Radiation from early black holes - I: effects on the neutral inter-galactic medium

TL;DR

This paper explores how early black hole X-ray emissions could heat and partially ionize the intergalactic medium before reionization, potentially producing observable 21-cm signals and affecting galaxy formation.

Contribution

It models various black hole populations to assess their impact on the IGM and predicts observable 21-cm signals from early black hole activity.

Findings

Black hole radiation can heat the IGM to 10^3-10^4 K.

Black holes induce a 21-cm signal detectable by upcoming experiments.

Black hole emission raises the critical mass for star formation at z<~10.

Abstract

In the pre-reionization Universe, the regions of the inter-galactic medium (IGM) which are far from luminous sources are the last to undergo reionization. Until then, they should be scarcely affected by stellar radiation; instead, the X-ray emission from an early black hole (BH) population can have much larger influence. We investigate the effects of such emission, looking at a number of BH model populations (differing for the cosmological density evolution of BHs, the BH properties, and the spectral energy distribution of the BH emission). We find that BH radiation can easily heat the IGM to 10^3-10^4 K, while achieving partial ionization. The most interesting consequence of this heating is that BHs are expected to induce a 21-cm signal (delta T_b ~ 20-30 mK at z<~12) which should be observable with forthcoming experiments (e.g. LOFAR). We also find that at z<~10 BH emission strongly increases the critical mass separating star-forming and non-star-forming halos.