Mapping Discrete Galaxies at Cosmic Dawn with 21 cm Observations

Abstract

At cosmic dawn, the 21 cm signal from intergalactic hydrogen was driven by Ly-α photons from some of the earliest stars, producing a spatial pattern that reflected the distribution of galaxies at that time. Due to the large foreground, it is thought that at around redshift 20 it is only observationally feasible to detect 21 cm fluctuations statistically, yielding a limited indirect probe of early galaxies. Here, we show that 21 cm images at cosmic dawn should actually be dominated by large (tens of comoving megaparsecs) high-contrast bubbles surrounding individual galaxies. We demonstrate this using a substantially upgraded seminumerical simulation code that realistically captures the formation and 21 cm effects of the small galaxies expected during this era. Small number statistics associated with the rarity of early galaxies, combined with the multiple scattering of photons in the blue wing of the Ly-α line, create the large bubbles, and also enhance the 21 cm power spectrum by a factor of 2–7 and add to it a feature that measures the typical brightness of galaxies. These various signatures of discrete early galaxies are potentially detectable with planned experiments, such as the Square Kilometer Array and the Hydrogen Epoch of Reionization Array, even if the early stars prove to be formed in dark matter halos with masses as low as 108 M ⊙, 10,000 times smaller than the Milky Way halo.