arXiv subjects

Paul R. Shapiro

Publications and source records attributed to Paul R. Shapiro.

At least 19 recordsLinked to original sources

arXiv2026

Accurate neural network emulator for primordial light element abundances

Big-Bang Nucleosynthesis (BBN) predictions of primordial light-element abundances offer a powerful probe of early-Universe physics. However, high-accuracy numerical BBN calculations have become a major computational bottleneck for large-scale cosmological inferences due to the complex nuclear network. Here we present BBNet, a fast and accurate deep learning emulator for primordial abundances. The training data are generated by full numerical calculations using two public BBN codes, PArthENoPE and AlterBBN, modified to accommodate extended cosmologies that include dark radiation and a stiff equation of state. The network employs a residual multi-head architecture to capture convoluted physical relationships. BBNet produces primordial helium-4 and deuterium abundances with negligible errors in milliseconds per sample, achieving a speed-up of up to $10^4$ times relative to first-principles solvers while remaining unbiased over wide parameter ranges. Therefore, our emulator can supersede traditional simplified numerical prescriptions that compromise accuracy for speed. Based on extensive assessments of its performance, we conclude that BBNet is an optimal solution to the theoretical prediction of primordial element abundances. It will serve as a reliable tool for precision cosmology and new-physics searches.

astro-ph.CO↗

arXiv2026

SageNet: Fast Neural Network Emulation of the Stiff-amplified Gravitational Waves from Inflation

Accurate modeling of the inflationary gravitational waves (GWs) requires time-consuming, iterative numerical integrations of differential equations to take into account their backreaction on the expansion history. To improve computational efficiency while preserving accuracy, we present SageNet (Stiff-Amplified Gravitational-wave Emulator Network), a deep learning framework designed to replace conventional numerical solvers. SageNet employs a Long Short-Term Memory architecture to emulate the present-day energy density spectrum of the inflationary GWs with possible stiff amplification, $Ω_\mathrm{GW}(f)$. Trained on a data set of 25,689 numerically generated solutions, SageNet allows accurate reconstructions of $Ω_\mathrm{GW}(f)$ and generalizes well to a wide range of cosmological parameters; 89.3% of the test emulations with randomly distributed parameters exhibit errors of under 4%. In addition, SageNet demonstrates its ability to learn and reproduce the artificial, adaptive sampling patterns in numerical calculations, which implement denser sampling of frequencies around changes of spectral indices in $Ω_\mathrm{GW}(f)$. The dual capability of learning both physical and artificial features of the numerical GW spectra establishes SageNet as a robust alternative to exact numerical methods. Finally, our benchmark tests show that SageNet reduces the computation time from tens of seconds to milliseconds, achieving a speed-up of ~$10^4$ times over standard CPU-based numerical solvers with the potential for further acceleration on GPU hardware. These capabilities make SageNet a powerful tool for accelerating Bayesian inference procedures for extended cosmological models. In a broad sense, the SageNet framework offers a fast, accurate, and generalizable solution to modeling cosmological observables whose theoretical predictions demand costly differential equation solvers.

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arXiv2026

Double-Peaked Ly$α$ Emission during Reionization Requires Nearby Voids and a Favorable Local Ionizing Background

Several Lyman-alpha (Ly$α$) emitters deep into the reionization era exhibit double-peaked Ly$α$ emission profiles, raising the question of how the intergalactic medium can transmit photons blueward of the Ly$α$ resonance at such high redshifts. To investigate this, we compute Ly$α$ transmission along sightlines originating from galaxies in the Cosmic Dawn III simulation and identify cases that closely reproduce the observed double-peaked emission. In these cases, the sightlines intersect highly underdense voids located a few comoving megaparsecs from the source galaxy. These voids allow photons emitted blueward of Ly$α$ to redshift through resonance without scattering while traversing them. The low opacity arises because the neutral hydrogen density scales with the square of the underlying gas density under ionization equilibrium, making sufficiently underdense regions with $\lesssim30~\%$ of cosmic mean density highly transmissive. Such voids naturally occur in the fluctuating cosmic density field, even in the vicinity of galaxies, and can also be associated with transmissive spikes in the Ly$α$ forest. We find that the global probability of observing double-peaked emission is $\sim0.5~\%$ during reionization at an 80\% global ionization fraction, while no cases are found at 60\% ionization. We also find that this probability depends sensitively on the local ionizing background intensity, increasing by a factor of $\sim10^3$ for a tenfold increase in intensity. Recent accretion episodes of supermassive black holes can further increase this probability. These results suggest that double-peaked Ly$α$ emission in high-$z$ galaxies can serve as a sensitive probe of the ionizing background during the late stages of cosmic reionization.

