Galactic wind X-ray heating of the intergalactic medium during the Epoch of Reionization

TL;DR

This study models the impact of galactic wind X-ray emission on heating the intergalactic medium during the Epoch of Reionization, predicting observable 21 cm signals that can distinguish between different heating mechanisms.

Contribution

It compares analytic wind models and a cosmological simulation to assess their effects on IGM heating and 21 cm signals during the EoR, incorporating radiative transfer and star formation physics.

Findings

Models predict detectable 21 cm signals during the EoR.

The FiBY simulation shows a prolonged absorption phase until reionization.

Additional heat sources like X-ray binaries can shift the transition to emission to higher redshifts.

Abstract

The diffuse soft X-ray emissivity from galactic winds is computed during the Epoch of Reionization (EoR). We consider two analytic models, a pressure-driven wind and a superbubble model, and a 3D cosmological simulation including gas dynamics from the First Billion Years (FiBY) project. The analytic models are normalized to match the diffuse X-ray emissivity of star-forming galaxies in the nearby Universe. The cosmological simulation uses physically motivated star formation and wind prescriptions, and includes radiative transfer corrections. The models and the simulation all are found to produce sufficient heating of the Intergalactic Medium to be detectable by current and planned radio facilities through 21 cm measurements during the EoR. While the analytic models predict a 21 cm emission signal relative to the Cosmic Microwave Background sets in by $z_{\rm trans} \simeq 8 - 10$, the predicted signal in the FiBY simulation remains in absorption until reionization completes. The 21 cm absorption differential brightness temperature reaches a minimum of $\Delta T \simeq -130$ to $-200$ mK, depending on model. Allowing for additional heat from high mass X-ray binaries pushes the transition to emission to $z_{\rm trans} \simeq 10 - 12$, with shallower absorption signatures having a minimum of $\Delta T \simeq -110$ to $-140$ mK. The 21 cm signal may be a means of distinguishing between the wind models, with the superbubble model favouring earlier reheating. While an early transition to emission may indicate X-ray binaries dominate the reheating, a transition to emission as early as $z_{\rm trans} > 12$ would suggest the presence of additional heat sources.