The present era of high precision cosmology requires a proper treatment of the physical and chemical phenomena occuring in the primordial plasma. For this reason, it is crucial to obtain a description as detailed as possible of the environment of the early universe and to discuss feasible strategies to test theoretical models with present and future observational instrumentation. The basic goals of the project that will be divided into two main areas of interests are listed below (refer to the figure for a block diagram illustrating the various components of the project)

1. From The Recombination Epoch (z∼ 1000) to the Formation of the First Structures (z∼ 10-30)
a.Chemical Evolution
b.Observational Signatures
c.Primordial Star Formation

2.The Impact of Metals and Dust from the First Stars
The massive stars that formed in the first stellar generation were short lived, implying that they quickly enriched the surrounding medium with newly synthesized heavy elements (mainly, CNO) and dust grains. In fact, only very few studies have reported the identification of gas clouds without any trace of heavy elements or with metallicities consistent with enrichment by primordial stars (e.g., LLS1134a and LLS0956B). The second part of the project will explore the consequences of this metal and dust enrichment on the evolution and fragmentation of the gas

a.Gas-phase chemistry
b.Grain Surface Chemistry
c.Observational Signatures

Finally, possible strategies to explore the chemistry in the first galaxies using the more sensitive available and upcoming instruments (e.g. ALMA, LOFAR, JWST and PIXIE) will be amply discussed.

Chemical Evolution

We will extend current calculations concerning the evolution of primordial chemistry including the contribution of non-thermal photons due to the recombination of hydrogen and helium using CosmoRec and adopting a new formalism for the modeling of non-equilibrium features in the primordial gas (Jens Chluba, Carla M. Coppola, Daniele Galli). Together with the experts of quantum chemistry (John Black, Evelyne Roueff, Jonathan Tennyson, external experts: Dario De Fazio, Francesco A. Gianturco), the reaction rates of the most critical formation/destruction and excitation processes will be updated and/or calculated in order to arrive at an accurate description of the chemical evolution of the expanding universe.

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Observational Signatures

The calculations performed as described in the Chemical Evolution section will be used to update previous estimates of the expected signatures of primordial molecules on the CMB, as well as the spectral distortions arising from the recombination photons. We will aim to extend these calculations into the epoch of re-ionization, including the expectation of H₂ line emission resulting from the collapse of primordial halos. As a result, we will provide the necessary sensitivities in order to probe observationally these early structures (Jens Chluba, Dominik Schleicher, Carla M. Coppola, Daniele Galli).

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Primordial Star Formation

The key ingredient in any realistic simulation of primordial star formation is a detailed modeling of chemistry and cooling during gravitational collapse. The resulting thermal evolution dictates the fragmentation scale with a direct impact on the mass spectrum of the first objects. Here, we will pursue an improved calculation including a non-equilibrium treatment and improved reaction rates discussed above for the modeling of these processes (Carla M. Coppola, Savino Longo). While for mini-halos the initial conditions are well understood, we will also explore the effects for the collapse of more massive halos and their attendant strong shocks that develop during virialization. The calculation of the atomic and molecular kinetics in the presence of supersonic shock waves will be carried out for comparison with 3D models (Carla M. Coppola, Daniele Galli, Savino Longo, Francesco Palla). Two different numerical approaches will be followed: one-zone and 3D-models (Kazuyuki Omukai, Dominik Schleicher, external expert: Andrea Ferrara). Finally, while previous calculations showed a strong dependence on the reaction rate of three-body processes, which are fundamental for H₂ formation at high densities, we will take advantage of the most recent determinations of this rate to investigate the impact on the primordial collapse and fragmentation (Kazuyuki Omukai, Francesco Palla). These results will be then used in the study of the observational signatures described above to derive the expected H₂ line emission from haloes of different masses.

Gas-phase chemistry

The kinetics of neutral and ionic partners containing C and O atoms and their molecular combinations will be investigated so that reliable final rates at the relevant temperatures can be established. Starting from theoretical calculations on state-to-state reaction rates, the kinetics for vibrational levels of the most abundant and relevant molecular species (namely, CO) will be determined under physical conditions typical for clouds of low metallicity. The gas-phase reactions will be modelled for very low metallicity clouds and the formation of CO starting from excited atomic states will be studied (Jonathan Tennyson, external experts: Vincenzo Aquilanti, Francesco A. Gianturco).

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Grain Surface Chemistry

Dust grains are excellent radiators and do provide strong cooling at high densities. They also act as efficient catalysts for surface reactions that initiate the formation of more complex species (including H₂). The effects of the presence of the first dust grains on molecular chemistry will be investigated in order to verify the existence of a transition in the mass scale of the objects formed in clouds of progressively higher metallicity/dust content (Kazuyuki Omukai, external expert: Raffaella Schneider). At present, it is still not completely clear whether the formation of the first low-mass stars requires a minimum level of enrichment in metals and dust grains. The fact that the C-enhanced metal poor stars of the Galaxy are more frequently found at lower metallicities has provided support to a transition in the mass scale driven by metal fine structure line cooling. However, the most recent observational findings on extremely metal poor stars have challenged this interpretation in favor of one dominated by dust cooling. Resolving this issue is one of the important goals of the project.

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Observational Signature

The early chemical enrichment can be observed both in primeval galaxies (i.e., Damped Lyman-α galaxies), in absorption along the lines of sight to high-redshift quasars, and in the extremely metal poor galactic stars which are the relics of the primordial ages of the Milky Way. The first two methods allow to uncover regions of the universe at z ∼6 with essentially primordial enrichment. Measurements of H₂ and HD abundances in these systems will allow us to compare the observed values with the predictions of the chemical evolution models and primordial nucleosynthesis, keeping into account the sensitivity of these molecules to the ambient conditions (UV background, low dust content, etc.) (Daniele Galli, Paolo Molaro, Francesco Palla). Traces of the early enrichment can also be found within the oldest stellar populations in the present universe. In particular, the chemical composition of low-mass dwarfs is the "fossil" record of the chemical composition of the gas out of which they were formed. This is because the atmosphere of dwarf stars is not mixed with the internal gas, thus reflecting the original composition. The recent discovery by Caffau and collaborators of SDSS J102915 with truly primitive composition with abundances in the range 10^-5-10^-4 of the solar value for all the elements measured in its spectrum has provided the strongest constraints on the processes leading to the formation of low-mass stars in extremely metal poor gas. The search for other stars similar to, or even of lower metallicity than SDSS J102915 is going on with a dedicated survey at ESO led by E. Caffau. We expect that significant results in this field will be obtained during the course of this project and that they will be amply debated at the ISSI meetings. We also plan to study the spectra of these EMP stars in order to verify the presence of possible signatures of the chemical non-equilibrium processes discussed in the previous section (Elisabetta Caffau, Carla M. Coppola, Paolo Molaro, Raffaella Schneider)

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