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arXiv2025

Dust-UV offsets in high-redshift galaxies in the Cosmic Dawn III simulation

Recent observations have revealed puzzling spatial disparities between ALMA dust continuum and UV emission as seen by HST and JWST in galaxies at $z=5-7$ (e.g. ALPINE and REBELS surveys), compelling us to propose a physical interpretation of such offsets. We investigate these offsets using the Cosmic Dawn III (CoDa III) simulation, a state-of-the-art fully coupled radiation-hydrodynamics cosmological simulation, which incorporates a dynamical dust model. First of all, we find that our simulated dust masses, while calibrated to match observed ones, yield unrealistically large UV attenuations. In fact, the bright-end galaxy UV Luminosity function is best reproduced using only 7.5\% of the dust content of CoDa III galaxies. With this recalibration, we obtain populations of massive galaxies matching ALPINE and REBELS magnitudes and UV slopes, but with smaller dust masses than observed. In this framework, we also find significant dust-UV offsets in massive, UV-bright galaxies ($\mathrm{M}_\mathrm{DM}> 10^{11.5}$ M$_\odot$, M$_*>10^{10}$ M$_\odot$, M$_{\rm AB1600}<-21.5$), reaching up to $\sim 2$ pkpc for the most massive systems. Our analysis reveals that these offsets primarily result from severe dust extinction in galactic centers rather than a misalignment between dust and stellar mass distributions. At the spatial resolution of CoDa III (1.65 pkpc at z=6), the dust remains in majority well-aligned with the bulk stellar component, and we predict the dust continuum should therefore align well with the stellar rest-frame NIR component, less affected by dust attenuation. This study highlights the importance of dust in shaping the appearance of early galaxies at UV wavelengths, even as early as in the Epoch of Reionization.

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arXiv2025

Hydrodynamics of ultralight complex scalar field dark matter and its impact on the growth of structure

The mass window of ultralight axion dark matter motivated by suppressing the growth of structure on subgalactic scales, $m\sim 10^{-22}\,\mathrm{eV}$, is now severely constrained by various observation data (e.g. Lyman-$α$ forest). As an attempt to reopen this mass window, we investigate an alternative ultralight dark matter candidate, the complex scalar field dark matter (SFDM). We derive the relativistic hydrodynamics of the complex SFDM in the framework of cosmological perturbation theory. Our formalism contains two novel ingredients uniquely associated with the complex SFDM model: the Eckart frame defined by the conserved Noether current, and the stiff gauge condition, $c_s^2\equiv (δP/δρ)|_s=1$. In the Eckart frame, the complex SFDM is effectively an imperfect fluid with a dissipative energy flux, distinguishing itself from axion dark matter. The energy flux can affect the growth of density fluctuations dynamically. Meanwhile, we apply the stiff gauge condition to find new constitutive equations for the complex SFDM. We revisit the homogeneous evolution of the complex SFDM and present illustrative early-stage solutions for perturbations of the complex SFDM in a simplified setting. We demonstrate the effects of varying the model parameters on the evolution of the perturbation variables.

