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Systematic uncertainties that have been subdominant in past large-scale structure (LSS) surveys are likely to exceed statistical uncertainties of current and future LSS data sets, potentially limiting the extraction of cosmological information. Here we present a general framework (Principal Component Analysis - PCA - marginalization) to consistently incorporate systematic effects into a likelihood analysis. This technique naturally accounts for degeneracies between nuisance parameters and can substantially reduce the dimension of the parameter space that needs to be sampled. As a practical application, we apply PCA marginalization to account for baryonic physics as an uncertainty in cosmic shear tomography. Specifically, we use COSMOLIKE to run simulated likelihood analyses on three independent sets of numerical simulations, each covering a wide range of baryonic scenarios differing in cooling, star formation, and feedback mechanisms. We simulate a Stage III (Dark Energy Survey) and Stage IV (Large Synoptic Survey Telescope/Euclid) survey and find a substantial bias in cosmological constraints if baryonic physics is not accounted for. We then show that PCA marginalization (employing at most three to four nuisance parameters) removes this bias. Our study demonstrates that it is possible to obtain robust, precise constraints on the dark energy equation of state even in the presence of large levels of systematic uncertainty in astrophysical processes. We conclude that the PCA marginalization technique is a powerful, general tool for addressing many of the challenges facing the precision cosmology programme. [ABSTRACT FROM AUTHOR]

It has been recently shown that the relative velocity between the dark matter and the baryons (vbc) at the time of recombination can affect the structure formation in the early universe. We statistically quantify this effect using large cosmological simulations. We use three different high-resolution sets of simulations (with separate transfer functions for baryons and dark matter) that vary in box size, particle number, and the value of the relative velocity between dark matter and baryons. We show that the total number density of halos is suppressed by ~20% at z = 25 for vbc = 1σvbc, where σvbc is the variance of the relative velocity, while for vbc = 3.4σvbc the relative suppression at the same redshift reaches 50%, remaining at or above the 30% level all the way to z = 11. We also find a high abundance of "empty halos," i.e., halos that have gas fraction below half of the cosmic mean baryonic fraction ...b. Specifically, we find that for vbc = 1σvbc all halos below 105 M⊙ are empty at z ≥ 19. The high abundance of empty halos results in significant delay in the formation of gas-rich minihalos and the first galaxies. [ABSTRACT FROM AUTHOR]

The close relation of star formation with molecular gas indicated by observations and assumed in recent models implies that the efficiency with which galaxies convert their gas into stars depends on gas metallicity. This is because abundance of molecular hydrogen is sensitive to abundance of dust, which catalyzes formation of H, and helps to shield it from dissociating radiation. In this study, we point out that in the absence of significant] pre-enrichment by Population III stars forming out of zero metallicity gas, such H2-based star formation is expected to leave an imprint in the form of bi-modality in the metallicity distribution among dwarf galaxies and in the metallicity distribution of stars within individual galaxies. The bi-modality arises because when gas metallicity (and dust abundance) is low, formation of molecular gas is inefficient, the gas consumption timescale is long, and star formation and metal enrichment proceed slowly. When metallicity reaches a critical threshold value star formation and enrichment accelerate, which leads to rapid increase in both stellar mass and metallicity of galaxies. We demonstrate this process both using a simple analytical model and full cosmological simulations. In contrast, the observed metallicity distributions of dwarf galaxies or stars within them are not bi-modal. We argue that this discrepancy points to substantial early stochastic pre-enrichment by Population llI stars to levels Z ~ 10-2 Zʘ in dense, star-forming regions of early galaxies. [ABSTRACT FROM AUTHOR]

