Spatially Extended 21 cm Signal from Strongly Clustered Uv and X-Ray Sources in the Early Universe

Summary

Abstract

Ahn and collaborators predict the 21 cm signal produced by a strongly clustered group of early galaxies called Rarepeak. Ultraviolet radiation couples neutral hydrogen to the gas temperature, while X-rays heat the surrounding intergalactic medium and change whether the 21 cm signal appears in emission or absorption. The signal extends across roughly 10 arcminutes and could be imaged by the Square Kilometre Array (SKA) between redshifts z = 13 and 17 with about 1,000 hours of observation.

1. Introduction

• The redshifted 21 cm line allows astronomers to study neutral hydrogen during cosmic dawn and the beginning of reionization.
• Ultraviolet radiation can create an absorption signal, while X-rays from Population III binaries can heat the gas and produce emission.
• The paper asks whether a large cluster of early radiation sources could create a spatially extended and detectable 21 cm signal.

2. Rarepeak Simulation and Radiation Transfer

• Rarepeak is an overdense region containing thousands of halos and more than 10,000 Population III stars and remnants by z = 15.
• The model includes ultraviolet radiation from stars and X-rays from Population III binary systems containing accreting black holes.
• Several X-ray energies and a combined spectrum are tested to see how efficiently the radiation travels through and heats the IGM.

3. The 21 cm Signal from Rarepeak

• Rarepeak produces a large, roughly spherical 21 cm feature with a central emission region that can be surrounded by an absorption trough.
• Lower-energy X-rays heat the nearby IGM efficiently and can weaken or erase the surrounding absorption signal.
• Higher-energy X-rays interact less with the gas, allowing a stronger absorption trough to remain around the central region.

4. Detectability with Radio Telescopes

• SKA could image Rarepeak at redshifts between approximately 13 and 17 using a two-arcminute beam and about 1,000 hours of observation.
• Real-space imaging is more promising than searching for Rarepeak through the overall 21 cm power spectrum.
• Earlier radio arrays would have difficulty separating the signal of rare peaks from signals produced by other sources and ordinary density fluctuations.

5. Summary and Discussion

• The shape of the 21 cm signal provides information about the balance between ultraviolet coupling and X-ray heating in early galaxies.
• A visible absorption trough would suggest that the surrounding IGM remained cold, while its absence would indicate stronger X-ray heating.
• Imaging regions like Rarepeak could help constrain the luminosity and X-ray spectra of some of the universe’s earliest sources.

Best Figures

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