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arXiv2025

Multimodality in the Search for New Physics in Pulsar Timing Data and the Case of Kination-amplified Gravitational-wave Background from Inflation

We investigate the kination-amplified inflationary gravitational-wave background (GWB) interpretation of the signal recently reported by various pulsar timing array (PTA) experiments. Kination is a post-inflationary phase in the expansion history dominated by the kinetic energy of some scalar field, characterized by a stiff equation of state $w=1$. Within the inflationary GWB model, we identify two modes which can fit the current data sets (NANOGrav and EPTA) with equal likelihood: the kination-amplification (KA) mode and the ordinary, non-kination-amplification (no-KA) mode. The multimodality of the likelihood motivates a Bayesian analysis with nested sampling. We analyze the free spectra of current PTA data and mock free spectra constructed with higher signal-to-noise ratios, using nested sampling. The analysis of the mock spectrum designed to be consistent with the best fit to the NANOGrav 15 yr (NG15) data successfully reveals the expected bimodal posterior for the first time while excluding the reheating mode that appears in the fit to the current NG15 data, making a case for our correct treatment of potential multimodal posteriors arising from future PTA data sets. The resultant Bayes factor is $B\equiv Z_\mathrm{no-KA}/Z_\mathrm{KA}=2.9\pm1.9$, indicating comparable statistical significance between the two modes. Given the theoretical model-building challenges of producing highly blue-tilted primordial tensor spectra, the KA mode has the advantage of requiring a less blue primordial spectrum, compared with the no-KA mode. The synergy between future cosmic microwave background polarization, pulsar timing and laser interferometer measurements of gravitational waves will help resolve the ambiguity implied by the multimodal posterior in PTA-only searches.

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arXiv2025

Line Intensity Mapping Prediction from the Cosmic Dawn (CoDa) III Simulation for H$α$ from Galaxies and the Intergalactic Medium during the Epoch of Reionization

The evolution of large-scale structure, galaxies and the intergalactic medium (IGM) during the Epoch of Reionization (EoR) can be probed by upcoming Line Intensity Mapping (LIM) experiments, which sample in redshift and direction without needing to resolve individual galaxies. We predict the intensity and sources of hydrogen H$α$ emission, dominated by radiative recombination following ionization by UV from the same massive stars that caused reionization, down to redshift 4.6, using the largest fully-coupled, radiation-hydro simulation of galaxy formation and reionization to date, Cosmic Dawn (CoDa) III. We compute the mean intensity and Voxel Intensity Distribution (VID) vs. redshift, including the relative contributions of galaxies and IGM. This will provide mock data to guide and interpret LIM experiments such as NASA's SPHEREx and proposed Cosmic Dawn Intensity Mapper (CDIM).

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arXiv2025

Constraining Reionization with Lyα Damping-Wing Absorption in Galaxy Spectra: A Machine Learning Model Based on Reionization Simulations

Recently, NIRSpec PRISM/CLEAR observations by JWST have begun providing rest-frame UV continuum measurements of galaxies at $z\gtrsim7$, revealing signatures of Ly$α$ damping-wing (DW) absorption by the intergalactic medium (IGM). We develop a methodology to constrain the global ionization fraction of the IGM $(Q_{\rm HII})$ using low-resolution spectra, employing the random forest classification (RFC) method. We construct mock spectra using the simulated galaxies and the IGM from the Cosmic Dawn II simulation and train RFC models to estimate $Q_{\rm HII}$ at the redshift of the source and to detect the presence of a damped Ly$α$ absorber (DLA). We find that individual galaxy spectra with spectral bins between 1220 and 1270 Å and with signal-to-noise ratios greater than 20 can place tight constraints on $Q_{\rm HII}$, provided the UV continuum is accurately modeled. This method is particularly effective for the early phase of reionization ($Q_{\rm HII}<50\%$), when the IGM opacity is high in the DW. As a demonstration, we apply our model to existing NIRSpec PRISM/CLEAR spectra, placing upper bounds of $Q_{\rm HII}=59.6\%$, $5.6\%$, and $18.5\%$ at $z=7.7,~9.4,$ and $10.6$, respectively, with $68\%$ confidence, though several modeling uncertainties remain to be discussed. These constraints favor late-starting reionization models, where $\gtrsim 80\%$ of the IGM is ionized after $z=8$. We conclude that high-SNR observations of carefully selected targets around $z\sim7-9$ can effectively constrain reionization models.

