Hydrodynamics and radiation in galaxies of the early universe – ORAGE

The Epoch of Reionization (EoR) is finally coming under the scrutiny of our instruments. The EoR is a period in the history of the universe spanning the first billion years (from z=20-30 to z=6). It begins when the light emitted by the first stars born from the gravitational collapse of primordial density fluctuations, start to ionize the surrounding intergalactic medium (IGM). The resulting ionized regions grow until they overlap, finally confining neutral hydrogen into small dense clumps embedded in vast diffuse regions of ionized gas by z~6. During this process the large scale properties of the universe are strongly connected to the small-scale physics of galaxy formation, making it complex to understand and model.
Modeling the EoR, however, is vital as observations are becoming available and will improve drastically in terms of quality and quantity in the next few years. To mention but a few: observations with the WFC3 on HST have unveiled the galaxy luminosity function at z~8 and will improve much with the JWST, and 21cm observations of the neutral intergalactic medium are just around the corner, with LOFAR starting its survey in 2013-2014 and the SKA being built for 2020. The main driver of the ORAGE project is to make a large contribution to our understanding of the EoR though numerical simulations, giving us the theoretical understanding necessary to interpret the up-coming observations.

The key process setting the pace of global reionization is the photon budget in primordial galaxies. How many are produced, how many escape into the IGM, how many are absorbed locally and possibly photo-evaporate the forming galaxy? Observations are scarce and the few existing simulations do not reach a consensus. The first task of ORAGE is to achieve a robust modeling of this photon budget. To this end, we will run radiative hydrodynamics (RHD) simulations. The members of this project have developed two (LICORICE and RAMSES-RT) of the less than ten existing codes for RHD in a cosmological context.
We will develop a convergence project consisting in a series of tests in a cosmological setting and of increasing complexity in terms of modeling. We will run them with both codes and work on numerical methods until we reach convergent results. We will then set up a web site offering the tests for download and the possibility for developers to upload results of other codes, existing of new.

Then, running a suite of very high-resolution simulations in sufficient volumes, we will study the properties of the sources of reionization. We intend to establish beyond current uncertainties how the fraction of ionizing continuum escaping from primordial galaxies depends on various environmental properties such as the mass of the host halo. Quantifying this behavior as a function fesc(M,…) will be an invaluable tool for interpreting observations or reusing as sub-grid physics in large scales simulations. We will also derive the stellar continuum and Lyman-alpha line emissions of a sample of objects at different z. This post-processing, using state-of-art population synthesis and detailed radiative transfer in the Lyman-alpha line, will provide observable quantities usable as templates to interpret observations with MUSE/VLT and JWST.

We will also use our highly resolved simulations to study the reionization of the Milky Way. The satellites of the MW can serve as an archeological probe of the EoR: the impact of reionization on their properties can still be observed today. Moreover, reionization can be a solution to the well-known missing satellites problem. While RHD is necessary to study the formation of the first galaxies, it has not yet been included in studies of the reionization of the MW: we intend to do it.

Finally we will implement the progresses made in our understanding of the small scale processes (such as fesc(M) ) into large scale simulations designed to produce robust simulated 21cm data cubes in preparation for observation by the SKA.