THE STATE OF STAR FORMATION AND THE INTERGALACTIC MEDIUM ATz∼ 6

Abstract

In the context of stellar reionization in the standard cold dark matter model, we analyze observations at z ∼ 6 and are able to draw three significant conclusions with respect to star formation and the state of the intergalactic medium (IGM) at z ∼ 6. (1) An initial stellar mass function (IMF) more efficient, by a factor of 10–20, in producing ionizing photons than the standard Salpeter IMF is required at z ∼ 6. This may be achieved by having either (a) a metal-enriched IMF with a lower mass cutoff of ⩾30 M☉ or (b) 2%–4% of stellar mass being Population III massive metal-free stars at z ∼ 6. While there is no compelling physical reason or observational evidence to support (a), (b) could plausibly be fulfilled by continued existence of some pockets of uncontaminated, metal-free gas for star formation. (2) The volume-weighted neutral fraction of the IGM of at z = 5.8 inferred from the SDSS observations of QSO absorption spectra provides enough information to ascertain that reionization is basically complete with at most ∼0.1%–1% of IGM that is unionized at z = 5.8. (3) Barring some extreme evolution of the IMF, the neutral fraction of the IGM is expected to rise quickly toward high redshift from the point of H ii bubble percolation, with the mean neutral fraction of the IGM expected to reach 6%–12% at z = 6.5, 13%–27% at z = 7.7, and 22%–38% at z = 8.8.