Presentation #126.04 in the session Elliptical and Other Galaxies.
A fundamental requirement to reionizing the Universe is that a sufficient fraction of the ionizing photons emitted by the first stars and galaxies successfully escapes into the intergalactic medium to transform the neutral hydrogen into a hot and ionized plasma. However, due to the scarcity of direct high-redshift observational data, the main mechanisms driving the Epoch of Reionization remain uncertain. The THESAN project combines the state-of-the-art AREPO-RT radiation-hydrodynamic solver with the IllustrisTNG galaxy formation model to self-consistently simulate both small-scale galaxy physics and large-scale reionization processes throughout a large patch of the Universe (Lbox= 95.5 cMpc). The suite successfully matches key observational data, and thus serves as an excellent tool to investigate the physics during the Epoch of Reionization. In this work, we analyze the highest-resolution THESAN simulation with a Monte-Carlo radiative transfer solver, the Cosmic Lya Transfer code COLT, to calculate the ionizing escape fractions of galaxies at z > 5.5. The large simulation volume allows the formation of numerous massive galaxies (Mhalo > 1010 Msun), which are often statistically underrepresented in previous comparable studies but are believed to be the driving forces in the popular late-reionization model. We find that THESAN escape fractions are typically maximal for low mass haloes (Mhalo < 109 Msun) but decrease rapidly at that point to ~ 1% with a slight increase for the most massive haloes with a low-dust fraction. Moreover, we find a strong dependence of the escape fraction on combinations of star-formation rate, gas mass reservoir, and physical escape size. Most importantly, we conclude that massive galaxies (Mhalo > 1010 Msun) play the dominant role in driving reionization below z~8 due to their high level of intrinsic star formation even though the median escape fractions in this range are relatively low.