Searching for the reionization sources

Using a reionization model simultaneously accounting for a number of experimental data sets, we investigate the nature and properties of reionization sources. Such a model predicts that hydrogen reionization starts at z ~ 15, is initially driven by metal-free (Population III) stars, and is 90 per cent complete by z ~ 8. We find that a fraction fγ > 80 per cent of the ionizing power at z >= 7 comes from haloes of mass M < 109Msolar predominantly harbouring Population III stars; a turnover to a Population II dominated phase occurs shortly after, with this population, residing in M > 109Msolar haloes, yielding fγ ~ 60 per cent at z = 6. Using Lyman-break broad-band dropout techniques, J-band detection of sources contributing to 50 per cent (90 per cent) of the ionizing power at z ~ 7.5 requires reaching a magnitude J110,AB = 31.2 (31.7), where ~ 15 (30) (Population III) sources arcmin-2 are predicted. We conclude that z > 7 sources tentatively identified in broad-band surveys are relatively massive (M ~ 109Msolar) and rare objects which are only marginally (~1 per cent) adding to the reionization photon budget.