First estimates of ionised Helium clumping factors
26 May 2015
Estimating the Intergalactic Medium (IGM) ionisation level of a region needs proper treatment of the reionisation process for a large representative volume of the Universe. The clumping factor, a parameter which accounts for the effect of recombinations in unresolved, small-scale structures, aids in achieving the required accuracy for the reionisation history even in simulations with low spatial resolution. Helium, along with Hydrogen, plays an important role in determining the temperature and ionisation structure of the IGM during reionisation, yet there exists no estimate of the Helium clumping factors.
Former CAASTRO postdoc Dr Akila Jeeson-Daniel at the University of Melbourne, together with international colleagues Dr Benedetta Ciardi (Max Planck Institute for Astrophysics, Garching) and Dr Luca Graziani (INAF Osservatorio Astronomico di Roma, Rome) published a paper estimating the redshift evolution of ionised Hydrogen and Helium clumping factors for the first time. They used a suite of small high resolution CRASH radiative transfer simulations of box sizes 2.2 – 8.8 h-1 Mpc (comoving) and grid sizes 323 – 1283 to estimate clumping factors exploring different definitions using IGM density thresholds (gas overdensities < 100), minimum ionisation level thresholds (xi = 0.1, 0.5, 0.9) and using temperature dependent recombination rates.
In their simulations, Hydrogen reionisation was completed by z ~ 6 while the Helium reionisation process extends to lower redshifts z ~ 3. As the ionised gas around sources increases in volume, the clumping factor increases. Clumping factors of HII, HeII and HeIII mostly have values in the range 1.5 – 4, similar to that of the total gas. The clumping factors of HeIII mostly have higher values compared to HeII due to the highly spatially inhomogeneous distribution really close to the ionising sources. HeII clumping factors show much higher values towards low redshifts (z < 5) when larger volumes of HeII around sources get converted to HeIII. It was found that the mean dimensionless density of the simulation volume plays a role in the determination of clumping factors. In most cases, clumping factors show a positive correlation with the mean dimensionless density, except for HeII during the later stages of reionisation when it significantly starts converting to HeIII. These estimates of clumping factors would prove very useful to improve the accuracy of future reionisation simulations.