arXiv subjects

Garrelt Mellema

Publications and source records attributed to Garrelt Mellema.

At least 109 records · Page 6Linked to original sources

arXiv2009

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 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 radiative transfer tests (i.e. for fixed, non-evolving density fields). This is the second paper of the Cosmological Radiative Transfer (RT) Comparison Project, in which we compare 9 independent RT codes directly coupled to gasdynamics on 3 relatively simple astrophysical hydrodynamics problems: (5) the expansion of an H II region in a uniform medium; (6) an ionization front (I-front) in a 1/r^2 density profile with a flat core, and (7), 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.

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arXiv2009

Detection and extraction of signals from the epoch of reionization using higher order one-point statistics

Detecting redshifted 21cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock datacubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21cm signal. We argue that some of these features - such as a dip at the onset of reionization, followed by a rise towards its later stages - may be generic, and give us a promising route to a statistical detection of reionization.

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arXiv2008

Effect of the intergalactic environment on the observability of Ly-alpha emitters during reionization

Observations of high-redshift Ly-alpha sources are a major tool for studying the high-redshift Universe. We discuss the effect of the reionizing intergalactic medium on the observability of Ly-alpha sources based on large simulations of early structure formation with radiative transfer. This takes into account self-consistently the reionization history, density, velocity and ionization structures and nonlinear source clustering. We find that all fields are highly anisotropic and as a consequence there are very large variations in opacity among the different lines-of-sight. The velocity effects, from both infall and source peculiar velocity are most important for the luminous sources, affecting the line profile and depressing the bright end of the luminosity function. The line profiles are generally asymmetric and the line centers of the luminous sources are always absorbed due to the high density of the local IGM. For both luminous and average sources the damping wing effects are of similar magnitude and remain significant until fairly late. The ionizing flux in the ionized patch surrounding a high density peak is generally strongly dominated, particularly at late times, by the cluster of faint sources, rather than the central massive galaxy. The IGM absorption does not change appreciably the correlation function of sources at high redshift. Our derived luminosity function assuming constant mass-to-light ratio provides an excellent match to the shape of the observed luminosity function at z=6.6 with faint-end slope of alpha=-1.5. The resulting mass-to-light ratio implies that the majority of sources responsible for reionization are too faint to be observed by the current surveys. (abridged)

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arXiv2008

Fast Large-Scale Reionization Simulations

We present an efficient method to generate large simulations of the Epoch of Reionization (EoR) without the need for a full 3-dimensional radiative transfer code. Large dark-matter-only simulations are post-processed to produce maps of the redshifted 21cm emission from neutral hydrogen. Dark matter haloes are embedded with sources of radiation whose properties are either based on semi-analytical prescriptions or derived from hydrodynamical simulations. These sources could either be stars or power-law sources with varying spectral indices. Assuming spherical symmetry, ionized bubbles are created around these sources, whose radial ionized fraction and temperature profiles are derived from a catalogue of 1-D radiative transfer experiments. In case of overlap of these spheres, photons are conserved by redistributing them around the connected ionized regions corresponding to the spheres. The efficiency with which these maps are created allows us to span the large parameter space typically encountered in reionization simulations. We compare our results with other, more accurate, 3-D radiative transfer simulations and find excellent agreement for the redshifts and the spatial scales of interest to upcoming 21cm experiments. We generate a contiguous observational cube spanning redshift 6 to 12 and use these simulations to study the differences in the reionization histories between stars and quasars. Finally, the signal is convolved with the LOFAR beam response and its effects are analyzed and quantified. Statistics performed on this mock data set shed light on possible observational strategies for LOFAR.

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arXiv2008

The Inhomogeneous Background of H2 Dissociating Radiation During Cosmic Reionization

The first, self-consistent calculations of the cosmological H_2 dissociating UV background produced during the epoch of reionization (EOR) by the sources of reionization are presented. Large-scale radiative transfer simulations of reionization trace the impact of all the ionizing starlight on the IGM from all the sources in our simulation volume down to dwarf galaxies of mass ~ 10^8 solar mass, 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 background in the Lyman-Werner band of H_2 between 11 and 13.6 eV.

