Kyungjin Ahn

Cosmological radiative transfer comparison project - II. The radiation-hydrodynamic tests

The development of radiation hydrodynamical methods that are able to follow gas dynamics and radiative transfer (RT) self-consistently is key to the solution of many problems in numerical astrophysics. Such fluid flows are highly complex, rarely allowing even for approximate analytical solutions against which numerical codes can be tested. An alternative validation procedure is to compare different methods against each other on common problems, in order to assess the robustness of the results and establish a range of validity for the methods. Previously, we presented such a comparison for a set of pure RT tests (i.e. for fixed, non-evolving density fields). This is the second paper of the Cosmological Radiative Transfer Comparison Project, in which we compare nine independent RT codes directly coupled to gas dynamics on three relatively simple astrophysical hydrodynamics problems: (i) the expansion of an H ii region in a uniform medium, (ii) an ionization front in a 1/r2 density profile with a flat core and (iii) the photoevaporation of a uniform dense clump. Results show a broad agreement between the different methods and no big failures, indicating that the participating codes have reached a certain level of maturity and reliability. However, many details still do differ, and virtually every code has showed some shortcomings and has disagreed, in one respect or another, with the majority of the results. This underscores the fact that no method is universal and all require careful testing of the particular features which are most relevant to the specific problem at hand.

THE INHOMOGENEOUS BACKGROUND OF H2-DISSOCIATING RADIATION DURING COSMIC REIONIZATION

The first, self-consistent calculations are presented of the cosmological, H2-dissociating UV background produced during the epoch of reionization by the sources of reionization. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the intergalactic medium (IGM) from all the sources in our simulation volume down to dwarf galaxies of mass ~108 M sun, identified by very high resolution N-body simulations, including the self-regulating effect of IGM photoheating on dwarf galaxy formation. The UV continuum emitted below 13.6 eV by each source is then transferred through the same IGM, attenuated by atomic H Lyman series resonance lines, to predict the evolution of the inhomogeneous radiation background in the Lyman-Werner (LW) bands of H2 between 11 and 13.6 eV. On average, the intensity of this LW background is found to rise to the threshold level at which dissociation suppresses H2 cooling and star formation inside minihalos, long before reionization is complete. Spatial variations in the LW background are found which result from the clustering of sources associated with large-scale structure formation, such that intensity fluctuations correlate with matter density fluctuations. As a result, the LW background rises to the threshold level for H2 suppression earlier in the vicinity of the reionization sources and their H II regions.

Simulating cosmic reionization: How large a volume is large enough?

We present the largest-volume (425 Mpc h(-1) = 607 Mpc on a side) full radiative transfer simulation of cosmic reionization to date. We show that there is significant additional power in density fluctuations at very large scales. We systematically investigate the effects this additional power has on the progress, duration and features of reionization and on selected reionization observables. We find that comoving volume of similar to 100 Mpc h(-1) per side is sufficient for deriving a convergent mean reionization history, but that the reionization patchiness is significantly underestimated. We use jackknife splitting to quantify the convergence of reionization properties with simulation volume. We find that sub-volumes of similar to 100 Mpc h(-1) per side or larger yield convergent reionization histories, except for the earliest times, but smaller volumes of similar to 50 Mpc h(-1) or less are not well converged at any redshift. Reionization history milestones show significant scatter between the sub-volumes, as high as Delta z similar to 1 for similar to 50 Mpc h(-1) volumes. If we only consider mean-density sub-regions the scatter decreases, but remains at Delta z similar to 0.1-0.2 for the different size sub-volumes. Consequently, many potential reionization observables like 21-cm rms, 21-cm PDF skewness and kurtosis all showgood convergence for volumes of similar to 200 Mpc h(-1), but retain considerable scatter for smaller volumes. In contrast, the three-dimensional 21-cm power spectra at large scales (k < 0.25 h Mpc(-1)) do not fully converge for any sub-volume size. These additional large-scale fluctuations significantly enhance the 21-cm fluctuations, which should improve the prospects of detection considerably, given the lower foregrounds and greater interferometer sensitivity at higher frequencies.

