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A characteristic size of 10 Mpc for the ionized bubbles at the end of cosmic reionization


The first galaxies to appear in the Universe at redshifts z > 20 created ionized bubbles in the intergalactic medium of neutral hydrogen left over from the Big Bang. The ionized bubbles grew with time, surrounding clusters of dwarf galaxies1,2 and eventually overlapped quickly throughout the Universe over a narrow redshift interval near z ≈ 6. This event signalled the end of the reionization epoch when the Universe was a billion years old. Measuring the size distribution of the bubbles at their final overlap phase is a focus of forthcoming programmes to observe highly redshifted radio emission from atomic hydrogen. Here we show that the combined constraints of cosmic variance and light travel time imply an observed bubble size at the end of the overlap epoch of 10 physical Mpc, and a scatter in the observed redshift of overlap along different lines-of-sight of 0.15. This scatter is consistent with observational constraints from recent spectroscopic data on the farthest known quasars. This implies that future radio experiments should be tuned to a characteristic angular scale of 0.5 degrees and have a minimum frequency bandwidth of 8 MHz for an optimal detection of 21-cm flux fluctuations near the end of reionization.

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Figure 1: The distances to the observed surface of bubble overlap (SBO) and surface of Lyα transmission (SLT) fluctuate on the sky.
Figure 2: Constraints on the scatter in the SBO redshift and the characteristic size of isolated bubbles at the final overlap stage, RSBO (see Fig. 1).


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This work was supported in part by ARC, NSF and NASA.

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Correspondence to J. Stuart B. Wyithe or Abraham Loeb.

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Wyithe, J., Loeb, A. A characteristic size of 10 Mpc for the ionized bubbles at the end of cosmic reionization. Nature 432, 194–196 (2004).

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