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arXiv2024

The Thermal Sunyaev-Zel'dovich Effect from the Epoch of Reionization

The thermal Sunyaev-Zel'dovich (tSZ) effect arises from inverse Compton scattering of low energy photons onto thermal electrons, proportional to the integrated electron pressure, and is usually observed from galaxy clusters. However, we can expect that the Epoch of Reionization (EoR) also contributes to this signal, but that contribution has not been previously evaluated. In this work we analyse a suite of fully-coupled radiation-hydrodynamics simulations based on RAMSES-CUDATON to calculate and study the tSZ signal from the Reionization Epoch. We construct lightcones of the electron pressure in the intergalactic medium for $6<z$ to calculate the resulting Compton y-parameters. We vary the box sizes, resolutions and star formation parameters to investigate how these factors affect the tSZ effect. We produce plots of maps and distributions of y, as well as angular temperature power spectra of the tSZ signal obtained from integrating the lightcones constructed for each simulation. We find that the tSZ signal from reionization is generally sub-dominant to the post-reionization one at larger scales ($\ell< 10^4$), but can contribute non-trivially and potentially contaminate the measured signals. At scales probed by current experiments like SPT ($\ell\sim10^3-10^4$), we find that the tSZ signal power spectrum from reionization contributes at roughly a percent level compared to the current templates, with the quadratic Doppler effect contributing an additional $\sim10\%$ to the tSZ signal. At smaller scales the tSZ from reionization peaks and can potentially dominate the total signal and is thus a potentially much more important contribution to take into account in any future, more sensitive experiments.

astro-ph.CO↗

arXiv2024

Dispersion Measures of Fast Radio Bursts through the Epoch of Reionization

Dispersion measures (DM) of fast radio bursts (FRBs) probe the density of electrons in the intergalactic medium (IGM) along their lines-of-sight, including the average density versus distance to the source and its variations in direction. While previous study focused on low-redshift, FRBs are potentially detectable out to high redshift, where their DMs can, in principle, probe the epoch of reionization (EOR) and its patchiness. We present the first predictions from large-scale, radiation-hydrodynamical simulation of fully-coupled galaxy formation and reionization, using Cosmic Dawn (``CoDa")~II to model the density and ionization fields of the universe down to redshifts through the end of the EOR at $z_{re}\approx6.1$. Combining this with an N-body simulation CoDa~II--Dark Matter of the fully-ionized epoch from the EOR to the present, we calculate the mean and standard deviation of FRB DMs as functions of their source redshift. The mean and standard deviation of DM increase with redshift, reaching a plateau by $z(x_{HII}\lesssim0.25)\gtrsim8$, i.e. well above $z_{re}$. The mean-DM asymptote $\mathcal{DM}_{max} \approx 5900~\mathrm{pc\, cm^{-3}}$ reflects the end of the EOR and its duration. The standard deviation there is $σ_{DM, max}\approx497 ~\mathrm{pc\, cm^{-3}}$, reflecting inhomogeneities of both patchy reionization and density. Inhomogeneities in ionization during the EOR contribute $\mathcal{O}(1$ per cent) of this value of $σ_{DM,max}$ from FRBs at redshifts $z\gtrsim 8$. Current estimates of FRB rates suggest this may be detectable within a few years of observation.

