Journal Article

X-rays and hard ultraviolet radiation from the first galaxies: ionization bubbles and 21-cm observations

Aparna Venkatesan and Andrew Benson

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 417, issue 3, pages 2264-2275
Published in print November 2011 | ISSN: 0035-8711
Published online October 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19407.x
X-rays and hard ultraviolet radiation from the first galaxies: ionization bubbles and 21-cm observations

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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 ultraviolet (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 quasi-stellar object 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. We find that the 21-cm signature of X-ray versus UV ionization could be distinct, with the emission signal expected from X-rays alone occurring at smaller scales than that from UV radiation, resulting from the inherently different spatial scales at which X-ray and UV ionization/heating manifests. This difference is time-dependent and becomes harder to distinguish with an increasing X-ray contribution to the total ionizing photon production. Such differing scale-dependent contributions from X-ray and UV photons may therefore ‘blur’ the 21-cm signature of the percolation of ionized bubbles around early haloes (depending on whether a cosmic X-ray or UV background is built up first) and affect the interpretation of 21-cm data constraints on reionization.

Keywords: stars: Population III; galaxies: high-redshift; intergalactic medium; quasars: general; cosmology: theory; dark ages, reionization, first stars

Journal Article.  10345 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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