Formation of the first galaxies and
the connection to reionization

With Harvard graduate student Rohan Naidu, we have investigate what current constraints of the neutral fraction in the universe in the universe imply for escape of ionizing photons of galaxies (Naidu, Tacchella, et al. 2020). Using the Tacchella et al. 2018 galaxy model, we find that bright galaxies need to play in important role in driving reionization, since only those galaxies can lead to a rather late, but rapid reionization history (see figure on the right). We propose a physically motivated model relating the escape fraction of galaxies to the star-formation rate surface density. Since the star-formation rate surface densities of galaxies falls by ~2.5 dex between z=8 and z=0, our model explains the humble upper limits on escape fractions at lower redshifts and its required evolution to escape fractions of 20% at z>6. Within this model, strikingly, <5% of galaxies with stellar masses of > 1e8 account for 80% of the reionization budget. We predict that bright Lyman-Continuum leakers (like COLA1) will become increasingly common toward z∼6 and that the drivers of reionization do not lie hidden across the faint end of the luminosity function but are already known to us.

With Harvard under-graduate student Diana Khimey, we have studies how the future observations by the James Webb Space Telescope (JWST) can constrain the nature of dark matter. In a Warm Dark Matter universe the formation of the first structure is expected to be delayed relative to a Cold Dark Matter universe. In Khimey, Bose & Tacchella 2021, we use a semi-empirical model of galaxy formation (Tacchella et al. 2018) to investigate the extent to which uncertainties in the implementation of baryonic physics may be degenerate with the predictions of two different models of dark matter: Cold Dark Matter (CDM) and a 7 keV sterile neutrino, which behaves as Warm Dark Matter (WDM). Our models are calibrated to the observed UV luminosity function at 𝑧 = 4 using two separate dust attenuation prescriptions, which manifest as high and low star formation efficiency in low mass halos. We find that while at fixed star formation efficiency, there are marked differences in the abundance of faint galaxies in the two dark matter models at high-z, these differences are mimicked easily by varying the star-formation efficiency in the same dark matter model (see figure on the left). Our results suggest that JWST will likely be more informative in constraining baryonic processes operating in high-z galaxies than it will be in constraining the nature of dark matter.