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arXiv2024

Impact of Self-shielding Minihalos on the Ly$α$ Forest at High Redshift

Dense gas in minihalos with masses of $10^6-10^8~M_\odot$ can shield themselves from reionization for $\sim100$ Myr after being exposed to the UV background. These self-shielded systems, often unresolved in cosmological simulations, can introduce strong absorption in quasar spectra. This paper is the first systematic study on the impact of these systems on the Ly$α$ forest. We first derive the HI column density profile of photoevaporating minihalos by conducting 1D radiation-hydrodynamics simulations. We utilize these results to estimate the Ly$α$ opacity from minihalos in a large-scale simulation that cannot resolve self-shielding. When the ionization rate of the background radiation is $0.03\times10^{-12}~{\rm s}^{-1}$, as expected near the end of reionization at $z\sim5.5$, we find that the incidence rate of damped Ly$α$ absorbers increases by a factor of $\sim2-4$ compared to at $z=4.5$. The Ly$α$ flux is, on average, suppressed by $\sim 3\%$ of its mean due to minihalos. The absorption features enhance the 1D power spectrum up to $\sim5\%$ at $k\sim0.1~h~{\rm Mpc}^{-1}~({\rm or}~10^{-3}~{\rm km}^{-1}~{\rm s})$, which is comparable to the enhancement caused by inhomogeneous reionization. The flux is particularly suppressed in the vicinity of large halos along the line-of-sight direction at separations of up to $10~h^{-1}~{\rm Mpc}$ at $r_\perp\lesssim2~h^{-1}~{\rm Mpc}$. However, these effects become much smaller for higher ionizing rates ($\gtrsim0.3\times10^{-12}~{\rm s}^{-1}$) expected in the post-reionization Universe. Our findings highlight the need to consider minihalo absorption when interpreting the Ly$α$ forest at $z\gtrsim5.5$. Moreover, the sensitivity of these quantities to the ionizing background intensity can be exploited to constrain the intensity itself.

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arXiv2024

Multiple Beads-on-a-string: Dark Matter-Deficient Galaxy Formation in a Mini-bullet Satellite-satellite Galaxy Collision

Dark matter-deficient galaxies (DMDGs) discovered in the survey of ultra-diffuse galaxies (UDGs), in apparent conflict with standard CDM, may be produced by high-velocity galaxy-galaxy collisions, the $\textit{Mini-bullet}$ scenario. Recent observations of an aligned trail of $7-11$ UDGs near NGC1052, including DMDGs DF2 and DF4, suggesting a common formation event, $\sim8.9\pm1.5$ Gyr ago, provide a test. Hydro/N-body simulations, supplemented by galaxy orbit integrations, demonstrate that satellite-satellite collisions outside the host-galaxy virial radius can reproduce the observed UDGs in the NGC1052 group. A trail of $\sim10$ DMDGs is shown to form, including two massive ones that replicate the observed motions of DF2 and DF4. The linear relation, $v=Ax+v_{0}$, conjectured previously to relate positions ($x$) and velocities ($v$) of the aligned DMDGs as a signature of the collision event, is approximately obeyed, but individual DMDGs can deviate significantly from it. The progenitors whose collision spawned the trail of DMDGs survive the collision without, themselves, becoming DMDGs. We predict one progenitor is located at the end of the trail, testable by observing the difference between its stars, formed pre-collision, from those of the DMDGs, formed post-collision. By contrast, stellar ages and metallicities of the DMDGs are nearly identical. We further offer a hint that the tidal field of host NGC1052 may contribute to making DMDGs diffuse. $Λ$CDM simulation in a 100 cMpc box finds our required initial conditions $\sim10$ times at $z<3$. These results indicate current observations are consistent with the $\textit{Mini-bullet}$ scenario.

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arXiv2023

Cosmic Variance and the Inhomogeneous UV Luminosity Function of Galaxies During Reionization

When the first galaxies formed and starlight escaped into the intergalactic medium to reionize it, galaxy formation and reionization were both highly inhomogeneous in time and space, and fully-coupled by mutual feedback. To show how this imprinted the UV luminosity function (UVLF) of reionization-era galaxies, we use our large-scale, radiation-hydrodynamics simulation CoDa II to derive the time- and space-varying halo mass function and UVLF, from $z\simeq6$-15. That UVLF correlates strongly with local reionization redshift: earlier-reionizing regions have UVLFs that are higher, more extended to brighter magnitudes, and flatter at the faint end than later-reionizing regions observed at the same $z$. In general, as a region reionizes, the faint-end slope of its local UVLF flattens, and, by $z=6$ (when reionization ended), the global UVLF, too, exhibits a flattened faint-end slope, `rolling-over' at $M_\text{UV}\gtrsim-17$. CoDa II's UVLF is broadly consistent with cluster-lensed galaxy observations of the Hubble Frontier Fields at $z=6$-8, including the faint end, except for the faintest data point at $z=6$, based on one galaxy at $M_\text{UV}=-12.5$. According to CoDa II, the probability of observing the latter is $\sim5\%$. However, the effective volume searched at this magnitude is very small, and is thus subject to significant cosmic variance. We find that previous methods adopted to calculate the uncertainty due to cosmic variance underestimated it on such small scales by a factor of 2-4, primarily by underestimating the variance in halo abundance when the sample volume is small.

