SPICE: the connection between cosmic reionisation and stellar feedback in the first galaxies

Abstract

Abstract We present SPICE, a new suite of radiation-hydrodynamic, cosmological simulations targeting the epoch of reionisation. The goal of these simulations is to systematically probe a variety of stellar feedback models, including ”bursty” and ”smooth” forms of supernova energy injection, as well as poorly-explored physical scenarios such as hypernova explosions and radiation pressure on dust. We show that even subtle differences in the behaviour of supernova feedback drive profound differences in reionisation histories, with burstier forms of feedback causing earlier reionisation. However, we also find that some global galaxy properties, such as the dust-attenuated luminosity functions and star formation main sequence, remain degenerate between models. In particular, we show that stellar feedback and its strength determine the morphological mix of galaxies emerging by z = 5 and that the reionisation history is inextricably connected to intrinsic properties such as galaxy kinematics and morphology. While star-forming, massive disks are prevalent if supernova feedback is ”smooth”, ”bursty” feedback preferentially generates dispersion-dominated systems. Different modes of feedback produce different strengths of outflows, altering the interstellar/circumgalactic medium in different ways, and in turn strongly affecting the escape of Lyman continuum (LyC) photons. We establish a correlation between galaxy morphology and LyC escape fraction, revealing that dispersion-dominated systems have escape fractions 10-50 times higher than their rotation-dominated counterparts at all redshifts. At the same intrinsic luminosity, dispersion-dominated systems should thus preferentially generate large HII regions as compared to their rotation-dominated counterparts. Since dispersion-dominated systems are more prevalent if stellar feedback is more explosive, reionisation occurs earlier in our simulation with burstier feedback. We argue that statistical samples of post-reionisation galaxy morphologies (using both stellar and gaseous components) probed with telescopes such as JWST, ALMA and MUSE can constrain stellar feedback at z > 5 and models of cosmic reionisation.