Detecting the rise and fall of the first stars by their impact on cosmic reionization

The intergalactic medium was reionized before redshift z ∼ 6, most likely by starlight which escaped from early galaxies. The very first stars formed when hydrogen molecules (H2) cooled gas inside the smallest galaxies, minihalos of mass between 105 and 108 solar masses, before redshift z ∼ 40. Minihalo stars then started reionization but could not finish it before the rising background of H2-dissociating soft-ultraviolet starlight choked them off. We confirm this hypothesis by the first large-scale radiative transfer simulations to include minihalo sources and their suppression. We show that reionization began much earlier with minihalo sources than without, and was greatly extended, which boosts the intergalactic electron-scattering optical depth and the large-angle polarization fluctuations of the cosmic microwave background significantly. Although within current WMAP uncertainties, this boost should be readily detectable by Planck. If reionization ended as late as z ov 7, as suggested by other observations, Planck will thereby see the signature of the first stars at high redshift, currently undetectable by any other probe. We also show that minimal reionization models satisfying both the late reionization condition, zov . 7, and the large optical depth condition, τes & 0.085, can be distinguished from our fiducial model under the same constraints by Planck at high confidence level.

Can 21-cm observations discriminate between high-mass and low-mass galaxies as reionization sources?

The prospect of detecting the first galaxies by observing their impact on the intergalactic medium as they reionized it during the first billion years leads us to ask whether such indirect observations are capable of diagnosing which types of galaxies were most responsible for reionization. We attempt to answer this by considering a set of large-scale radiative transfer simulations of reionization in sufficiently large volumes to make statistically meaningful predictions of observable signatures, while also directly resolving all atomically-cooling halos down to 10^8 M_solar. We focus here on predictions of the 21-cm background, to see if upcoming observations are capable of distinguishing a universe ionized primarily by high-mass halos from one in which both high-mass and low-mass halos are responsible, and to see how these results depend upon the uncertain source efficiencies. We find that 21-cm fluctuation power spectra observed by the first generation EoR/21-cm radio interferometer arrays should be able to distinguish the case of reionization by high-mass halos alone from that by both high- and low-mass halos, together. Some reionization scenarios yield very similar power spectra and rms evolution and thus can only be discriminated by their different mean reionization history and 21-cm PDF distributions. We find that the skewness of the 21-cm PDF distribution smoothed over LOFAR-like window shows a clear feature correlated with the rise of the rms due to patchiness. Measurements of the mean photoionization rates are sensitive to the average density of the regions being studied and therefore could be strongly skewed in certain cases. (abridged)

Redshift-space distortion of the 21-cm background from the epoch of reionization – I. Methodology re-examined

The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper presents a detailed treatment of the effects of peculiar velocity on the 21cm signal. (1) We show that properly accounting for finite optical depth eliminates the unphysical divergence of 21cm brightness temperature in the IGM overdense regions found in previous work that employed the usual optically-thin approximation. (2) We show that previous attempts to circumvent this divergence by capping the velocity gradient result in significant errors in the power spectrum on all scales. (3) We further show that the observed power spectrum in redshift-space remains finite even in the optically-thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (4) We also show that the linear theory for redshift-space distortion results in a ~30% error in the power spectrum at the observationally relevant wavenumber range, at the 50% ionized epoch. (5) We describe and test two numerical schemes to calculate the 21cm signal from reionization simulations which accurately incorporate peculiar velocity in the optically-thin approximation. One is particle-based, the other grid-based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can achieve sufficient accuracy. [Abridged]

Light-cone effect on the reionization 21-cm power spectrum

Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of low-frequency radio telescopes is to measure the 3D 21-cm power spectrum. However, the 21-cm signal could evolve substantially along the line-of-sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverses to the LOS originated from different look-back times and could therefore be statistically different. Using numerical simulations we investigate this so-called light-cone effect on the spherically averaged 3D 21-cm power spectrum. For this version of the power spectrum, we find that the effect mostly ‘averages out’ and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21-cm signal and its rms variations along the LOS direction. Nevertheless, changes up to ∼50 per cent at large scales are possible. In general, the power is enhanced/suppressed at large/small scales when the effect is included. The cross-over mode below/above which the power is enhanced/suppressed moves towards larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations

