The 21cm Signature of Early Relic \HII Regions

TL;DR

This paper models the 21 cm brightness of relic extHII regions around the first stars, showing they could produce detectable signals in the early universe's 21 cm background, despite individual patches being unresolved.

Contribution

It provides the first detailed simulation-based predictions of the 21 cm signatures of relic extHII regions from the first stars.

Findings

Relic extHII regions can reach brightness temperatures of ~100 mK.

Individual patches are unresolved by current telescopes, but collective signals may be detectable.

Relic extHII regions influence the 21 cm background fluctuations.

Abstract

We calculate the spin temperature and 21 cm brightness of early \HII regions around the first stars. We use outputs from cosmological radiation-hydrodynamics simulations of the formation and evolution of early \HII regions. In the pre-reionization era, \HII regions around massive primordial stars have diameters of a few kpc. The gas within the \HII regions is almost fully ionized, but begins recombining after the central stars die off. The relic \HII regions are then seen as bright {\it emission} sources in hydrogen 21 cm. We make brightness temperature maps of the \HII regions, accounting for radiative coupling with Lyman-$\alpha$ photons in a simplified manner. The spin temperature in the relic \HII region is close to the gas kinetic temperature, generally several hundred to several thousand degrees. We show that the relic \HII region can be as bright as $\delta T_{\rm b} \sim 100$ mK in differential temperature against the cosmic microwave background for an angular resolution of sub-arcseconds. While individual early \HII patches will not be identified by currently planned radio telescopes, the collective fluctuations from early \HII regions might imprint signatures in the 21 cm background.