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arXiv2008

The Theory and Simulation of the 21-cm Background from the Epoch of Reionization

The redshifted 21-cm line of distant neutral H atoms provides a probe of the cosmic ``dark ages'' and the epoch of reionization (``EOR'') which ended them. The radio continuum produced by this redshifted line can be seen in absorption or emission against the CMB at meterwaves, yielding information about the thermal and ionization history of the universe and the primordial density perturbation spectrum that led to galaxy and large-scale structure formation. Observing this 21-cm background is a great challenge. A new generation of low-frequency radio arrays is currently under development to search for this background. Accurate theoretical predictions of the spectrum and anisotropy of this background, necessary to guide and interpret future observations, are also quite challenging. It is necessary to model the inhomogeneous reionization of the intergalactic medium and determine the spin temperature of the 21-cm transition and its variations in time and space as it decouples from the temperature of the CMB. Here, we focus on just a few of the predictions for the 21-cm background from the EOR, based on our newest, large-scale simulations of patchy reionization. These simulations are the first with enough N-body particles (from 5 to 29 billion) and radiative transfer rays to resolve the formation of and trace the ionizing radiation from each of the millions of dwarf galaxies believed responsible for reionization, down to 10^8 M_solar, in a cubic volume large enough (90 and 163 comoving Mpc on a side) to make meaningful statistical predictions of the fluctuating 21-cm background. (abridged)

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arXiv2008

Simulating Cosmic Reionization

The Cosmic Dark Ages and the Epoch of Reionization constitute a crucial missing link in our understanding of the evolution of the intergalactic medium and the formation and evolution of galaxies. Due to the complex nature of this global process it is best studied through large-scale numerical simulations. This presents considerable computational challenges. The dominant contributors of ionizing radiation were dwarf galaxies. These tiny galaxies must be resolved in very large cosmological volumes in order to derive their clustering properties and the corresponding observational signatures correctly, which makes this one of the most challenging problems of numerical cosmology. We have recently performed the largest and most detailed simulations of the formation of early cosmological large-scale structures and their radiative feedback leading to cosmic reionization. This was achieved by running extremely large (up to 29 billion-particle) N-body simulations of the formation of the Cosmic Web, with enough particles and sufficient force resolution to resolve all the galactic halos with total masses larger than 10^8 Solar masses in computational volumes of up to (163 Mpc)^3. These results were then post-processed by propagating the ionizing radiation from all sources by using fast and accurate ray-tracing radiative transfer method. Both of our codes are parallelized using a combination of MPI and OpenMP and to this date have been run efficiently on up to 2048 cores (N-body) and up to 10000 cores (radiative transfer) on the newly-deployed Sun Constellation Linux Cluster at the Texas Advanced Computing Center. In this paper we describe our codes, parallelization strategies, scaling and some preliminary scientific results. (abridged)

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arXiv2007

Character and detectability of the dark ages and the epoch of reionization: the view from the simulations

Direct detection of the Dark Ages and the Epoch of Reionization (EOR) is among the main scientific objectives of all current and future low-frequency radio facilities. In this paper we summarize and discuss recent results, based on state-of-the-art numerical simulations, regarding the fundamental EOR properties and its observability with current and future radio arrays, like the Giant Metrewave Radio Telescope (GMRT), the Low Frequency Array (LOFAR), the 21-CM Array (21CMA), the Murchison Widefield Array (MWA) and the Square Kilometre Array (SKA). Results show that the optimal observational frequencies for statistical detection are 140-160 MHz. The signals are strongly non-Gaussian at late times. The correlation widths between 21-cm maps at neighbouring frequencies are short, of order 300-800 kHz, which should help with the cleaning of the strong foregrounds. Direct comparison of the resolutions and expected sensitivities of GMRT and MWA indicate that their optimal sensitivity ranges are similar, at scales k~0.2-0.4 h/Mpc, however, all else being equal the former should require shorter integration times due to its significantly larger collecting area.

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arXiv2007

Self-regulated reionization

Recently, we have presented the first large-scale radiative transfer simulations of reionization. Here we present new simulations which extend the source halo mass range downward to 10^8M_solar, to capture the full range of halo masses thought to be primarily responsible for reionization by their star formation following atomic hydrogen radiative cooling and gravitational collapse. Haloes below about 10^9M_solar, however, are subject to Jeans-mass filtering in the ionized regions, which suppresses their baryonic content and their ability to release ionizing radiation. By including these smaller-mass haloes but accounting for their suppression, too, we find that reionization is ``self-regulating,'' as follows. As the mean ionized fraction rises, so does the fraction of the volume within which suppression occurs. Hence, the degree of suppression is related to the mean ionized fraction. Since low-mass haloes with high emissivity achieve a given mean ionized fraction earlier than do those with low efficiency, Jeans-mass filtering compensates for the difference in the emissivity of the suppressible haloes in these two cases. As a result, in the presence of lower-mass source haloes, reionization begins earlier, but the later stages of reionization and the time of overlap are dictated by the efficiency of the higher-mass haloes, independent of the efficiency of the suppressible, lower-mass haloes. Reionization histories consistent with current observational constraints are shown to be achievable with standard stellar sources in haloes above 10^8M_solar. Neither minihalos nor exotic sources are required, and the phenomenon of ``double reionization'' previously suggested does not occur. (abridged)

