The transition from the "dark ages" in the history of the Universe to "cosmic dawn" saw the formation of the first autonomous sources of radiation, stars and black holes. Excitingly, we are at the verge of a revolution of research into the transition from the dark ages to cosmic dawn. The launch of the James Webb Space Telescope (JWST) in October 2021 will enable an unprecedented epochal leap forward. JWST will offer the very first opportunity to find and fully characterize the first galaxies and to explore the nature of the sources of reionization, with the ultimate goal of detecting the signatures of the very first generation of stars. Determining these physical properties will rely heavily on comparison with models and simulations tailored to the data. In this project, we propose to perform dedicated hydrodynamical cosmological simulations to guide the interpretation of these complex data sets, allowing to extract at best their physical content. Close collaboration with observers involved in current and future high-z galaxy surveys with JWST is envisaged.
The advanced theoretical models that we are developing will be an essential tool to exploit the enormous wealth of data coming from JWST. In particular, members of our collaboration (see the box below) are key participants of two JWST Early Release Science (ERS) programs, which are committed to release data and pipelines in very short time ( 6. In particular, it will allow us to measure:
- the UV luminosity density at z > 9, solving a long-standing tension between existing analysis;
- the size distribution of z > 7 galaxies, which is an interesting result by itself and it is also crucial to determine the faint slope of the UV luminosity function, that is subject to morphology-dependent corrections for incompleteness;
- the UV luminosity function up to z > 10, and especially its faint end. Our goal is to constrain the slope of the luminosity function and the presence of possible cut-offs from ultra-faint galaxy samples in the GLASS lensed fields. At the same time, a consistent measurement of its bright end will be made possible by the clean selection of high-z galaxies in the CEERS area.
In addition, exploiting the capability of JWST of detecting galaxies in the optical rest-frame, we will be able to estimate dust absorption on the entire sample at z > 5, using advanced SED-fitting tools along with information on the stellar mass, SFR, age and SFH. We will be able to detect dust-enshrouded galaxies at z > 5, which are missed by UV selection. Combining these measurements, we will obtain a sound and unbiased estimate of the SFR density at z = 7 - 12 (see Fig. 4, right).
Finally, the third approach is based on the direct measurement of the stellar mass. So far, measurements obtained at z > 6 are unreliable, since they are not based on accurate rest-frame optical bands and secure spectroscopic/photometric redshifts - exactly what the JWST surveys will provide for the very first time at z > 6.
This enormous wealth of data will be an essential benchmark for our theoretical models. Using zoom-in simulations, we will constrain the physics of feedback-regulated star formation by comparing model predictions with observations of the faint-end slope of the UV LF and with the inferred physical properties of the galaxies at the low-mass end of the galaxy MF (see Fig. 4, left). Accurate knowledge of SFR, stellar mass, gas metallicity for large samples of galaxies at z > 6 will tightly constrain the SFH of individual galaxies predicted by CAT and dustyGADGET cosmological simulations, and its dependence on stellar mass, redshift and on the environment where the galaxy is residing.
In addition, JWST data will enable us to measure the dust content and gas metallicity for hundreds of galaxies at z > 6 by combining NIRCam photometry and NIRSpec nebular emission lines (rest-frame optical and UV), using photoionization models and empirical calibrations. JWST photometric surveys will deliver high S/N data in 4 (3) bands in the rest-frame UV for a thousand (hundredth) z > 7 (> 9) galaxies, by which we will reliably measure the UV slope, which is a sensitive proxy of the dust content and (to a lesser extent) metallicity of the stellar population. While slopes -2.5) measure the level of dust enrichment in the ISM (see Fig. 5, left). We note that the detectability of Pop III stars with JWST is still debated, depending on the specific model adopted, but it is obvious that the detection of primeval galaxies hosting Pop III stars (which would rely also on the absence of metal lines and prominent HeII, all detectable with JWST) would be a major breakthrough in astrophysics. Using CAT we will explore the sensitivity of the model predictions on the unknown Pop III stellar IMF, while dustyGADGET zoom-in simulations will provide tight constraints on the physics governing the efficiency and duration of Pop III star formation. We will obtain key information on the metal and dust content of z > 6 galaxies which is a key diagnostic of theoretical models, providing tight constraints on the SFHs and the integrated effects of inflows/outflows, particularly in the early Universe. The ultimate goal is to identify galaxies of different metallicities to be compared with the detailed models of dust formation in the early Universe provided by dustyGADGET (see Fig. 5, right).