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We present a detailed comparison of three different simulations of the epoch of reionization (EoR). The radiative transfer simulation (C-2-RAY) among them is our benchmark. Radiative transfer codes can produce realistic results, but are computationally expensive. We compare it with two seminumerical techniques: one using the same haloes as C-2-RAY as its sources (Sem-Num), and one using a conditional Press-Schechter scheme (CPS+GS). These are vastly more computationally efficient than C-2-RAY, but use more simplistic physical assumptions. We evaluate these simulations in terms of their ability to reproduce the history and morphology of reionization. We find that both Sem-Num and CPS+GS can produce an ionization history and morphology that is very close to C-2-RAY, with Sem-Num performing slightly better compared to CPS+GS. We also study different redshift-space observables of the 21-cm signal from EoR: the variance, power spectrum and its various angular multipole moments. We find that both seminumerical models perform reasonably well in predicting these observables at length scales relevant for present and future experiments. However, Sem-Num performs slightly better than CPS+GS in producing the reionization history, which is necessary for interpreting the future observations. The CPS+GS scheme, however, has the advantage that it is not restricted by the mass resolution of the dark matter density field., AuthorCount:7

Dark halo substructure may reveal itself through secondary, small-scale gravitational lensing effects on light sources that are macrolensed by a foreground galaxy. Here, we explore the prospects of using Very Long Baseline Interferometry (VLBI) observations of multiply-imaged quasar jets to search for submilliarcsecond-scale image distortions produced by various forms of dark substructures in the 10(3)-10(8) M-circle dot mass range. We present lensing simulations relevant for the angular resolutions attainable with the existing European VLBI Network, the global VLBI array and an upcoming observing mode in which the Atacama Large Millimeter Array (ALMA) is connected to the global VLBI array. While observations of this type would not be sensitive to standard cold dark matter subhaloes, they can be used to detect the more compact forms of halo substructure predicted in alternative structure formation scenarios. By mapping approximately five strongly lensed systems, it should be possible to detect or robustly rule out primordial black holes in the 10(3)-10(6) M-circle dot mass range if they constitute greater than or similar to 1 per cent of the dark matter in these lenses. Ultracompact minihaloes are harder to detect using this technique, but 10(6)-10(8) M-circle dot ultracompact minihaloes could in principle be detected if they constitute greater than or similar to 10 per cent of the dark matter., AuthorCount:11

A commonly accepted method to measure the in situ gas diffusivity of topsoil is based on the diffusion of a test gas from an open-bottomed chamber that is inserted a few centimeters into the soil. The aim of this study is to develop an easy-to-use approach to determine the gas diffusivity of topsoil at sites where the chamber cannot be inserted into the soil. In the existing method, the diffusion of gas into the soil is a quasi-one-dimensional problem with an analytical solution for gas concentration in the chamber as a function of time t. This one-dimensionality is no longer given in the case of a chamber placed on top of the soil. In the newly suggested method, simulated time series of gas concentrations in a chamber inserted into the soil (Case 1) were compared to those for a chamber placed on top of the soil (Case 2), with equal gas diffusion coefficients for the soil, D-s. We determined a relationship that can be used to convert the chamber concentration for a given D-s at each t in Case 2 to Case 1. The values of D-s can be found iteratively. The method was validated using D-s values different from those used to calculate the function parameters. Practical field measurements of gas diffusivity were also conducted, which show that, on average, both methods yield the same results for an individual site., AuthorCount:3

We present a study of the impact of a bright quasar on the redshifted 21-cm signal during the epoch of reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H ii regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H ii region substantially beyond those of large H ii regions produced by clustered stellar sources alone. However, the quasar H ii region is found to be more spherical. We next investigate the prospects of detecting such H ii regions in the redshifted 21-cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H ii regions with radii similar to 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 h of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21-cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21-cm data can set new constraints on quasar lifetimes., AuthorCount:5

We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of H ii regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FoF) method, the spherical average (SPA) method and the power spectrum (PS) of the ionized fraction. The latter three methods are complementary: while the FoF method captures the size distribution of the small-scale H ii regions, which contribute only a small amount to the total ionization fraction, the SPA method provides a smoothed measure for the average size of the H ii regions constituting the main contribution to the ionized fraction, and the PS does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H ii regions is the average size which results if we divide the total volume of the H ii regions by their total surface area (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of the size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization and the minimum halo mass for sources to form. We find that the suppression of ionizing sources within ionized regions slows the growth of H ii regions and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex, as indicated by the evolution of the Euler characteristic of the ionized regions. We find the density and ionized, authorCount :5

We present a study of the impact of a bright quasar on the redshifted 21cm signal during theEpoch of Reionization (EoR). Using three different cosmological radiative transfer simula-tions, we investigate if quasars are capable of substantially changing the size and morphologyof the H II regions they are born in. We choose stellar and quasar luminosities in a way that isfavourable to seeing such an effect. We find that even the most luminous of our quasar modelsis not able to increase the size of its native H II region substantially beyond those of largeH II regions produced by clustered stellar sources alone. However, the quasar H II region isfound to be more spherical. We next investigate the prospects of detecting such H II regionsin the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matchedfilter technique. We find that H II regions with radii ∼ 25 comoving Mpc or larger shouldhave a sufficiently high detection probability for 1200 hours of integration time. Although thematched filter can in principle distinguish between more and less spherical regions, we findthat when including realistic system noise this distinction can no longer be made. The strongforegrounds are found not to pose a problem for the matched filter technique. We also demon-strate that when the quasar position is known, the redshifted 21cm data can still be used toset upper limits on the ionizing photon rate of the quasar. If both the quasar position and itsluminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.