The Impact of the First Galaxies on Cosmic Dawn and Reionization.

The formation of the first galaxies during cosmic dawn and reionization (at redshifts $z=5-30$), triggered the last major phase transition of our universe, as hydrogen evolved from cold and neutral to hot and ionized. The 21-cm line of neutral hydrogen will soon allow us to map these cosmic milestones and study the galaxies that drove them. To aid in interpreting these observations, we upgrade the public code 21cmFAST, improving the treatment of feedback in molecular-cooling galaxies. We introduce a new, flexible parametrization of the additive feedback from: (i) an inhomogeneous, $H_2$-dissociating (Lyman-Werner; LW) background; and (ii) dark matter -- baryon relative velocities. We demonstrate that our flexible model can recover results from recent, small-scale hydrodynamical simulations. We perform a large (1.5 comoving Gpc on a side), "best-guess" simulation as the 2021 installment of the Evolution of 21-cm Structure (EOS) project. This improves on the previous EOS release by using an updated galaxy model that reproduces the observed UV luminosity functions (UVLFs), and by including an additional population of molecular-cooling galaxies. The resulting 21-cm global signal and power spectrum are significantly weaker than in the 2016 EOS releases, due to a more rapid evolution of the star-formation rate density required to match the UVLFs. Nevertheless, we forecast high signal-to-noise detections for both HERA and the SKA. We demonstrate how the stellar-to-halo mass relation of the unseen, first galaxies can be inferred from the evolution of 21-cm fluctuations. Finally, we show that the spatial modulation of X-ray heating due to the relative velocities provides a unique acoustic signature that is detectable at $z \approx 10-15$ in our fiducial model. Ours are the first public simulations with joint inhomogeneous LW and relative-velocity feedback across cosmic dawn and reionization.