concentrating on discontinuities, we find that high-temperature regions of width $25 \, h^{-1}\, {\rm Mpc}$ and temperature $20\, 000$ K can be fairly easily detected and characterized. We show an example where multiple sightlines are combined to yield tomographic images of hot bubbles. Deep learning techniques may be useful in this way to help us understand the complex temperature structure of the intergalactic medium around the time of helium reionization.

Cosmic Reionization on Computers: Evolution of the Flux Power Spectrum

Abstract We explore the evolution of the flux power spectrum in the Cosmic Reionization On Computers simulations. We find that, contrary to some previous studies, the shape of the flux power spectrum is rather insensitive to the timing of reionization. However, the amplitude of the flux power spectrum does strongly evolve with time, and that evolution is almost perfectly correlated with the timing of reionization. We show how such correlation can be used in a (futuristic) measurement to determine the redshift of overlap of ionized bubbles.

Prospects for kSZ2–Galaxy Cross-correlations during Reionization

Abstract We explore a new approach for extracting reionization-era contributions to the kinetic Sunyaev–Zel’dovich (kSZ) effect. Our method utilizes the cross-power spectrum between filtered and squared maps of the cosmic microwave background (CMB) and photometric galaxy surveys during the Epoch of Reionization (EoR). This kSZ2–galaxy cross-power spectrum statistic has been successfully detected at lower redshifts (z ≲ 1.5). Here we extend this method to z ≳ 6 as a potential means to extract signatures of patchy reionization. We model the expected signal across multiple photometric redshift bins using seminumeric simulations of the reionization process. In principle, the cross-correlation statistic robustly extracts reionization-era contributions to the kSZ signal, while its redshift evolution yields valuable information regarding the timing of reionization. Specifically, the model cross-correlation signal near ℓ ∼ 1000 peaks during the early stages of the EoR, when about 20% of the volume of the universe is ionized. Detectable ℓ modes mainly reflect squeezed-triangle configurations of the related bispectrum, quantifying correlations between the galaxy overdensity field on large scales and the smaller-scale kSZ power. We forecast the prospects for detecting this signal using future wide-field samples of Lyman-break galaxies from the Roman Space Telescope and next-generation CMB surveys including the Simons Observatory, CMB-S4, and CMB-HD. We find that a roughly 13σ detection is possible for CMB-HD and Roman after summing over all ℓ modes. We discuss the possibilities for improving this approach and related statistics, with the aim of moving beyond simple detections to measure the scale and redshift dependence of the cross-correlation signals.

Thethesanproject: properties of the intergalactic medium and its connection to reionization-era galaxies

ABSTRACTThe high-redshift intergalactic medium (IGM) and the primeval galaxy population are rapidly becoming the new frontier of extragalactic astronomy. We investigate the IGM properties and their connection to galaxies at z ≥ 5.5 under different assumptions for the ionizing photon escape and the nature of dark matter, employing our novel thesan radiation-hydrodynamical simulation suite, designed to provide a comprehensive picture of the emergence of galaxies in a full reionization context. Our simulations have realistic ‘late’ reionization histories, match available constraints on global IGM properties, and reproduce the recently observed rapid evolution of the mean free path of ionizing photons. We additionally examine high-z Lyman-α transmission. The optical depth evolution is consistent with data, and its distribution suggests an even-later reionization than simulated, although with a strong sensitivity to the source model. We show that the effects of these two unknowns can be disentangled by characterizing the spectral shape and separation of Lyman-α transmission regions, opening up the possibility to observationally constrain both. For the first time in simulations, thesan reproduces the modulation of the Lyman-α flux as a function of galaxy distance, demonstrating the power of coupling a realistic galaxy formation model with proper radiation hydrodynamics. We find this feature to be extremely sensitive on the timing of reionization, while being relatively insensitive to the source model. Overall, thesan produces a realistic IGM and galaxy population, providing a robust framework for future analysis of the high-z Universe.

The correlation of high-redshift galaxies with the thermal Sunyaev–Zel’dovich effect traces reionization

ABSTRACT We explore a potential new probe of reionization: the cross-correlation of high-redshift galaxies with maps of the thermal Sunyaev–Zel’dovich (tSZ) effect. We consider two types of high-redshift galaxies: Lyman break galaxies (LBGs) and Lyman-α emitters (LAEs). LBGs and LAEs will be detected in large numbers at high redshift (z ≈ 4–7) by ongoing and future surveys. We consider a future LBG sample from The Rubin Observatory Legacy Survey of Space and Time, and a selection of LAEs modelled after the Subaru SILVERRUSH programme, but covering a much larger sky fraction. The tSZ effect is sensitive to a line-of-sight integral of the ionized gas pressure, and is measured across large patches of sky using multifrequency cosmic microwave background (CMB) surveys. We consider forecast tSZ maps from CMB Stage 4 and more futuristic observations. Using a suite of hydrodynamical simulations, we show that LBGs and LAEs are correlated with the tSZ signal from reionization. The cross-spectra between LBGs/LAEs with tSZ maps contain information about the reionization history of the Universe, such as the distribution of bubble sizes, and could be used to directly measure the timing of reionization. The amplitude of the signal is small, however, and its detectability is hindered by low-redshift contributions to tSZ maps and by instrumental noise. If the low-redshift contribution to the observed tSZ signal is suppressed by masking of massive haloes, a combination of overlapping futuristic CMB and galaxy surveys could probe this signal.

