Joseph Smidt

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.

Near-infrared background anisotropies from diffuse intrahalo light of galaxies

Unresolved anisotropies of the cosmic near-infrared background radiation are expected to have contributions from the earliest galaxies during the epoch of reionization and from faint, dwarf galaxies at intermediate redshifts. Previous measurements were unable to pinpoint conclusively the dominant origin because they did not sample spatial scales that were sufficiently large to distinguish between these two possibilities. Here we report a measurement of the anisotropy power spectrum from subarcminute to one-degree angular scales, and find the clustering amplitude to be larger than predicted by the models based on the two existing explanations. As the shot-noise level of the power spectrum is consistent with that expected from faint galaxies, a new source population on the sky is not necessary to explain the observations. However, a physical mechanism that increases the clustering amplitude is needed. Motivated by recent results related to the extended stellar light profile in dark-matter haloes, we consider the possibility that the fluctuations originate from intrahalo stars of all galaxies. We find that the measured power spectrum can be explained by an intrahalo light fraction of 0.07 to 0.2 per cent relative to the total luminosity in dark-matter haloes of 109 to 1012 solar masses at redshifts of about 1 to 4.

CMB contraints on primordial non-Gaussianity from the bispectrum (fNL) and trispectrum (gNL and τNL) and a new consistency test of single-field inflation

We outline the expected constraints on non-Gaussianity from the cosmic microwave background with current and future experiments, focusing on both the third (f{sub NL}) and fourth-order (g{sub NL} and {tau}{sub NL}) amplitudes of the local configuration or non-Gaussianity. The experimental focus is the skewness (two-to-one) and kurtosis (two-to-two and three-to-one) power spectra from weighted maps. In addition to a measurement of {tau}{sub NL} and g{sub NL} with WMAP 5-year data, our study provides the first forecasts for future constraints on g{sub NL}. We describe how these statistics can be corrected for the mask and cut-sky through a window function, bypassing the need to compute linear terms that were introduced for the previous-generation non-Gaussianity statistics, such as the skewness estimator. We discus the ratio A{sub NL}={tau}{sub NL}/(6f{sub NL}/5){sup 2} as an additional test of single-field inflationary models and discuss the physical significance of each statistic. Using these estimators with WMAP 5-Year V+W-band data out to l{sub max}=600 we constrain the cubic order non-Gaussianity parameters {tau}{sub NL}, and g{sub NL} and find -7.4<g{sub NL}/10{sup 5}<8.2 and -0.6<{tau}{sub NL}/10{sup 4}<3.3 improving the previous COBE-based limit on {tau}{sub NL}<10{sup 8} nearly 4 orders of magnitude with WMAP.

On the origin of near-infrared extragalactic background light anisotropy.

A diffuse cosmic glow is not primordial A cumulative map of all photons ever emitted by any star or galaxy is a highly desirable historical record of the universes evolution. For this reason, cosmologists have sought to measure this diffuse distribution of light: the extragalactic background light. Zemcov et al. sent up a rocket to measure the fluctuations in this faint background and found largescale fluctuations greater than known galaxies alone should produce (see the Perspective by Moseley). Stars tidally stripped from their host galaxies are the most likely culprit, rather than unknown primordial galaxies. Science, this issue p. 732; see also p. 696 Emission fluctuations that trace the cosmic history are most consistent with the light from intrahalo stars at low redshift. [Also see Perspective by Moseley] Extragalactic background light (EBL) anisotropy traces variations in the total production of photons over cosmic history and may contain faint, extended components missed in galaxy point-source surveys. Infrared EBL fluctuations have been attributed to primordial galaxies and black holes at the epoch of reionization (EOR) or, alternately, intrahalo light (IHL) from stars tidally stripped from their parent galaxies at low redshift. We report new EBL anisotropy measurements from a specialized sounding rocket experiment at 1.1 and 1.6 micrometers. The observed fluctuations exceed the amplitude from known galaxy populations, are inconsistent with EOR galaxies and black holes, and are largely explained by IHL emission. The measured fluctuations are associated with an EBL intensity that is comparable to the background from known galaxies measured through number counts and therefore a substantial contribution to the energy contained in photons in the cosmos.

The Near-infrared Background Intensity and Anisotropies during the Epoch of Reionization

A fraction of the extragalactic near-infrared (near-IR) background light involves redshifted photons from the ultraviolet (UV) emission from galaxies present during reionization at redshifts above 6. The absolute intensity and the anisotropies of the near-IR background provide an observational probe of the first-light galaxies and their spatial distribution. We estimate the extragalactic background light intensity during reionization by accounting for the stellar and nebular emission from first-light galaxies. We require the UV photon density from these galaxies to generate a reionization history that is consistent with the optical depth to electron scattering from cosmic microwave background measurements. We also require the bright-end luminosity function (LF) of galaxies in our models to reproduce the measured Lyman-dropout LFs at redshifts of 6-8. The absolute intensity is about 0.1-0.4 nW m–2 sr–1 at the peak of its spectrum at ~1.1 μm. We also discuss the anisotropy power spectrum of the near-IR background using a halo model to describe the galaxy distribution. We compare our predictions for the anisotropy power spectrum to existing measurements from deep near-IR imaging data from Spitzer/IRAC, Hubble/NICMOS, and AKARI. The predicted rms fluctuations at tens of arcminute angular scales are roughly an order of magnitude smaller than the existing measurements. While strong arguments have been made that the measured fluctuations do not have an origin involving faint low-redshift galaxies, we find that measurements in the literature are also incompatible with galaxies present during the era of reionization. The measured near-IR background anisotropies remain unexplained with an unknown origin.

