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Galaxy formation in the first billion years mark a time of great
upheaval in the history of the Universe: as the first sources of light,
these galaxies ended the 'cosmic dark ages' and produced the first
photons that could break apart the hydrogen atoms suffusing all of space
starting the process of cosmic reionization. As the earliest building
blocks, the galaxies that formed in the first billion years also
determine the physical properties of all subsequent galaxy populations.
However fully coupling galaxy formation with large-scale reionization
remains a massive computational challenge as a result of the range in
(mass and volume) scales required. I will start by introducing the
Astraeus framework that is the largest (230 Mpc box) and most highly
resolved (DM resolution mass of 10^6.9 solar masses) simulation to fully
couple a semi-analytic model of galaxy formation with a semi-numerical
scheme for reionization (feedback) to shed light on galaxy formation at
cosmic dawn.I will then show how cross-correlations of 21cm data with
the underlying galaxy population, in the forthcoming era of 21cm
cosmology, will yield tantalising constraints on the average
intergalactic medium ionization state as well as the reionization
topology (outside-in versus inside-out). Finally, time permitting, I
will try to give a flavour of how the assembly of early galaxies,
accessible with the forthcoming James Webb Space Telescope, can provide
a powerful testbed for Dark Matter models beyond "Cold Dark Matter".