Abstract
We investigate the effect of helium on hydrogen reionisation using a hydrodynamical simulation combined with the cosmological radiative transfer code CRASH. The simulations are run in a 35.12/h comoving Mpc box using a variety of assumptions for the amplitude and power-law extreme-UV (EUV) spectral index, alpha, of the ionising emissivity. We use an empirically motivated prescription for ionising sources which ensures all of the models are consistent with constraints on the Thomson scattering optical depth and the hydrogen photo-ionisation rate at z=6. The inclusion of helium slightly delays reionisation due to the small number of ionising photons which reionise neutral helium instead of hydrogen. However, helium has a significant impact on the thermal state of the IGM. Models with alpha=3 produce IGM temperatures at the mean density at z=6 which are about 20 % higher compared to models without helium photo-heating. Harder EUV indices produce even larger IGM temperature boosts. A comparison to recent observational estimates of the IGM temperature at z=5 - 6 suggests that hydrogen reionisation was primarily driven by pop-II stellar sources with a soft EUV index, alpha<3. We also find that faint, as yet undetected galaxies, characterised by a luminosity function with a steepening faint-end slope and an increasing Lyman continuum escape fraction (fesc=0.5), are required to reproduce the ionising emissivity used in our simulations at z>6. Finally, we note there is some tension between recent observational constraints which indicate the IGM is > 10% neutral by volume z=7, and estimates of the ionising emissivity at z=6 which indicate only between 1 and 3 ionising photons are emitted per hydrogen atom over a Hubble time. This tension may be alleviated by either a lower neutral fraction at z=7 or an IGM which still remains a few % neutral by volume at z=6.
Highlights
The last decade has witnessed the establishment of the two key pieces of observational evidence which presently shape our empirical understanding of the hydrogen reionisation epoch
We note there is some tension between recent observational constraints which indicate the intergalactic medium (IGM) is > 10 per cent neutral by volume z ∼ 7, and estimates of the ionising emissivity at z = 6 which indicate only 1–3 ionising photons are emitted per hydrogen atom over a Hubble time at z = 6
The inclusion of helium in these models is clearly important, adding an additional τe ∼ 0.02 to the total optical depth for E 1.2-α1.8. This is largely because of the extra electrons liberated by the reionisation of helium, but will be partly due to the higher IGM temperatures which arise from He II photo-heating; the temperature dependence of the H II recombination rate, αHII ∝ T −0.7, means higher temperatures will produce a slight increase in the H II fraction and the electron number density
Summary
The last decade has witnessed the establishment of the two key pieces of observational evidence which presently shape our empirical understanding of the hydrogen reionisation epoch. In order for hydrogen reionisation to complete by z = 6 and simultaneously match observational constraints from the CMB and the background photo-ionisation rate at z ≤ 6, reionisation must be an extended process where the ionising emissivity increases at z > 6 (Miralda-Escude 2003; Meiksin 2005; Bolton & Haehnelt 2007; Haardt & Madau 2012; McQuinn et al 2011).
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