Cosmological Ionization Fronts Research Articles

Lyman-α polarization from cosmological ionization fronts. Part II. Implications for intensity mapping

This is the second paper in a series whose aim is to predict the power spectrum of intensity and polarized intensity from cosmic reionization fronts. After building the analytic models for intensity and polarized intensity calculations in paper I, here we apply these models to simulations of reionization. We construct a geometric model for identifying front boundaries, calculate the intensity and polarized intensity for each front, and compute a power spectrum of these results. This method was applied to different simulation sizes and resolutions, so we ensure that our results are convergent.We find that the power spectrum of fluctuations at z = 8 in a bin of width Δz = 0.5 (λ/Δλ = 18) is Δℓ ≡ [ℓ(ℓ + 1)C ℓ/2π]1/2 is 3.2 × 10-11 erg s-1 cm-2 sr-1 for the intensity I, 7.6 × 10-13 erg s-1 cm-2 sr-1 for the E-mode polarization, and 5.8 × 10-13 erg s-1 cm-2 sr-1 for the B-mode polarization at ℓ = 1.5 × 104.After computing the power spectrum, we compare results to detectable scales and discuss implications for observing this signal based on a proposed experiment. We find that, while fundamental physics does not exclude this kind of mapping from being attainable, an experiment would need to be highly ambitious and require significant advances to make mapping Lyman-α polarization from cosmic reionization fronts a feasible goal.

Lyman-α polarization from cosmological ionization fronts. Part I. Radiative transfer simulations

In this paper, we present the formalism of simulating Lyman-α emission and polarization around reionization (z = 8) from a plane-parallel ionization front. We accomplish this by using a Monte Carlo method to simulate the production of a Lyman-α photon, its propagation through an ionization front, and the eventual escape of this photon. This paper focuses on the relation of the input parameters of ionization front speed U, blackbody temperature T bb, and neutral hydrogen density n HI, on intensity I and polarized intensity P as seen by a distant observer. The resulting values of intensity range from 3.18 × 10-14 erg/cm2/s/sr to 1.96 × 10-9 erg/cm2/s/sr , and the polarized intensity ranges from 5.73 × 10-17 erg/cm2/s/sr to 5.31 × 10-12 erg/cm2/s/sr. We found that higher T bb, higher U, and higher n HI contribute to higher intensity, as well as polarized intensity, though the strongest dependence was on the hydrogen density. The dependence of viewing angle of the front is also explored. We present tests to support the validity model, which makes the model suitable for further use in a following paper where we will calculate the intensity and polarized intensity power spectrum on a full reionization simulation.

A Multistep Algorithm for the Radiation Hydrodynamical Transport of Cosmological Ionization Fronts and Ionized Flows

Radiation hydrodynamical transport of ionization fronts (I-fronts) in the next generation of cosmological reionization simulations holds the promise of predicting UV escape fractions from first principles as well as investigating the role of photoionization in feedback processes and structure formation. We present a multistep integration scheme for radiative transfer and hydrodynamics for accurate propagation of I-fronts and ionized flows from a point source in cosmological simulations. The algorithm is a photon-conserving method that correctly tracks the position of I-fronts at much lower resolutions than nonconservative techniques. The method applies direct hierarchical updates to the ionic species, bypassing the need for the costly matrix solutions required by implicit methods while retaining sufficient accuracy to capture the true evolution of the fronts. We review the physics of ionization fronts in power-law density gradients, whose analytical solutions provide excellent validation tests for radiation coupling schemes. The advantages and potential drawbacks of direct and implicit schemes are also considered, with particular focus on problem time-stepping, which if not properly implemented can lead to morphologically plausible I-front behavior that nonetheless departs from theory. We also examine the effect of radiation pressure from very luminous central sources on the evolution of I-fronts and flows.

The Impact of Small‐Scale Structure on Cosmological Ionization Fronts and Reionization

The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale gas inhomogeneities due to structure formation. These inhomogeneities include both collapsed minihalos, which are generally self-shielding, and lower density structures, which are not. The minihalos are dense and sufficiently optically thick to trap intergalactic ionization fronts, blocking their path and robbing them of ionizing photons until the minihalo gas is expelled as an evaporative wind. The lower density structures do not trap these fronts, but they can slow them down by increasing the overall recombination rate in the intergalactic medium (IGM). In this paper we study the effects of both types of inhomogeneities, including nonlinear clustering effects, and we find that both IGM clumping and collapsed minihalos have significant yet qualitatively different impacts on reionization. While the number density of minihalos on average increases strongly with time, the density of minihalos inside H II regions around ionizing sources is largely constant. Thus the impact of minihalos is essentially to decrease the number of ionizing photons available to the IGM at all epochs, which is equivalent to a reduction in the luminosity of each source. On the other hand, the effect of IGM clumping increases strongly with time, slowing down reionization and extending it. Thus while the impact of minihalos is largely degenerate with the unknown source efficiency, IGM clumping can help significantly in reconciling the recent observations of cosmic microwave background polarization with quasar absorption spectra at z ~ 6, which together point to an early but extended reionization epoch.

Effects of small-scale structure on the progress and duration of reionization

The propagation of cosmological ionization fronts during the reionization of the universe is strongly influenced by small-scale (∼ kpc) gas inhomogeneities caused by struc- ture formation. In this paper we study this important effect by performing detailed radiative- hydrodynamic simulations of photoevaporation of cosmological minihalos (MHs) and incorpo- rating the results into a semi-analytical model of reionization, which also includes the effect of mean intergalactic medium (IGM) clumping and the nonlinear clustering of minihalos. We find that small-scale structures have a significant effect on the process of reionization, slowing it down and extending it in time. This can help in understanding the recent observations by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, which point to an early and extended reionization epoch.

Generation of the Primordial Magnetic Fields during Cosmological Reionization

We investigate the generation of magnetic fields by the Biermann battery in cosmological ionization fronts, using new simulations of the reionization of the universe by stars in protogalaxies. Two mechanisms are primarily responsible for magnetogenesis: (1) the breakout of ionization fronts from protogalaxies and (2) the propagation of ionization fronts through the high-density neutral filaments that are part of the cosmic web. The first mechanism is dominant prior to overlapping of ionized regions (z ≈ 7), whereas the second continues to operate even after that epoch. However, after overlap the field strength increase is largely due to the gas compression occurring as cosmic structures form. As a consequence, the magnetic field at z ≈ 5 closely traces the gas density, and it is highly ordered on megaparsec scales. The mean mass-weighted field strength is B0 ≈ 10-19 G in the simulation box. There is a relatively well-defined, nearly linear correlation between B0 and the baryonic mass of virialized objects, with B0 ≈ 10-18 G in the most massive objects (M ≈ 109 M☉) in our simulations. This is a lower limit, as lack of numerical resolution prevents us from following small-scale dynamical processes that could amplify the field in protogalaxies. Although the field strengths we compute are probably adequate as seed fields for a galactic dynamo, the field is too small to have had significant effects on galaxy formation, on thermal conduction, or on cosmic-ray transport in the intergalactic medium. It could, however, be observed in the intergalactic medium through innovative methods based on analysis of γ-ray burst photon arrival times.