ReIonization and Signatures of Early Stars

Understanding how the first stars and galaxies formed and drove the Universe's final phase transition by re-ionizing intergalactic hydrogen is one of the long-standing goals of extragalactic astronomy.

In the past two decades, we have discovered this ``Epoch of Reionization'' ended around one billion years after the Big Bang (redshift, z < 6), but the process is still poorly understood.

We are currently undergoing a radical shift in our ability to observe this epoch with the new James Webb Space Telescope (JWST).

Early JWST observations have discovered a surprising excess of luminous z > 9 galaxies, implying stars may have formed differently in the early universe than we had previously expected. However, we can only observe the brightest galaxies directly.

Enhanced star formationin the bulk of the early galaxy population should have produced an early start to reionization, but this is stillvery poorly constrained observationally.

Excitingly, JWST finally enables us to chart reionization out to itsearliest stages and infer the properties of the first stars.In RISES, I propose to analyse groundbreaking new datasets I am accruing as PI on JWST with state-of-the-art statistical inference frameworks to connect observations of the bright galaxies visible with JWST to the evolution of the intergalactic medium.The key aims of this proposal are to: (1) test what drove the enhancement of star formation in the first bright galaxies; (2) establish when reionization began, to infer whether star formation was enhanced in the bulk of early galaxies; and (3) understand how reionization proceeded by connecting galaxies to ionized regions for the first time.These measurements will push the boundaries of our knowledge of structure formation in the early universe, and enable potential breakthroughs in our understanding of the formation of the first galaxies, the regulation of star formation at the highest redshifts and the evolution of the intergalactic medium.