SPICE connects stellar feedback in the first galaxies and cosmic reionisation

The first billion years saw the transformation of a cold neutral Universe to a hot and ionised one. This Epoch of Reionisation is thought to come about from stellar radiation from the first galaxies. Understanding the nature of the galaxies that drove reionisation remains a key question. Scientists at MPA have designed a novel suite of simulations to systematically understand how different modes of energy and mass injection from stars affect the first galaxies. According to these new models, subtle differences in the behaviour of stellar feedback drive profound differences in the morphologies of galaxies and the speed at which they ionise the universe. Combining these findings with the latest observations will help constrain feedback models in the first billion years of the Universe.

Some 300,000 years after the Big Bang, for the first time, protons and free electrons formed hydrogen and helium atoms – the Universe was mostly neutral. In the following billion years, however, the vast majority of hydrogen became ionised. This transition from a neutral to an ionised state is known as the "epoch of reionisation". For this to happen, ionising photons had to escape from galaxies. Massive stars emit ionising radiation and at the end of their lives, they die in supernova explosions, releasing mass and energy into gas around them. These processes are termed "stellar feedback", and change the physical structure of the so-called interstellar medium. How this stellar feedback changes the properties of the first galaxies and in particular the gas within these galaxies is poorly understood, but it is key to interpret the recent flood of observations of the early Universe from the James Webb Space telescope.

In the last decade, advancements in high performance computing and theoretical models have allowed scientists to reproduce observations of the first billion years of the Universe. Even though these sophisticated simulations were able to combine radiation and hydrodynamics, due to the computational costs, they typically explore only one of the many possible models of stellar feedback. A team of researchers at MPA led by Aniket Bhagwat, therefore, developed a new suite of simulations called SPICE to systematically test different models and, in particular, to look at connections between stellar feedback and the properties of galaxies that drive reionisation, which could be tested through observations.

The SPICE simulations follow the hydrodynamical evolution of gas, including key processes like star formation and supernova explosions, while tracking the propagation of radiation emitted by those stars. In particular, they include three different modes of stellar feedback: "bursty", where supernovae explode in clustered bursts; "smooth", where supernova explosions are spread out in time; or a mix of the two. And indeed, the simulations show that differences in the strength and behaviour of feedback can have dramatic effects on the morphology of the galaxies. Bursty feedback preferentially produces red and passive galaxies (elliptical-like), whereas smooth feedback produces mostly galaxies that are blue and star forming (spiral-like).

What implications does this have for the process of reionisation? The researchers connected the morphologies of the galaxies to the fraction of ionising photons that manage to leave the galaxy; this quantity is called the “escape fraction”. They find that elliptical-like galaxies show a much higher escape fraction than spiral-like ones (by a factor of 20-50). Why does this happen? The bursts of energy due to the combined supernova explosions are able to substantially disturb the gas and punch low-density holes in the interstellar medium, through which the ionising photons can escape easily. Without bursts, the feedback is unable to disturb the gas enough to allow photons to escape en masse. Therefore, bursty feedback models allow for faster reionisation.

Overall, the new SPICE simulations demonstrate for the first time how sensitive cosmic reionisation and galaxy morphologies are to the mode of stellar feedback, marking a step forward in our understanding of the first billion years of the Universe.

Simulations and data analysis were carried out on the RAVEN and COBRA supercomputers at MPCDF.