X-rays and hard UV radiation From the First Galaxies: Ionization Bubbles and 21 cm Observations

The first stars and quasars are known sources of hard ionizing radiation in the first billion years of the Universe. We examine the joint effects of X-rays and hard UV radiation from such first-light sources on the hydrogen and helium reionization of the intergalactic medium (IGM) at early times, and the associated heating. We study the growth and evolution of individual H II, He II and He III regions around early galaxies with first stars and/or QSO populations. We find that in the presence of helium-ionizing radiation, X-rays may not dominate the ionization and thermal history of the IGM at z ∼ 10–20, contributing relatively modest increases to IGM ionization, and heating up to ∼ 103–105 K in IGM temperatures. We also calculate the 21 cm signal expected from a number of scenarios with metal-free starbursts and quasars in varying combinations and masses at these redshifts. The peak values for the spin temperature reach ∼ 104–105 K in such cases. The maximum values for the 21 cm brightness temperature are around 30–40 mK in emission, while the net values of the 21 cm absorption signal range from ∼ a few to 60 mK on scales of 0.01–1 Mpc