1,721,253 research outputs found
The first cosmic structures and their effects
No full textDespite much recent theoretical and observational progress in our knowledge of the early universe, many fundamental questions remain only partially answered. Here, we review the latest achievements and persisting problems in the understanding of first cosmic structure formation
CRASH : a radiative transfer scheme
No full textWe present a largely improved version of crash, a 3D radiative transfer code that treats the effects of ionizing radiation propagating through a given inhomogeneous H/He cosmological density field on the physical conditions of the gas. The code, based on a Monte Carlo technique, self-consistently calculates the time evolution of gas temperature and ionization fractions due to an arbitrary number of point/extended sources and/or diffuse background radiation with given spectra. In addition, the effects of diffuse ionizing radiation following recombinations of ionized atoms have been included. After a complete description of the numerical scheme, to demonstrate the performance, accuracy, convergence and robustness of the code, we present four different test cases designed to investigate specific aspects of radiative transfer: (i) a pure-hydrogen isothermal Strömgren sphere; (ii) realistic Strömgren spheres; (iii) multiple overlapping point sources; and (iv) shadowing of background radiation by an intervening optically thick layer. When possible, detailed quantitative comparison of the results against either analytical solutions or 1D standard photoionization codes has been made, and shows a good level of agreement. For more complicated tests the code yields physically plausible results, which could be eventually checked only by comparison with other similar codes. Finally, we briefly discuss future possible developments and cosmological applications of the code
Radiative transfer of ionizing radiation through gas and dust: the stellar source case
No full textWe present a new dust extension to the Monte Carlo radiative transfer code CRASH, which enables it to simulate the propagation of ionizing radiation through mixtures of gas and dust. The newcode is applied to study the impact of dust absorption on idealized galacticHII regions and on small-scale reionization. We find that HII regions are reduced in size by the presence of dust, while their inner temperature and ionization structure remain largely unaffected. In the small-scale reionization simulation, dust hardens ionization fronts and delays the overlap of ionized bubbles. This effect is found to depend only weakly on the assumed abundance of dust in underdense regions
Early reionization by the first galaxies
Full text linkLarge-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, τe∼ 0.17. The reionization of the Universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results (Ωm= 0.3, ΩΛ= 0.7, h= 0.7, Ωb= 0.04, n= 1 and σ8= 0.9). Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with ionizing photon production as expected for zero-metallicity stars distributed according to a standard stellar initial mass function (IMF) (1061 photons per unit solar mass of formed stars) and with a moderate photon escape fraction from galaxies (5 per cent), produces τe= 0.104, which is within 1.0 to 1.5σ of the ‘best’WMAP value. Values of up to 0.16 can be produced by taking larger escape fractions or a top-heavy IMF. The data do not require a separate populations of ‘miniquasars’ or of stars forming in objects with total masses below 109 M⊙. Reconciling such early reionization with the observed Gunn–Peterson troughs in z > 6 quasars may be challenging. Possible resolutions of this problem are discussed
CRASH3: cosmological radiative transfer through metals
No full textHere we introduce CRASH3, the latest release of the 3D radiative transfer code CRASH. In its current implementation, CRASH3 integrates into the reference algorithm the code CLOUDY to evaluate the ionization states of metals, self-consistently with the radiative transfer through H and He. The feedback of the heavy elements on the calculation of the gas temperature is also taken into account, making CRASH3 the first 3D code for cosmological applications which treats self-consistently the radiative transfer through an inhomogeneous distribution of metalenriched gas with an arbitrary number of point sources and/or a background radiation. The code has been tested in idealized configurations, as well as in a more realistic case of multiple sources embedded in a polluted cosmic web. Through these validation tests, the new method has been proven to be numerically stable and convergent. We have studied the dependence of the results on a number of physical quantities such as the source characteristics (spectral range and shape, intensity), the metal composition, the gas number density and metallicity. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
