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The brighter galaxies reionised the Universe.

Sharma, M. and Theuns, T. and Frenk, C. and Bower, R. and Crain, R. and Schaller, M. and Schaye, J. (2016) 'The brighter galaxies reionised the Universe.', Monthly notices of the Royal Astronomical Society : letters., 458 (1). L94-L98.


Hydrogen in the Universe was (re)ionized between redshifts z ≈ 10 and z ≈ 6. The nature of the sources of the ionizing radiation is hotly debated, with faint galaxies below current detection limits regarded as prime candidates. Here, we consider a scenario in which ionizing photons escape through channels punctured in the interstellar medium by outflows powered by starbursts. We take account of the observation that strong outflows occur only when the star formation density is sufficiently high, and estimate the galaxy-averaged escape fraction as a function of redshift and luminosity from the resolved star formation surface densities in the EAGLE cosmological hydrodynamical simulation. We find that the fraction of ionizing photons that escape from galaxies increases rapidly with redshift, reaching values of 5–20 per cent at z > 6, with the brighter galaxies having higher escape fractions. Combining the dependence of escape fraction on luminosity and redshift with the observed luminosity function, we demonstrate that galaxies emit enough ionizing photons to match the existing constraints on reionization while also matching the observed ultraviolet-background post-reionization. Our findings suggest that galaxies above the current Hubble Space Telescope detection limit emit half of the ionizing radiation required to reionize the Universe.

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Publisher statement:This article has been published in Monthly Notices of the Royal Astronomical Society ©: 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:09 February 2016
Date deposited:07 April 2016
Date of first online publication:11 February 2016
Date first made open access:07 April 2016

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