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arXiv2007

Current models of the observable consequences of cosmic reionization and their detectability

A number of large current experiments aim to detect the signatures of the Cosmic Reionization at redshifts z>6. Their success depends crucially on understanding the character of the reionization process and its observable consequences and designing the best strategies to use. We use large-scale simulations of cosmic reionization to evaluate the reionization signatures at redshifted 21-cm and small-scale CMB anisotropies in the best current model for the background universe, with fundamental cosmological parameters given by WMAP 3-year results (WMAP3). We find that the optimal frequency range for observing the ``global step'' of the 21-cm emission is 120-150 MHz, while statistical studies should aim at 140-160 MHz, observable by GMRT. Some strongly-nongaussian brightness features should be detectable at frequencies up to ~190 MHz. In terms of sensitivity-signal trade-off relatively low resolutions, corresponding to beams of at least a few arcminutes, are preferable. The CMB anisotropy signal from the kinetic Sunyaev-Zel'dovich effect from reionized patches peaks at tens of muK at arcminute scales and has an rms of ~1 muK, and should be observable by the Atacama Cosmology Telescope and the South Pole Telescope. We discuss the various observational issues and the uncertainties involved, mostly related to the poorly-known reionization parameters and, to a lesser extend, to the uncertainties in the background cosmology.

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arXiv2007

Reionization: Characteristic Scales, Topology and Observability

Recently the numerical simulations of the process of reionization of the universe at z>6 have made a qualitative leap forward, reaching sufficient sizes and dynamic range to determine the characteristic scales of this process. This allowed making the first realistic predictions for a variety of observational signatures. We discuss recent results from large-scale radiative transfer and structure formation simulations on the observability of high-redshift Ly-alpha sources. We also briefly discuss the dependence of the characteristic scales and topology of the ionized and neutral patches on the reionization parameters.

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arXiv2007

Simulating Reionization: Character and Observability

In recent years there has been considerable progress in our understanding of the nature and properties of the reionization process. In particular, the numerical simulations of this epoch have made a qualitative leap forward, reaching sufficiently large scales to derive the characteristic scales of the reionization process and thus allowing for realistic observational predictions. Our group has recently performed the first such large-scale radiative transfer simulations of reionization, run on top of state-of-the-art simulations of early structure formation. This allowed us to make the first realistic observational predictions about the Epoch of Reionization based on detailed radiative transfer and structure formation simulations. We discuss the basic features of reionization derived from our simulations and some recent results on the observational implications for the high-redshift Ly-alpha sources.

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arXiv2007

Dependence of the Local Reionization History on Halo Mass and Environment: Did Virgo Reionize the Local Group?

The reionization of the Universe has profound effects on the way galaxies form and on their observed properties at later times. Of particular importance is the relative timing of the reionization history of a region and its halo assembly history, which can affect the nature of the first stars formed in that region, the properties and radial distribution of its stellar halo, globular cluster population and its satellite galaxies. We distinguish two basic cases for the reionization of a halo - internal reionization, whereby the stars forming in situ reionize their host galaxy, and external reionization, whereby the progenitor of a galaxy is reionized by external radiation before its own stars are able to form in sufficient numbers. We use a set of large-scale radiative transfer and structure formation simulations, based on cosmologies derived from both WMAP 1-year and WMAP 3-year data, to evaluate the mean reionization redshifts and the probability of internal/external reionization for Local Group-like systems, galaxies in the field and central cD galaxies in clusters. We find that these probabilities are strongly dependent on the underlying cosmology and the efficiency of photon production, but also on the halo mass. There is a rapid transition between predominantly external and predominantly internal reionization at a mass scale of 1.0e12 Msun (corresponding roughly to L*galaxies), with haloes less massive than this being reionized preferentially from distant sources. We provide a fit for the reionization redshift as a function of halo mass, which could be helpful to parameterize reionization in semi-analytical models of galaxy formation on cosmological scales. We find no statistical correlation between the reionization history of field galaxies and their environment.