The 21 cm Background from the Cosmic Dark Ages: Minihalos and the Intergalactic Medium before Reionization

The H atoms inside minihalos (i.e., halos with virial temperatures Tvir ≤ 104 K, in the mass range roughly from 104 to 108 M☉) during the cosmic dark ages in a ΛCDM universe produce a redshifted background of collisionally pumped 21 cm line radiation that can be seen in emission relative to the cosmic microwave background (CMB). Previously, we used semianalytical calculations of the 21 cm signal from individual halos of different mass and redshift and the evolving mass function of minihalos to predict the mean brightness temperature of this 21 cm background and its angular fluctuations. Here we use high-resolution cosmological N-body and hydrodynamic simulations of structure formation at high redshift (z 8) to compute the mean brightness temperature of this background from both minihalos and the intergalactic medium (IGM) prior to the onset of Lyα radiative pumping. We find that the 21 cm signal from gas in collapsed, virialized minihalos dominates over that from the diffuse shocked gas in the IGM.

Implications of WMAP 3 Year Data for the Sources of Reionization

New results on the anisotropy of the cosmic microwave background (CMB) and its polarization based on the first 3 years of data from the Wilkinson Microwave Anisotropy Probe (WMAP) have revised the electron scattering optical depth downward from τes = 0.17 to τes = 0.09 ± 0.03. This implies a shift of the effective reionization redshift from zr 17 to zr 11. Previous attempts to explain the high redshift of reionization inferred from the WMAP 1 year data have led to widespread speculation that the sources of reionization must have been much more efficient than those associated with the star formation observed at low redshift. This is consistent, for example, with the suggestion that early star formation involved massive, Population III stars that early on produced most of the ionizing radiation escaping from halos. It is therefore tempting to interpret the new WMAP results as implying that we can now relax those previous high demands on the efficiency of the sources of reionization and perhaps even turn the argument around as evidence against such high efficiency. We show that this is not the case, however. The new WMAP results also find that the primordial density fluctuation power spectrum has a lower amplitude, σ8, and departs substantially from the scale-invariant spectrum. We show that these effects combine to cancel the impact of the later reionization implied by the new value of τes on the required ionizing efficiency per collapsed baryon. The delay of reionization is surprisingly well matched by a comparable delay (by a factor of ~1.4 in scale factor) in the formation of the halos responsible for reionization.

Formation and evolution of self-interacting dark matter haloes

We have derived the first, fully-cosmological, similarity solutions for CDM halo formation in the presence of nongravitational collisionality, which provides an analytical theory of the effect of the self-interacting dark matter (SIDM) hypothesis on halo density profiles. Collisions transport heat inward, which produces a constant-density core, while continuous infall pumps energy into the halo to stabilize the core against gravothermal catastrophe. These solutions improve upon earlier attempts to model the formation and evolution of SIDM halos, offer deeper insight than existing N-body experiments, and yield a more precise determination of the dependence of halo density profile on the value of the CDM self-interaction cross section. Different solutions arise for different values of the dimensionless collisionality parameter Q = s rho_b r_v \~ r_v/l_mfp, where s is the scattering cross section per unit mass, rho_b is the cosmic mean matter density, r_v is halo virial radius and l_mfp is the collision mean free path. The maximum flattening of central density occurs for an intermediate value of Q, Q_th, at which the halo is maximally relaxed to isothermality. The density profiles with constant-density cores preferred by dwarf and LSB rotation curves are best fit by the maximally-flattened (Q=Q_th) solution. If we assume that dwarfs and LSB galaxies formed at their typical collapse epoch in LCDM, then the value of s which makes Q=Q_th is s ~ 200 cm^{2}/g, much higher than previous estimates, s ~ 0.5-5 cm^{2}/g, based on N-body experiments. If s is independent of collision velocity, then the same value s ~ 200 cm^{2}/g would make Q>Q_th for clusters, which typically formed only recently, resulting in relatively less flattening of their central density profile and a smaller core.