Using a high-resolution cosmological simulation of reionization, we have examined the differing structures formed by gas and dark matter at a redshift of 5.1. Baryon-rich regions form a small number of filaments, which connect the largest galaxies in the simulation. More detailed examination of the 10 largest galaxies reveals long, slender gaseous filaments about five proper kpc in width radiating from the galaxy centers. Extending out from each filament are a few smooth, thin, nearly planar gaseous sheets. By contrast, the dark matter concentrates into quasi-spherical bodies. The results have implications for our understanding of structure formation in the early Universe and of the Lyman α forest. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We use numerical simulations of a cosmological volume to study the X-ray ionization and heating of the intergalactic medium (IGM) by an early population of accreting black holes (BHs). By considering theoretical limits on the accretion rate and observational constraints from the X-ray background and faint X-ray source counts, we find that the maximum value of the optical depth to Thompson scattering which can be produced using these models is, in agreement with previous semi-analytic results. The redshifted soft X-ray background produced by these early sources is important in producing a fully ionized atomic hydrogen in the low-density intergalactic medium before stellar reionization at redshift. As a result, stellar re-ionization is characterized by an almost instantaneous‘overlap phase’ of H iiregions. The background also produces a second He iire-ionization at about redshift 3 and maintains the temperature of the IGM at about 10 000 K even at low redshifts.If the spectral energy distribution of these sources has a non-negligible high-energy power-law component, the luminosity in the soft X-ray band of the‘typical’ galaxies hosting intermediate-mass accreting BHs is maximum atz∼ 15 and is about one or two orders of magnitude below the sensitivity limit of theChandraDeep Field. We find that about a thousand of these sources may be present per square arcmin of the sky, producing potentially detectable fluctuations. We also estimate that a few rare objects, not present in our small simulated volume, could be luminous enough to be visible in theChandraDeep Field. TheXEUSandConstellation-Xsatellites will be able to detect more of these sources that, if radio loud, could be used to study the 21-cm forest in absorption.A signature of an early X-ray pre-ionization is the production of secondary cosmic microwave background (CMB) anisotropies on small angular scales (<1 arcmin). We find that in these models the power spectrum of temperature fluctuations increases with decreasing angular scale (at∼1-arcsec scales), while for stellar re-ionization scenarios the power decreases on smaller scales. We also show that the redshifted 21-cm radiation from neutral hydrogen can be marginally detected in emission at redshifts. At a redshift of aboutz∼ 30 a stronger and narrower (in redshift space) signal in absorption against the CMB, that is peculiar to these models, could be detectable. [ABSTRACT FROM AUTHOR]

We have developed an algorithm, calledvoboz(VOronoi BOund Zones), to find haloes in anN-body dark matter simulation; it has as little dependence on free parameters as we can manage. By using the Voronoi diagram, we achieve non-parametric,‘natural’ measurements of each particle's density and set of neighbours. We then eliminate much of the ambiguity in merging sets of particles together by identifying every possible density peak, and measuring the probability that each does not arise from Poisson noise. The main halo in a cluster tends to have a high probability, while its subhaloes tend to have lower probabilities. The first parameter invobozcontrols the subtlety of particle unbinding, and may be eliminated if one is cavalier with processor time; even if one is not, the results saturate to the parameter-free answer when the parameter is sufficiently small. The only parameter that remains, an outer density cut-off, does not influence whether or not haloes are identified, nor does it have any effect on subhaloes; it only affects the masses returned for supercluster haloes. [ABSTRACT FROM AUTHOR]

By comparing the Point Source Catalogue Redshift (PSC z) galaxy power spectrum with the results of nested pure dark matter N-body simulations, we try to understand how infrared-selected galaxies populate dark matter haloes, paying special attention to the method of halo identification in the simulations. We thus test the hypothesis that baryonic physics negligibly affects the distribution of galaxies down to the smallest scales yet observed. We are successful in reproducing the PSC z power spectrum on scales , near our resolution limit, by imposing a central density cut-off on simulated haloes, which gives a rough minimum mass and circular velocity of haloes in which PSC z galaxies formed. [ABSTRACT FROM AUTHOR]

We investigate possible systematic errors in the recent measurement of the matter power spectrum from the Lyman-alpha forest by Croft et al. We find that for a large set of prior cosmological models the Croft et al. result holds quite well, with systematic errors being comparable with random ones, when a dependence of the recovered-matter power spectrum on the cosmological parameters at z∼3 is taken into account. We find that peculiar velocities cause the flux power spectrum to be smoothed over about 100–300 km s[sup -1] , depending on scale. Consequently, the recovered-matter power spectrum is a smoothed version of the underlying true power spectrum. Uncertainties in the recovered power spectrum are thus correlated over about 100–300 km s[sup -1] . As a side effect, we find that residual fluctuations in the ionizing background, while having almost no effect on the recovered-matter power spectrum, significantly bias estimates of the baryon density from the Lyman-alpha forest data. We therefore conclude that the Croft et al. result provides a powerful new constraint on cosmological parameters and models of structure formation. [ABSTRACT FROM AUTHOR]

This article presents information regarding the evolution of universe. Since the first analogue simulation by Erik Holmberg, who used the inverse-square law of light to mimic gravity, numerical cosmology has made remarkable progress, abstract particles and digital supercomputers have now replaced light bulbs and photometers as tools for measuring gravitational forces. An alternative approach to this kind of semi-analytical modelling is to use a hydrodynamic simulation code, which follows the complex swirl of cosmic gas from the largest scales down to the tiny and ultra-dense molecular clouds where stars are born.

