Reionisation: the role of Globular Clusters

In this talk I discuss the role of proto-globular clusters as the dominant sources of radiation that reionised hydrogen in the intergalactic medium (IGM) at redshift z ∼ 6. Observations at lower redshift indicate that only a small fraction, 〈fesc〉, of hydrogen ionising radiation emitted from massive stars can escape unabsorbed by the galaxy into the IGM. High redshift galaxies are expected to be more compact and gas rich than present day galaxies, consequently 〈fesc〉 from their disks or spheroids might have been very small. But if the sites of star formation in the galaxies are off-centre and if the star formation efficiency of the proto-clusters is high, then the mean 〈fesc〉 calculated for these objects only, is expected to be close to unity. Here I argue that this mode of star formation is consistent with several models for globular clusters formation. Using simple arguments based on the observed number of globular cluster systems in the local universe and assuming that the oldest globular clusters formed before reionisation and had 〈fesc〉 ∼ 1, I show that they produced enough ionising photons to reionise the IGM at z ∼ 6. I also emphasise that globular cluster formation might have been the dominant mode of star formation at redshifts from 6 to 12.