Heating the Intergalactic medium by X-Rays from Population III Binaries in High-redshift Galaxies

Summary

Abstract

Xu and collaborators investigate how X-rays produced by Population III binary systems could heat and ionize the early intergalactic medium (IGM). Their models show that X-rays below 1 keV mainly affect gas near their sources, while higher-energy photons travel farther and contribute to a widespread X-ray background. These X-rays could heat the IGM above 100 K by redshift z = 10, but they do not produce enough free electrons to strongly affect the cosmic microwave background.

1. Introduction

• Population III binaries may contain black holes that produce strong X-ray radiation while collecting matter from companion stars.
• Unlike ultraviolet radiation, X-rays can travel long distances through the mostly neutral early universe.
• The paper asks how these X-rays affect the temperature and ionization state of the IGM.

2. Simulation and X-Ray Binary Model

• The authors use the Rarepeak simulation, which contains more than 13,000 Population III stars and remnants in an overdense region.
• They model Population III binaries containing a black hole that produces X-rays by accreting matter from its companion.
• Several photon energies between 300 eV and 3 keV are tested to determine how far the X-rays travel and how strongly they interact with gas.

3. X-Ray Luminosity from Rarepeak

• The total X-ray luminosity increases with the Population III star-formation rate, reaching about 8 × 10⁴² erg s⁻¹ by z = 15.
• Most of the X-ray emission comes from Population III binaries in relatively small halos.
• Longer binary lifetimes produce more X-ray radiation because the black holes remain active for a longer period.

4. X-Ray Heating, Ionization, and Escape

• Low-energy X-rays are absorbed near their sources, where they efficiently heat and ionize the surrounding gas.
• Photons around 1 keV travel into the wider IGM and provide a balance between long-distance travel and efficient heating.
• Higher-energy photons travel farther but interact too weakly with the gas to produce much heating or ionization.

5. Effects of X-Rays on the IGM

• X-ray heating begins to overcome cooling from cosmic expansion at about z = 16.
• Most models heat the average IGM above 100 K by z = 10 and to several hundred kelvin by z = 6.
• X-rays increase the number of free electrons only slightly, so their effect on the cosmic microwave background is small.

6. Discussion and Conclusions

• Population III binaries may have been an important source of X-rays before later sources such as active galactic nuclei became common.
• X-rays are much more effective at heating the early IGM than fully ionizing it.
• This early heating could affect the 21 cm signal produced by neutral hydrogen and help astronomers study the universe before reionization.

Best Figures

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