Recherche et étude des premières galaxies dans l’univers – TG_REGALDIS

Constraining the abundance and the properties of the first galaxies is an important challenge for modern cosmology. Distant star-forming sources at redshifts z~7-12 could have been responsible for a significant part of the cosmic reionization. Detailed studies of these 'primordial' systems require the use of new ground-based and space facilities (e.g. Herschel, VLT/Hawk-I, GTC/EMIR, HST/ WFC3), in preparation for future surveys with ALMA. The goal of our project is precisely to take advantage of our privileged access to these key facilities (recently granted or guaranteed time) to build up the first representative spectroscopic sample of galaxies at redshift z>7, together with an additional sample of candidates based on photometric redshifts and wide multi-wavelength coverage. The method proposed is similar to the one successfully developed in our pilot program with ISAAC/VLT (Pello et al. 2004, Richard et al. 206) and HST/ACS+NICMOS (Kneib et al 2004, Richard et al 2008), using lensing clusters as natural gravitational telescopes. We propose a systematic exploration of this new observationally unknown territory. Multi-wavelength coverage should allow us to determine the physical properties of extremely distant sources, such as star formation rate, extinction, stellar population, and possibly also metallicity and IMF. Various other inferences (e.g. on luminosity functions and luminosity density) will also be made from the pure photometric observations of high-z galaxy candidates. The expected results, probing the Universe at ages of ~ 450-800 Myr, should have profound implications on our knowledge of distant possibly even primeval galaxies, galaxy formation, cosmological reionization, and the early Universe. New deep imaging data of blank fields (CFHT/WIRCAM WUDS Survey) and eight lensing clusters will be used, coming from our observing programs recently accepted with Spitzer, Herschel, LABOCA/APEX, and HAWK-I/VLT. We have also access to Chandra X-ray observations, in addition to a large database, including HST images, from our own observing programs and archives. The project also benefits from the new generation of near-IR spectrographs (Flamingos2/Gemini-S and EMIR/GTC) for the precise redshift determination and subsequent emission-line studies. Galaxy candidates at z>7 will be selected from deep optical and near-IR imaging. Other multi-wavelength data (including X-rays, IR, mm bands) will be used to further characterize them. Strong lensing clusters are more efficient than present blank fields for detailed studies of galaxies in the z~7-12 domain. However, both blank and lensing fields are needed in order to constrain the Luminosity Function, allowing us to achieve a complete view of the first star-forming galaxies. Present day ultra-deep surveys (either blank or lensing fields) are dramatically small in terms of effective surface. For this reason, increasing the number of lensing and blank fields with ultra-deep near-IR photometry, as proposed here, is essential to get tighter constraints on the abundance and physical properties of z>7 starburst galaxies. Gathering a representative sample of galaxies at z~7-12 is also of great interest for future observations of primeval galaxies with ALMA.