Resistive Heating Induced by Streaming Cosmic Rays Around a Galaxy in the Early Universe
It is expected that cosmic rays (CRs) escape from high-redshift galaxies at redshift $z\sim 10 \, - \, 20$ because CRs are accelerated by supernova remnants of the first stars. Although ultraviolet and X-ray photons are widely considered the main source of heating of the intergalactic medium, CRs can also contribute to it. When the CRs propagate in the intergalactic medium, in addition to the heating process due to CR ionization, resistive heating occurs due to the electron return current induced by the streaming CRs. We evaluate the heating rate around a galaxy as a function of the distance from the galaxy. We find that the resistive heating induced by CRs dominates over the other heating processes in the vicinity of the galaxy $r \lesssim 10^2 \, \mathrm{kpc}$ until the temperature reaches $T\sim 10^4 \, \mathrm{K}$. We also recalculate the strength of the magnetic field generated by streaming CRs under the presence of X-ray heating and show that achieved strength can be about $1$ order of magnitude smaller when the X-ray heating is included. The presence of the "first" CRs could be confirmed from the characteristic signature of CR heating imprinted on the $21$-$\mathrm{cm}$ line map in future radio observations.