Daniel J. Whalen

Cosmological radiative transfer codes comparison project - I. The static density field tests

Radiative transfer (RT) simulations are now at the forefront of numerical astrophysics. They are becoming crucial for an increasing number of astrophysical and cosmological problems; at the same time their computational cost has come within reach of currently available computational power. Further progress is retarded by the considerable number of different algorithms (including various flavours of ray tracing and moment schemes) developed, which makes the selection of the most suitable technique for a given problem a non-trivial task. Assessing the validity ranges, accuracy and performances of these schemes is the main aim of this paper, for which we have compared 11 independent RT codes on five test problems: (0) basic physics; (1) isothermal H II region expansion; (2) H II region expansion with evolving temperature; (3) I-front trapping and shadowing by a dense clump and (4) multiple sources in a cosmological density field. The outputs of these tests have been compared and differences analysed. The agreement between the various codes is satisfactory although not perfect. The main source of discrepancy appears to reside in the multifrequency treatment approach, resulting in different thicknesses of the ionized-neutral transition regions and the temperature structure. The present results and tests represent the most complete benchmark available for the development of new codes and improvement of existing ones. To further this aim all test inputs and outputs are made publicly available in digital form.

The Destruction of Cosmological Minihalos by Primordial Supernovae

We present numerical simulations of primordial supernovae in cosmological minihalos at z ~ 20. We consider Type II supernovae, hypernovae, and pair instability supernovae (PISN) in halos from 6.9 × 105 to 1.2 × 107 M☉, those in which Population III stars are expected to form via H2 cooling. Our simulations are the first to follow the evolution of the blast from a free expansion on spatial scales of 10−4 pc until its approach to pressure equilibrium in the relic H II region of the progenitor, ~1000 pc. Supernovae in H II regions first expand adiabatically and then radiate strongly upon collision with baryons ejected from the halo during its photoevaporation by the progenitor. In contrast to previous findings, supernovae in neutral halos promptly emit most of their kinetic energy as X-rays, but retain enough momentum to seriously disrupt the halo. Explosions in H II regions escape into the IGM, but neutral halos confine the blast and its metals. In H II regions, a prompt second generation of stars may form in the remnant at radii of 100-200 pc. Explosions confined by massive halos instead recollapse, with infall rates in excess of 10−2 M☉ yr−1 that heavily contaminate their interior. This fallback may either fuel massive black hole growth at very high redshifts or create the first globular clusters with radii of 10-20 pc at the center of the halo. Our findings suggest that the first primitive galaxies may therefore have formed sooner, with greater numbers of stars and distinct chemical signatures, than in current models.

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.

How the First Stars Regulated Local Star Formation. I. Radiative Feedback

We present numerical simulations of how a 120 -->M☉ primordial star regulates star formation in nearby cosmological halos at -->z ~ 20 by photoevaporation. Our models include nine-species primordial chemistry and self-consistent multifrequency conservative transfer of UV photons with all relevant radiative processes. Whether or not new stars form in halos clustered around a Population III star ultimately depends on their core densities and proximity to the star. Diffuse halos with central densities below 2-3 cm−3 are completely ionized and evaporated anywhere in the cluster. Evolved halos with core densities above 2000 cm−3 are impervious to both ionizing and Lyman-Werner flux at most distances from the star and collapse as quickly as they would in its absence. Star formation in halos of intermediate density can be either promoted or suppressed depending on how the ionization front (I-front) remnant shock compresses, deforms, and enriches the core with H2. We find that the 120 -->M☉ star photodissociates H2 in most halos in the cluster, but that catalysis by H− restores it a few hundred kiloyears after the death of the star, with little effect on star formation. Our models exhibit significant departures from previous one-dimensional, spherically symmetric simulations, which are prone to serious errors due to unphysical geometric focusing effects.