CRASH3: Cosmological radiative transfer through metals
No full textHere we introduce CRASH3, the latest release of the 3D radiative transfer code CRASH. In its current implementation, CRASH3 integrates into the reference algorithm the code CLOUDY to evaluate the ionization states of metals, self-consistently with the radiative transfer through H and He. The feedback of the heavy elements on the calculation of the gas temperature is also taken into account, making CRASH3 the first 3D code for cosmological applications which treats self-consistently the radiative transfer through an inhomogeneous distribution of metalenriched gas with an arbitrary number of point sources and/or a background radiation. The code has been tested in idealized configurations, as well as in a more realistic case of multiple sources embedded in a polluted cosmic web. Through these validation tests, the new method has been proven to be numerically stable and convergent. We have studied the dependence of the results on a number of physical quantities such as the source characteristics (spectral range and shape, intensity), the metal composition, the gas number density and metallicity. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society
Gravitational Wave Signals from the Collapse of the First Stars
Full text linkWe study the gravitational wave emission from the first stars, which are assumed to be very massive objects (VMOs). We take into account various feedback (both radiative and stellar) effects regulating the collapse of objects in the early Universe and thus derive the VMO initial mass function and formation rate. If the final fate of VMOs is to collapse, leaving very massive black hole remnants, then the gravitational waves emitted during each collapse would be seen as a stochastic background. The predicted spectral strain amplitude in a critical density cold dark matter (CDM) universe peaks in the frequency range ν~5×10-4-5×10-3Hz, where it has a value in the range ~10-20-10-19Hz-1/2, and might be detected by the Laser Interferometer Space Antenna (LISA). The expected emission rate is roughly 4000eventyr-1, resulting in a stationary discrete sequence of bursts, i.e. a shot-noise signal
X-ray ionization of the intergalactic medium by quasars
No full textWe investigate the impact of quasars on the ionization of the surrounding intergalactic medium (IGM) with the radiative transfer code CRASH4, now accounting for X-rays and secondary electrons. After comparing with analytic solutions, we post-process a cosmic volume (≈1.5 × 104 Mpc3 h-3) containing a ULAS J1120+0641-like quasar (QSO) hosted by a 5 × 1011M⊙h-1 dark matter (DM) halo. We find that (i) the average H II region (R ~3.2 pMpc in a lifetime tf = 107 yr) is mainly set by UV flux, in agreement with semi-analytic scaling relations; (ii) a largely neutral (x H II < 0.001), warm (T ~103 K) tail extends up to few Mpc beyond the ionization front, as a result of the X-ray flux; and (iii) LyC-opaque inhomogeneities induce a line of sight (LOS) scatter in R as high as few physical Mpc, consistent with the DLA scenario proposed to explain the anomalous size of the ULAS J1120+0641 ionized region. On the other hand, with an ionization rate Nγ,0 ~1057 s-1, the assumed DLA clustering and gas opacity, only one LOS shows an H II region compatible with the observed one.We deduce that either the ionization rate of the QSO is at least one order of magnitude lower or the ULAS J1120+0641 bright phase is shorter than 107 yr
Metal and molecule cooling in simulations of structure formation
No full textCooling is the main process leading to the condensation of gas in the dark matter potential wells and consequently to star and structure formation. In a metal-free environment, the main available coolants are H, He, H2 and HD; once the gas is enriched with metals, these also become important in defining the cooling properties of the gas. We discuss the implementation in GADGET-2 of molecular and metal cooling at temperatures lower than 104 K, following the time-dependent properties of the gas and pollution from stellar evolution. We have checked the validity of our scheme by comparing the results of some test runs with previous calculations of cosmic abundance evolution and structure formation, finding excellent agreement. We have also investigated the relevance of molecule and metal cooling in some specific cases, finding that inclusion of HD cooling results in a higher clumping factor of the gas at high redshifts, while metal cooling at low temperatures can have a significant impact on the formation and evolution of cold objects
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