We use recent proper motion measurements of the tangential velocity of M31, along with its radial velocity and distance, to derive the likelihood of the sum of halo masses of the Milky Way and M31. This is done using a sample of halo pairs in the Bolshoi cosmological simulation of ΛCDM cosmology selected to match the properties and the environment of the Local Group. The resulting likelihood gives an estimate of the sum of the masses of MMW, 200c + MM31, 200c = (90% confidence interval). This estimate is consistent with individual mass estimates for the Milky Way and M31 and is consistent, albeit somewhat on the low side, with the mass estimated using the timing argument. We show that although the timing argument is unbiased on average for all pairs, for pairs constrained to have radial and tangential velocities similar to that of the Local Group the argument overestimates the sum of masses by a factor of 1.6. Using similar technique, we estimate the total dark matter mass enclosed within 1 Mpc from the Local Group barycenter to be (90% confidence interval). [ABSTRACT FROM AUTHOR]

Cosmic Reionization On Computers is a long-term program of numerical simulations of cosmic reionization. Its goal is to model fully self-consistently (albeit not necessarily from the first principles) all relevant physics, from radiative transfer to gas dynamics and star formation, in simulation volumes of up to 100 comoving Mpc, and with spatial resolution approaching 100 pc in physical units. In this method paper, we describe our numerical method, the design of simulations, and the calibration of numerical parameters. Using several sets (ensembles) of simulations in 20 h–1 Mpc and 40 h–1 Mpc boxes with spatial resolution reaching 125 pc at z = 6, we are able to match the observed galaxy UV luminosity functions at all redshifts between 6 and 10, as well as obtain reasonable agreement with the observational measurements of the Gunn-Peterson optical depth at z < 6. [ABSTRACT FROM AUTHOR]

We compare the results from several sets of cosmological simulations of cosmic reionization, produced under the Cosmic Reionization On Computers project, with existing observational data on the high-redshift Lyα forest and the abundance of Lyα emitters. We find good consistency with the observational measurements and previous simulation work. By virtue of having several independent realizations for each set of numerical parameters, we are able to explore the effect of cosmic variance on observable quantities. One unexpected conclusion we are forced into is that cosmic variance is unusually large at z > 6, with both our simulations and, most likely, observational measurements still not fully converged for even such basic quantities as the average Gunn-Peterson optical depth or the volume-weighted neutral fraction. We also find that reionization has little effect on the early galaxies or on global cosmic star formation history, because galaxies whose gas content is affected by photoionization contain no molecular (i.e., star-forming) gas in the first place. In particular, measurements of the faint end of the galaxy luminosity function by the James Webb Space Telescope are unlikely to provide a useful constraint on reionization. [ABSTRACT FROM AUTHOR]

We discuss the role of recombinations in the intergalactic medium, and the related concept of the clumping factor, during cosmic reionization. The clumping factor is, in general, a local quantity that depends on both the local overdensity and the scale below which the baryon density field can be assumed smooth. That scale, called the filtering scale, depends on over-density and local thermal history. We present a method for building a self-consistent analytical model of inhomogeneous reionization, assuming the linear growth rate of the density fluctuation, which simultaneously accounts for these effects. We show that taking into account the local clumping factor introduces significant corrections to the total recombination rate, compared to the model with a globally uniform clumping factor. [ABSTRACT FROM AUTHOR]

We study the high column density regime of the H I column density distribution function and argue that there are two distinct features: a turnover at NH I ≈ 1021 cm–2, which is present at both z = 0 and z ≈ 3, and a lack of systems above NH I ≈ 1022 cm–2 at z = 0. Using observations of the column density distribution, we argue that the H I-H2 transition does not cause the turnover at NH I ≈ 1021 cm–2 but can plausibly explain the turnover at NH I ≳ 1022 cm–2. We compute the H I column density distribution of individual galaxies in the THINGS sample and show that the turnover column density depends only weakly on metallicity. Furthermore, we show that the column density distribution of galaxies, corrected for inclination, is insensitive to the resolution of the H I map or to averaging in radial shells. Our results indicate that the similarity of H I column density distributions at z = 3 and 0 is due to the similarity of the maximum H I surface densities of high-z and low-z disks, set presumably by universal processes that shape properties of the gaseous disks of galaxies. Using fully cosmological simulations, we explore other candidate physical mechanisms that could produce a turnover in the column density distribution. We show that while turbulence within giant molecular clouds cannot affect the damped Lyα column density distribution, stellar feedback can affect it significantly if the feedback is sufficiently effective in removing gas from the central 2-3 kpc of high-redshift galaxies. Finally, we argue that it is meaningful to compare column densities averaged over ∼ kpc scales with those estimated from quasar spectra that probe sub-pc scales due to the steep power spectrum of H I column density fluctuations observed in nearby galaxies. [ABSTRACT FROM AUTHOR]

The baryonic Tully-Fisher relation is an important observational constraint on cosmological and galactic models. However, it is critical to keep in mind that in observations only stars and molecular and atomic gas are counted, while the contribution of the ionized gas is almost universally missed. The ionized gas is, however, expected to be present in the gaseous disks of dwarf galaxies simply because they are exposed to the cosmic ionizing background and to the stellar radiation that manages to escape from the central regions of the galactic disks into their outer layers. Such an expectation is, indeed, born out both by cosmological numerical simulations and by simple analytical models. [ABSTRACT FROM AUTHOR]