astro-ph.CO↗

arXiv2023

The Effect of Boundary Conditions on Structure Formation in Fuzzy Dark Matter

We illustrate the effect of boundary conditions on the evolution of structure in Fuzzy Dark Matter. Scenarios explored include the evolution of single, ground-state equilibrium solutions of the Schrödinger-Poisson system, the relaxation of a Gaussian density fluctuation, mergers of two equilibrium configurations, and the random merger of many solitons. For comparison, each scenario is evolved twice, with isolation boundary conditions and periodic boundary conditions, the two commonly used to simulate isolated systems and structure formation, respectively. Replacing isolation boundary conditions by periodic boundary conditions changes the domain topology and dynamics of each scenario, by affecting the outcome of gravitational cooling. With periodic boundary conditions, the ground-state equilibrium solution and Gaussian fluctuation each evolve toward the single equilibrium solitonic core of the isolated case, but surrounded by a tail, unlike the isolated versions. The case of head-on, binary mergers illustrates additional effects, caused by the pull suffered by the system due to the infinite network of periodic images along each direction of the domain. Binary merger with angular momentum is the first scenario we found in which the tail has a polynomial profile when using a periodic domain. Finally, the 3D merger of many, randomly-placed solitonic cores of different mass makes a solitonic core surrounded by a tail with power-law-like density profile, for periodic boundary conditions, while producing a core with a much sharper fall-off in the isolated case. This suggests that the conclusion of earlier work that the ground-state equilibrium solution is an attractor for the asymptotic state is true even in 3D and for general circumstances, but only if gravitational cooling is able to carry mass and energy off to infinity, which isolation boundary conditions allow, but periodic ones do not.

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arXiv2022

The short ionizing photon mean free path at z=6 in Cosmic Dawn III, a new fully-coupled radiation-hydrodynamical simulation of the Epoch of Reionization

Recent determinations of the mean free path of ionising photons (mfp) in the intergalactic medium (IGM) at $\rm z=6$ are lower than many theoretical predictions. To gain insight into this issue, we investigate the evolution of the mfp in our new massive fully coupled radiation hydrodynamics cosmological simulation of reionization: Cosmic Dawn III (CoDaIII). CoDaIII's scale ($\rm 94^3 \, cMpc^3$) and resolution ($\rm 8192^3$ grid) make it particularly suitable to study the evolution of the IGM during Reionization. The simulation was performed with RAMSES-CUDATON on Summit, and used 131072 processors coupled to 24576 GPUs, making it the largest Reionization simulation, and largest RAMSES simulation ever performed. A superior agreement with global constraints on Reionization is obtained in CoDaIII over CoDaII especially for the evolution of the neutral hydrogen fraction and the cosmic photo-ionization rate, thanks to an improved calibration, later end of reionization ($\rm z=5.6$), and higher spatial resolution. Analyzing the mfp, we find that CoDaIII reproduces the most recent observations very well, from $\rm z=6$ to $\rm z=4.6$. We show that the distribution of the mfp in CoDaIII is bimodal, with short (neutral) and long (ionized) mfp modes, respectively, due to the patchiness of reionization and the co-existence of neutral versus ionized regions during Reionization. The neutral mode peaks at sub-kpc to kpc scales of mfp, while the ionized mode peak evolves from $\rm 0.1 Mpc/h$ at $\rm z=7$ to $\sim 10$ Mpc/h at $\rm z=5.2$. Computing the mfp as the average of the ionized mode provides the best match to the recent observational determinations. The distribution reduces to a single neutral (ionized) mode at $\rm z>13$ ($\rm z<5$).