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arXiv2007

The Signature of Patchy Reionization in the Polarization Anisotropy of the CMB

The inhomogeneous ionization state of the universe when the first sources of ionizing radiation appeared should lead to anisotropies in the polarization of the cosmic microwave background. We use cosmological simulations of the process by which the first sources ionized the intergalactic medium to study the induced polarization anisotropies. We find that the polarization anisotropies have rms of order ~0.01 μK, and local peak values of ~0.1 μK, smaller than those due to gravitational lensing on small scales. The polarization direction is highly coherent over degree scales. This directional coherence is not expected from either primary anisotropy or gravitational lensing effects, making the largest signals due to inhomogeneous ionization relatively easy to isolate, should experiments achieve the necessary very low noise levels.

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arXiv2006

Reconstructing the Thomson Optical Depth due to Patchy Reionization with 21-cm Fluctuation Maps

Large fluctuations in the electron column density can occur during the reionization process. We investigate the possibility of deriving the electron density fluctuations through detailed mapping of the redshifted 21-cm emission from the neutral medium during reionization. We find that the electron-scattering optical depth and 21-cm differential brightness temperature are strongly anti-correlated, allowing optical depth estimates based entirely on redshifted 21-cm measurements. This should help isolate the CMB polarization fluctuations that are due to reionization, allowing both cleaning of the patchy reionization polarization signal as a contaminating source of confusion to other signals and a measurement of the primordial quadrupole that would be measured at various locations in the universe at the epoch of reionization. This latter application in principle allows mapping of the primordial density field at z~1100 over a large fraction of the Hubble volume.

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arXiv2006

The Kinetic Sunyaev-Zel'dovich Effect from Radiative Transfer Simulations of Patchy Reionization

We present the first calculation of the kinetic Sunyaev-Zel'dovich (kSZ) effect due to the inhomogeneous reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at l=2000-8000 with maximum values of l^2C_l~1\times10^{-12}. The peak scale is determined by the typical size of the ionized regions and roughly corresponds to the ionized bubble sizes observed in our simulations, ~5-20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above l=3000. This predicted kSZ signal at arcminute scales is sufficiently strong to be detectable by upcoming experiments, like the Atacama Cosmology Telescope and South Pole Telescope which are expected to have ~1' resolution and ~muK sensitivity. The extended and patchy nature of the reionization process results in a boost of the peak signal in power by approximately one order of magnitude compared to a uniform reionization scenario, while roughly tripling the signal compared with that based upon the assumption of gradual but spatially uniform reionization. At large scales the patchy kSZ signal depends largely on the ionizing source efficiencies and the large-scale velocity fields: sources which produce photons more efficiently yield correspondingly higher signals. The introduction of sub-grid gas clumping in the radiative transfer simulations produces significantly more power at small scales, and more non-Gaussian features, but has little effect at large scales. The patchy nature of the reionization process roughly doubles the total observed kSZ signal for l~3000-10^4 compared to non-patchy scenarios with the same total electron-scattering optical depth.

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arXiv2006

Simulating Cosmic Reionization at Large Scales II: the 21-cm Emission Features and Statistical Signals

We present detailed predictions for the redshifted 21cm signal from the epoch of reionization. These predictions are obtained from radiative transfer calculations on the results of large scale (100/h Mpc), high dynamic range, cosmological simulations. We consider several scenarios for the reionization history, of both early and extended reionization. From the simulations we construct and analyze a range of observational characteristics, from the global signal, via detailed images and spectra, to statistical representations of rms fluctuations, angular power spectra, and probability distribution functions to characterize the non-gaussianity of the 21cm signal. (abbreviated abstract)

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arXiv2006

The Kinetic Sunyaev-Zel'dovich Effect from Patchy Reionization: the View from the Simulations

We present the first calculation of the kinetic Sunyaev-Zel'dovich (kSZ) effect due to the inhomogeneus reionization of the universe based on detailed large-scale radiative transfer simulations of reionization. The resulting sky power spectra peak at l=2000-8000 with maximum values of l(l+1)C_l/(2pi)_{max}~4-7x10^{-13}. The scale roughly corresponds to the typical ionized bubble sizes observed in our simulations, of ~5-20 Mpc. The kSZ anisotropy signal from reionization dominates the primary CMB signal above l=3000. At large scales the patchy kSZ signal depends only on the source efficiencies. It is higher when sources are more efficient at producing ionizing photons, since such sources produce larger ionized regions, on average, than less efficient sources. The introduction of sub-grid gas clumping in the radiative transfer simulations produce significantly more power at small scales, but has little effect at large scales. The patchy reionization kSZ signal is dominated by the post-reionization signal from fully-ionized gas, but the two contributions are of similar order at scales l~3000-10^4, indicating that the kSZ anisotropies from reionization are an important component of the total kSZ signal at these scales.

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