Supermassive Seeds for Supermassive Black Holes

Recent observations of quasars powered by supermassive black holes (SMBHs) out to z & 7 constrain both the initial seed masses and the growth of the most massive black holes (BHs) in the early universe. Here we elucidate the implications of the radiative feedback from early generations of stars and from BH accretion for popular models for the formation and growth of seed BHs. We show that by properly accounting for (1) the limited role of mergers in growing seed BHs as inferred from cosmological simulations of early star formation and radiative feedback, (2) the sub-Eddington accretion rates of BHs expected at the earliest times, and (3) the large radiative efficiencies ǫ of the most massive BHs inferred from observations of active galactic nuclei at high redshift (ǫ & 0.1), we are led to the conclusion that the initial BH seeds may have been as massive as & 10 5 M⊙. This presents a strong challenge to the Population III seed model, which calls for seed masses of ∼ 100 M⊙ and, even with constant Eddington-limited accretion, requires ǫ . 0.09 to explain the highest-z SMBHs

Forming a Primordial Star in a Relic H II Region

There has been considerable theoretical debate over whether photoionization and supernova feedback from the first Population III stars facilitate or suppress the formation of the next generation of stars. We present results from an Eulerian adaptive mesh refinement simulation demonstrating the formation of a primordial star within a region ionized by an earlier nearby star. Despite the higher temperatures of the ionized gas and its flow out of the dark matter potential wells, this second star formed within 23 million years of its neighbors death. The enhanced electron fraction within the H II region catalyzes rapid molecular hydrogen formation that leads to faster cooling in the subsequent star-forming halos than in the first halos. This second generation primordial protostar has a much lower accretion rate because, unlike the first protostar, it forms in a rotationally supported disk of ~10-100 M☉. This is primarily due to the much higher angular momentum of the halo in which the second star forms. In contrast to previously published scenarios, such configurations may allow binaries or multiple systems of lower mass stars to form. These first high-resolution calculations offer insight into the impact of feedback upon subsequent populations of stars and clearly demonstrate how primordial chemistry promotes the formation of subsequent generations of stars even in the presence of the entropy injected by the first stars into the intergalactic medium.

Ionization Front Instabilities in Primordial H II Regions

Radiative cooling by metals in shocked gas mediates the formation of ionization front instabilities in the galaxy today that are responsible for a variety of phenomena in the interstellar medium, from the morphologies of nebulae to triggered star formation in molecular clouds. An important question in early reionization and chemical enrichment of the intergalactic medium is whether such instabilities arose in the H II regions of the first stars and primeval galaxies, which were devoid of metals. We present three-dimensional numerical simulations that reveal both shadow and thin-shell instabilities readily formed in primordial gas. We find that the hard UV spectra of Population III stars broadened primordial ionization fronts, causing H2 formation capable of inciting violent thin-shell instabilities in D-type fronts, even in the presence of intense Lyman-Werner flux. The high postfront gas temperatures associated with He ionization sustained and exacerbated shadow instabilities, unaided by molecular hydrogen cooling. Our models indicate that metals eclipsed H2 cooling in I-front instabilities at modest concentrations, from 1 × 10−3 to 1 × 10−2 Z☉. We conclude that ionization front instabilities were prominent in the H II regions of the first stars and galaxies, influencing the escape of ionizing radiation and metals into the early universe.

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 Early Evolution of Primordial Pair-instability Supernovae

The observational signatures of the first cosmic explosions and their chemical imprint on second-generation stars both crucially depend on how heavy elements mix within the star at the earliest stages of the blast. We present numerical simulations of the early evolution of Population III pair-instability supernovae with the new adaptive mesh refinement code CASTRO. In stark contrast to 15 - 40 Msun core-collapse primordial supernovae, we find no mixing in most 150 - 250 Msun pair-instability supernovae out to times well after breakout from the surface of the star. This may be the key to determining the mass of the progenitor of a primeval supernova, because vigorous mixing will cause emission lines from heavy metals such as Fe and Ni to appear much sooner in the light curves of core-collapse supernovae than in those of pair-instability explosions. Our results also imply that unlike low-mass Pop III supernovae, whose collective metal yields can be directly compared to the chemical abundances of extremely metal-poor stars, further detailed numerical simulations will be required to determine the nucleosynthetic imprint of very massive Pop III stars on their direct descendants.

Seeing the First Supernovae at the Edge of the Universe with JWST

The first stars ended the cosmic dark ages and created the first heavy elements necessary for the formation of planets and life. The properties of these stars remain uncertain, and it may be decades before individual Population III (Pop III) stars are directly observed. Their masses, however, can be inferred from their supernova explosions, which may soon be found in both deep-field surveys by the James Webb Space Telescope (JWST) and in all-sky surveys by the Wide Field Infrared Survey Telescope (WFIRST). We have performed radiation hydrodynamical simulations of the near-infrared signals of Pop III pair-instability supernovae in realistic circumstellar environments with Lyman absorption by the neutral intergalactic medium. We find that JWST and WFIRST will detect these explosions out to z {approx} 30 and 20, respectively, unveiling the first generation of stars in the universe.