We investigate the effect of primordial helium on hydrogen reionisation using a hydrodynamical simulation combined with the cosmological radiative transfer code CRASH .
The radiative transfer simulations are performed in a 35.12 h ^ { -1 } comoving Mpc box using a variety of assumptions for the amplitude and power-law extreme-UV ( EUV ) spectral index of the ionising emissivity at z > 6 .
We use an empirically motivated prescription for ionising sources which , by design , ensures all of the models are consistent with constraints on the Thomson scattering optical depth and the metagalactic hydrogen photo-ionisation rate at z \sim 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 during hydrogen reionisation .
Models with a soft EUV spectral index , \alpha = 3 , produce IGM temperatures at the mean density at z \sim 6 , T _ { 0 } \simeq 10500 K , which are \sim 20 per cent higher compared to models in which helium photo-heating is excluded .
Harder EUV indices produce even larger IGM temperature boosts by the end of hydrogen reionisation .
A comparison of these simulations to recent observational estimates of the IGM temperature at z \sim 5 – 6 suggests that hydrogen reionisation was primarily driven by population-II stellar sources with a soft EUV index , \alpha \lower 2.15 pt \hbox { $ \buildrel < \over { \sim } $ } 3 .
We also find that faint , as yet undetected galaxies , characterised by a luminosity function with a steepening faint-end slope ( \alpha _ { LF } \leq - 2 ) and an increasing Lyman continuum escape fraction ( f _ { esc } \sim 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 per cent neutral by volume z \sim 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 .
This tension may be alleviated by either a lower neutral fraction at z \sim 7 or an IGM which still remains a few per cent neutral by volume at z = 6 .