CMB contraints on primordial non-Gaussianity from the bispectrum (f(NL)) and trispectrum (g(NL) and tau(NL)) and a new consistency test of single-field inflation

We outline the expected constraints on non-Gaussianity from the cosmic microwave background with current and future experiments, focusing on both the third (f{sub NL}) and fourth-order (g{sub NL} and {tau}{sub NL}) amplitudes of the local configuration or non-Gaussianity. The experimental focus is the skewness (two-to-one) and kurtosis (two-to-two and three-to-one) power spectra from weighted maps. In addition to a measurement of {tau}{sub NL} and g{sub NL} with WMAP 5-year data, our study provides the first forecasts for future constraints on g{sub NL}. We describe how these statistics can be corrected for the mask and cut-sky through a window function, bypassing the need to compute linear terms that were introduced for the previous-generation non-Gaussianity statistics, such as the skewness estimator. We discus the ratio A{sub NL}={tau}{sub NL}/(6f{sub NL}/5){sup 2} as an additional test of single-field inflationary models and discuss the physical significance of each statistic. Using these estimators with WMAP 5-Year V+W-band data out to l{sub max}=600 we constrain the cubic order non-Gaussianity parameters {tau}{sub NL}, and g{sub NL} and find -7.4<g{sub NL}/10{sup 5}<8.2 and -0.6<{tau}{sub NL}/10{sup 4}<3.3 improving the previous COBE-based limit on {tau}{sub NL}<10{sup 8} nearly 4 orders of magnitude with WMAP.

SUPERMASSIVE POPULATION III SUPERNOVAE AND THE BIRTH OF THE FIRST QUASARS

The existence of supermassive black holes as early as z ~ 7 is one of the great, unsolved problems in cosmological structure formation. One leading theory argues that they are born during catastrophic baryon collapse in z ~?15 protogalaxies that form in strong Lyman-Werner UV backgrounds. Atomic line cooling in such galaxies fragments baryons into massive clumps that are thought to directly collapse to 104-105 M ? black holes. We have now discovered that some of these fragments can instead become supermassive stars that eventually explode as thermonuclear supernovae (SNe) with energies of ~1055 erg, the most energetic explosions in the universe. We have calculated light curves and spectra for supermassive Pop III SNe with the Los Alamos RAGE and SPECTRUM codes. We find that they will be visible in near-infrared all-sky surveys by Euclid out to z ~ 10-15 and by WFIRST and WISH out to z ~ 15-20, perhaps revealing the birthplaces of the first quasars.

Parameter Space of General Gauge Mediation

We study a subspace of General Gauge Mediation (GGM) models which generalize models of gauge mediation. We find superpartner spectra that are markedly different from those of typical gauge and gaugino mediation scenarios. While typical gauge mediation predictions of either a neutralino or stau next-to-lightest supersymmetric particle (NLSP) are easily reproducible with the GGM parameters, chargino and sneutrino NLSPs are generic for many reasonable choices of GGM parameters.

THE BIGGEST EXPLOSIONS IN THE UNIVERSE

Supermassive primordial stars are expected to form in a small fraction of massive protogalaxies in the early universe, and are generally conceived of as the progenitors of the seeds of supermassive black holes (BHs). Supermassive stars with masses of ∼ 55,000M⊙, however, have been found to explode and completely disrupt in a supernova (SN) with an energy of up to ∼ 10 55 erg instead of collapsing to a BH. Such events, ∼ 10,000 times more energetic than typical SNe today, would be among the biggest explosions in the history of the universe. Here we present a simulation of such a SN in two stages. Using the RAGE radiation hydrodynamics code we first evolve the explosion from an early stage through the breakout of the shock from the surface of the star until the blast wave has propagated out to several parsecs from the explosion site, which lies deep within an atomic cooling dark matter (DM) halo at z ≃ 15. Then, using the GADGET cosmological hydrodynamics code we evolve the explosion out to several kiloparsecs from the explosion site, far into the low-density intergalactic medium. The host DM halo, with a total mass of 4 × 10 7 M⊙, much more massive than typical primordial star-forming halos, is completely evacuated of high density gas after . 10Myr, although dense metal-enriched gas recollapses into the halo, where it will likely form second-generation stars with metallicities of ≃ 0.05Z⊙ after & 70Myr. The chemical signature of supermassive star explosions may be found in such long-lived second-generation stars today. Subject headings: Cosmology: theory — early universe — supernovae: general