Gravitational theory of cosmology, galaxies and galaxy clusters

A modified gravitational theory explains early universe and late time cosmology, galaxy and galaxy cluster dynamics. The modified gravity (MOG) theory extends general relativity (GR) by three extra degrees of freedom: a scalar field G, enhancing the strength of the Newtonian gravitational constant G_N, a gravitational, spin 1 vector graviton field phi _mu , and the effective mass mu of the ultralight spin 1 graviton. For t < t_mathrm{rec}, where t_mathrm{rec} denotes the time of recombination and re-ionization, the density of the vector graviton rho _phi > rho _b, where rho _b is the density of baryons, while for t > t_mathrm{rec} we have rho _b > rho _phi . The matter density is parameterized by Omega _M=Omega _b+Omega _phi +Omega _r where Omega _r=Omega _gamma +Omega _nu . For the cosmological parameter values obtained by the Planck Collaboration, the CMB acoustical oscillation power spectrum, polarization and lensing data can be fitted as in the Lambda CDM model. When the baryon density rho _b dominates the late time universe, MOG explains galaxy rotation curves, the dynamics of galaxy clusters, galaxy lensing and the galaxy clusters matter power spectrum without dominant dark matter.

How to quench a dwarf galaxy: The impact of inhomogeneous reionization on dwarf galaxies and cosmic filaments

ABSTRACT We use the SPHINX suite of high-resolution cosmological radiation hydrodynamics simulations to study how spatially and temporally inhomogeneous reionization impacts the baryonic content of dwarf galaxies and cosmic filaments. We compare simulations with and without stellar radiation to isolate the effects of radiation feedback from that of supernova, cosmic expansion, and numerical resolution. We find that the gas content of cosmic filaments can be reduced by more than 80 per cent following reionization. The gas inflow rates into haloes with ${M_{\rm vir}\lesssim 10^8\, \mathrm{M}_{\odot }}$ are strongly affected and are reduced by more than an order of magnitude compared to the simulation without reionization. A significant increase in gas outflow rates is found for halo masses ${M_{\rm vir}\lesssim 7\times 10^7\, \mathrm{M}_{\odot }}$. Our simulations show that inflow suppression (i.e. starvation), rather than photoevaporation, is the dominant mechanism by which the baryonic content of high-redshift dwarf galaxies is regulated. At fixed redshift and halo mass, there is a large scatter in the halo baryon fractions that is entirely dictated by the timing of reionization in the local region surrounding a halo which can change by Δz ≳ 3 at fixed mass. Finally, although the gas content of high-redshift dwarf galaxies is significantly impacted by reionization, we find that most haloes with ${M_{\rm vir}\lesssim 10^8\, \mathrm{M}_{\odot }}$ can remain self-shielded and form stars long after reionization, until their local gas reservoir is depleted, suggesting that Local Group dwarf galaxies do not necessarily exhibit star formation histories that peak prior to z = 6. Significantly larger simulation boxes will be required to capture the full process of reionization and understand how our results translate to environments not probed by our current work.

EDGE: The Origin of Scatter in Ultra-faint Dwarf Stellar Masses and Surface Brightnesses

Abstract We demonstrate how the least luminous galaxies in the universe, ultra-faint dwarf galaxies, are sensitive to their dynamical mass at the time of cosmic reionization. We select a low-mass ( ) dark matter halo from a cosmological volume, and perform zoom hydrodynamical simulations with multiple alternative histories using “genetically modified” initial conditions. Earlier-forming ultra-faints have higher stellar mass today, due to a longer period of star formation before their quenching by reionization. Our histories all converge to the same final dynamical mass, demonstrating the existence of extended scatter (≥1 dex) in stellar masses at fixed halo mass due to the diversity of possible histories. One of our variants builds less than 2% of its final dynamical mass before reionization, rapidly quenching in situ star formation. The bulk of its final stellar mass is later grown by dry mergers, depositing stars in the galaxy’s outskirts and hence expanding its effective radius. This mechanism constitutes a new formation scenario for highly diffuse ( , ), metal-poor ( ), ultra-faint ( ) dwarf galaxies within the reach of next-generation low surface brightness surveys.