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arXiv2022

Reionization time of the Local Group and Local-Group-like halo pairs

Patchy cosmic reionization resulted in the ionizing UV background asynchronous rise across the Universe. The latter might have left imprints visible in present day observations. Several numerical simulation-based studies show correlations between reionization time and overdensities and object masses today. To remove the mass from the study, as it may not be the sole important parameter, this paper focuses solely on the properties of paired halos within the same mass range as the Milky Way. For this purpose, it uses CoDaII, a fully-coupled radiation hydrodynamics reionization simulation of the local Universe. This simulation holds a halo pair representing the Local Group, in addition to other pairs, sharing similar mass, mass ratio, distance separation and isolation criteria but in other environments, alongside isolated halos within the same mass range. Investigations of the paired halo reionization histories reveal a wide diversity although always inside-out given our reionization model. Within this model, halos in a close pair tend to be reionized at the same time but being in a pair does not bring to an earlier time their mean reionization. The only significant trend is found between the total energy at z = 0 of the pairs and their mean reionization time: pairs with the smallest total energy (bound) are reionized up to 50 Myr earlier than others (unbound). Above all, this study reveals the variety of reionization histories undergone by halo pairs similar to the Local Group, that of the Local Group being far from an average one. In our model, its reionization time is ~625 Myr against 660+/-4 Myr (z~8.25 against 7.87+/-0.02) on average.

astro-ph.CO↗

arXiv2022

Scattering of Lyα Photons through the Reionizing Intergalactic Medium: I. Spectral Energy Distribution

During reionization, a fraction of galactic Ly$α$ emission is scattered in the intergalactic medium (IGM) and appears as a diffuse light extending megaparsecs from the source. We investigate how to probe the properties of the early galaxies and their surrounding IGM using this scattered light. We create a Monte Carlo algorithm to track individual photons and reproduce several test cases from previous literature. Then, we run our code on the simulated IGM of the CoDaII simulation. We find that the scattered light can leave an observable imprint on the emergent spectrum if collected over several square arcminutes. Scattering can redden the emission by increasing the path lengths of photons, but it can also make the photons bluer by upscattering them according to the peculiar motion of the scatterer. The photons emitted on the far blue side of the resonance appear more extended in both frequency and space compared to those emitted near the resonance. This provides a discriminating feature for the blueward emission, which cannot be constrained from the unscattered light coming directly from the source. The ionization state of the IGM also affects the scattered light spectrum. When the source is in a small HII region, the emission goes through more scatterings in the surrounding HI region regardless of the initial frequency and ends up more redshifted and spatially extended. This can result in a weakening of the scattered light toward high $z$ during reionization. Our results provide a framework for interpreting the scattered light to be measured by high-$z$ integral-field-unit surveys.

astro-ph.CO↗

arXiv2021

Crucial Factors for Lyα Transmission in the Reionizing Intergalactic Medium: Infall Motion, HII Bubble Size, and Self-shielded Systems

Using the CoDa II simulation, we study the Ly$α$ transmissivity of the intergalactic medium (IGM) during reionization. At $z>6$, a typical galaxy without an active galactic nucleus fails to form a proximity zone around itself due to the overdensity of the surrounding IGM. The gravitational infall motion in the IGM makes the resonance absorption extend to the red side of Ly$α$, suppressing the transmission up to roughly the circular velocity of the galaxy. In some sight lines, an optically thin blob generated by a supernova in a neighboring galaxy results in a peak feature, which can be mistaken for a blue peak. Redward of the resonance absorption, the damping-wing opacity correlates with the global IGM neutral fraction and the UV magnitude of the source galaxy. Brighter galaxies tend to suffer lower opacity because they tend to reside in larger HII regions, and the surrounding IGM transmits redder photons, which are less susceptible to attenuation, owing to stronger infall velocity. The HII regions are highly nonspherical, causing both sight-line-to-sight-line and galaxy-to-galaxy variation in opacity. Also, self-shielded systems within HII regions strongly attenuate the emission for certain sight lines. All these factors add to the transmissivity variation, requiring a large sample size to constrain the average transmission. The variation is largest for fainter galaxies at higher redshift. The 68\% range of the transmissivity is similar to or greater than the median for galaxies with $M_{\rm UV}\ge-21$ at $z\ge7$, implying that more than a hundred galaxies would be needed to measure the transmission to 10\% accuracy.

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