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| Paper: |
Keeping the Universe Ionised: Photoheating and the High-redshift Clumping Factor of the Intergalactic Gas |
| Volume: |
432, New Horizons in Astronomy: Frank N. Bash Symposium 2009 |
| Page: |
230 |
| Authors: |
Pawlik, A. H.; Schaye, J.; van Scherpenzeel, E. |
| Abstract: |
The critical star formation rate density required to keep the intergalactic
hydrogen ionised depends crucially on the average rate of recombinations in
the intergalactic medium (IGM). This rate is proportional to the clumping factor CIGM≡‹ρb2›IGM /
ρb2, where ρb and ρb are the
local and cosmic mean baryon density, respectively, and the brackets ‹
›IGM indicate spatial averaging over the recombining gas in the
IGM. We perform a suite of cosmological smoothed particle hydrodynamics
simulations to calculate the volume-weighted clumping factor of the IGM at
redshifts z≥6. We investigate the effect of photoionisation heating by a
uniform ultraviolet background and find that photoheating strongly reduces the
clumping factor as the increased pressure support smoothes out small-scale
density fluctuations. Even our most conservative estimate for the clumping
factor, CIGM = 6, is five times smaller than the clumping factor that
is usually employed to determine the capacity of star-forming galaxies to keep
the z≈6 IGM ionised. Our results imply that the observed population
of star-forming galaxies at z≈6 may be sufficient to keep the IGM
ionised, provided that the IGM was reheated at zga9 and that the fraction
of ionising photons that escape star-forming regions to ionise the IGM is
larger than 0.25. |
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