Dark-ages reionization and galaxy formation simulation – XVI. The thermal memory of reionization

Abstract Intergalactic medium temperature is a powerful probe of the epoch of reionization, as information is retained long after reionization itself. However, mean temperatures are highly degenerate with the timing of reionization, with the amount heat injected during the epoch, and with the subsequent cooling rates. We post-process a suite of semi-analytic galaxy formation models to characterize how different thermal statistics of the intergalactic medium can be used to constrain reionization. Temperature is highly correlated with redshift of reionization for a period of time after the gas is heated. However as the gas cools, thermal memory of reionization is lost, and a power-law temperature–density relation is formed, T = T0(1 + δ)1 − γ with γ ≈ 1.5. Constraining our model against observations of electron optical depth and temperature at mean density, we find that reionization likely finished at $z_{\rm {reion}} = 6.8 ^{+ 0.5} _{-0.8}$ with a soft spectral slope of $\alpha = 2.8 ^{+ 1.2} _{-1.0}$. By restricting spectral slope to the range [0.5, 2.5] motivated by population II synthesis models, reionization timing is further constrained to $z_{\rm {reion}} = 6.9 ^{+ 0.4} _{-0.5}$. We find that, in the future, the degeneracies between reionization timing and background spectrum can be broken using the scatter in temperatures and integrated thermal history.

Helium Reionization Simulations. III. The Helium Lyα Forest

Abstract In a previous paper we presented a new suite of hydrodynamic simulations with the aim of accurately capturing the process of helium ii reionization. In this paper, we discuss the observational signatures present in the He ii Lyα forest. We show that the effective optical depth of the volume-averaged τ eff alone is not sufficient for capturing the ionization state of helium ii, due to the large variance inherent in sightlines. However, the He ii flux probability density function (PDF) can be used to determine the timing of helium ii reionization. The amplitude of the one-dimensional flux power spectrum can also determine the ionization state of helium ii. We show that, even given the currently limited number of observations (∼50 sightlines), measurements of the flux PDF can yield information about helium ii reionization. Further, measurements using the one-dimensional power spectrum can provide clear indications of the timing of reionization, as well as the relative bias of sources of ionizing radiation.

The SPHINX Cosmological Simulations of the First Billion Years: the Impact of Binary Stars on Reionization★

We present the SPHINX suite of cosmological adaptive mesh refinement simulations, the first radiation-hydrodynamical simulations to simultaneously capture large-scale reionization and the escape of ionizing radiation from thousands of resolved galaxies. Our $5$ and $10$ co-moving Mpc volumes resolve haloes down to the atomic cooling limit and model the inter-stellar medium with better than $\approx10$ pc resolution. The project has numerous goals in improving our understanding of reionization and making predictions for future observations. In this first paper we study how the inclusion of binary stars in computing stellar luminosities impacts reionization, compared to a model that includes only single stars. Owing to the suppression of galaxy growth via strong feedback, our galaxies are in good agreement with observational estimates of the galaxy luminosity function. We find that binaries have a significant impact on the timing of reionization: with binaries, our boxes are $99.9$ percent ionized by volume at $z\approx 7$, while without them our volumes fail to reionize by $z=6$. These results are robust to changes in volume size, resolution, and feedback efficiency. The escape of ionizing radiation from individual galaxies varies strongly and frequently. On average, binaries lead to escape fractions of $\approx 7-10$ percent, about $3.5$ times higher than with single stars only. The higher escape fraction is a result of a shallower decline in ionizing luminosity with age, and is the primary reason for earlier reionization, although the higher integrated luminosity with binaries also plays a sub-dominant role.

Spatial fluctuations of the intergalactic temperature–density relation after hydrogen reionization

The thermal state of the post-reionization IGM is sensitive to the timing of reionization and the nature of the ionizing sources. We have modelled here the thermal state of the IGM in cosmological radiative transfer simulations of a realistic, extended, spatially inhomogeneous hydrogen reionization process, carefully calibrated with Ly-alpha forest data. We compare these with cosmological simulations run using a spatially homogeneous ionizing background. The simulations with a realistic growth of ionized regions and a realistic spread in reionization redshifts show, as expected, significant spatial fluctuations in the temperature-density relation (TDR) of the post-reionization IGM. The most recently ionized regions are hottest and exhibit a flatter TDR. In simulations consistent with the average TDR inferred from Ly-alpha forest data, these spatial fluctuations have a moderate but noticeable effect on the statistical properties of the Ly-alpha opacity of the IGM at z ~ 4-6. This should be taken into account in accurate measurements of the thermal properties of the IGM and the free-streaming of dark matter from Ly-alpha forest data in this redshift range. The spatial variations of the TDR predicted by our simulations are, however, smaller by about a factor two than would be necessary to explain the observed large spatial opacity fluctuations on large (> 50 comoving Mpc/h) scales at z > 5.5.