B. Ciardi

The first cosmic structures and their effects

Despite 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.

Cosmological radiative transfer codes comparison project - I. The static density field tests

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.

Foreground simulations for the LOFAR - Epoch of Reionization Experiment

Future high-redshift 21-cm experiments will suffer from a high degree of contamination, due both to astrophysical foregrounds and to non-astrophysical and instrumental effects. In order to reliably extract the cosmological signal from the observed data, it is essential to understand very well all data components and their influence on the extracted signal. Here we present simulated astrophysical foregrounds data cubes and discuss their possible statistical effects on the data. The foreground maps are produced assuming 5 degrees x 5 degrees windows that match those expected to be observed by the LOFAR epoch of reionization (EoR) key science project. We show that with the expected LOFAR-EoR sky and receiver noise levels, which amount to approximate to 52 mK at 150 MHz after 400 h of total observing time, a simple polynomial fit allows a statistical reconstruction of the signal. We also show that the polynomial fitting will work for maps with realistic yet idealized instrument response, i.e. a response that includes only a uniform uv coverage as a function of frequency and ignores many other uncertainties. Polarized Galactic synchrotron maps that include internal polarization and a number of Faraday screens along the line of sight are also simulated. The importance of these stems from the fact that the LOFAR instrument, in common with all current interferometric EoR experiments, has an instrumentally polarized response.

The transition from population III to population II-I star formation

We present results from the first cosmological simulations which study the onset of primordial, metal-free (population III), cosmic star formation and the transition to the present-day, metalrich star formation (population II-I), including molecular (H2, HD, etc.) evolution, tracing the injection of metals by supernovae (SNe) into the surrounding intergalactic medium and following the change in the initial mass function (IMF) according to the metallicity of the corresponding stellar population. Our investigation addresses the role of a wide variety of parameters (critical metallicity for the transition, IMF slope and range, SN/pair-instability SN metal yields, star formation threshold, resolution, etc.) on the metal-enrichment history and the associated transition in the star formation mode. All simulations present common trends. Metal enrichment is very patchy, with rare, unpolluted regions surviving at all redshifts, inducing the simultaneous presence of metal-free and metal-rich star formation regimes. As a result of the rapid pollution within high-density regions due to the first SN/pair-instability SN, local metallicity is quickly boosted above the critical metallicity for the transition. For this reason, population III stars dominate only during the very first stages of structure formation, with an average contribution to the total star formation rate that reaches a constant value of ∼10 −3 at redshift ∼11–13. If primordial supenovae consisted only of type II ones, the contribution would be ∼10 −1 . Interestingly, the above conclusions are independent from many poorly constrained

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.

Simulating intergalactic medium reionization

We have studied the IGM reionization process in its full cosmological context including structure evolution and a realistic galaxy population. We have used a combination of high-resolution N-body simulations (to describe the dark matter and diffuse gas component), a semi-analytic model of galaxy formation (to track the evolution of the sources of ionizing radiation) and the Monte Carlo radiative transfer code CRASH (to follow the propagation of ionizing photons into the IGM). The process has been followed in the largest volume ever used for this kind of study, a field region of the universe with a comoving length of L~20/h Mpc, embedded in a much larger cosmological simulation. To assess the effect of environment on the reionization process, the same radiative transfer simulations have been performed on a 10/h Mpc comoving box, centered on a clustered region. We find that, to account for the all ionizing radiation, objects with total masses of M~10^9 Msun must be resolved. In this case, the simulated stellar population produces a volume averaged ionization fraction x_v=0.999 by z~8, consistent with observations without requiring any additional sources of ionization. We also find that environment substantially affects the reionization process. In fact, although the simulated proto-cluster occupies a smaller volume and produces a higher number of ionizing photons, it gets totally ionized later. This is because high density regions, which are more common in the proto-cluster, are difficult to ionize because of their high recombination rates.

CRASH: a Radiative Transfer Scheme

We 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 Stromgren sphere; (ii) realistic Stromgren 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.

Metal and molecule cooling in simulations of structure formation

Cooling 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 ga s. We discuss the implementation in Gadget-2 of molecular and metal cooling at temperatures lower that 10 4 K, following the time dependent properties of the gas and pollution from stellar evolution. We have checked the validity of our scheme comparing the results of some test runs with previous calculations of cosmic abundance evolution and structure formation, find ing 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 significan t impact on the formation and evolution of cold objects.

Power spectrum extraction for redshifted 21-cm Epoch of Reionization experiments: the LOFAR case

One of the aims of the Low Frequency Array (LOFAR) Epoch of Reionization (EoR) project is to measure the power spectrum of variations in the intensity of redshifted 21-cm radiation from the EoR. The sensitivity with which this power spectrum can be estimated depends on the level of thermal noise and sample variance, and also on the systematic errors arising from the extraction process, in particular from the subtraction of foreground contamination. We model the extraction process using realistic simulations of the cosmological signal, the foregrounds and noise, and so estimate the sensitivity of the LOFAR EoR experiment to the redshifted 21-cm power spectrum. Detection of emission from the EoR should be possible within 360 h of observation with a single station beam. Integrating for longer, and synthesizing multiple station beams within the primary (tile) beam, then enables us to extract progressively more accurate estimates of the power at a greater range of scales and redshifts. We discuss different observational strategies which compromise between depth of observation, sky coverage and frequency coverage. A plan in which lower frequencies receive a larger fraction of the time appears to be promising. We also study the nature of the bias which foreground fitting errors induce on the inferred power spectrum and discuss how to reduce and correct for this bias. The angular and line-of-sight power spectra have different merits in this respect, and we suggest considering them separately in the analysis of LOFAR data.

The interplay between chemical and mechanical feedback from the first generation of stars

We study cosmological simulations of early structure formation, including non-equilibrium molecular chemistry, metal pollution from stellar evoluti on, transition from population III (popIII) to population II (popII) star formation, regulate d by a given critical metallicity, and feedback effects. We perform analyses of the properties of t he gas, and use the popIII and popII populations as tracers of the metallicity. This allow s us to investigate the properties of early metal spreading from the different stellar populatio ns and its interplay with pristine molecular gas, in terms of the initial mass function and crit ical metallicity. We find that, independently of the details about popIII modeling, after t he onset of star formation, regions enriched below the critical level are mostly found in isolat ed environments, while popII star formation regions are much more clumped. Typical star forming haloes, at z � 15 10, with masses between � 10 7 10 8 M⊙, show average SN driven outflow rates of up to � 10 −4 M⊙/yr in enriched gas, initially leaving the original star format ion regions almost devoid of metals. The polluted material, which is gravitati onally incorporated in over-dense environments on timescales of � 10 7 yr, is mostly coming from external, nearby star forming sites (“gravitational enrichment”). In parallel, the pris tine-gas inflow rates are some orders of magnitudes larger, between � 10 −3 10 −1 M⊙/yr. However, thermal feedback from SN destroys molecules within the pristine gas hindering its ab ility to cool and to condense into high-density star forming regions. Only the polluted mater ial incorporated via gravitational enrichment can continue to cool by atomic fine-structure tra nsitions on short time scales, short enough to end the initial popIII regime within less tha n 10 8 yr. Moreover, the interplay between the pristine, cold, infalling gas and the ejected, h ot, metal-rich gas leads to turbulent Reynolds numbers of the order of � 10 8 10 10 , and contributes to the suppression of pristine inflow rates into the densest, star forming regions.