The State of the Universe at z~6

TL;DR

This paper analyzes observations at z~6 to understand star formation and the IGM state, concluding that an enhanced IMF or Population III stars are needed for reionization, which is nearly complete by z=5.8, with a rapid increase in neutral fraction at higher redshifts.

Contribution

It proposes a revised stellar IMF or Population III stars as necessary for reionization at z~6, based on observational data and theoretical modeling.

Findings

An IMF more efficient by 10-20 times than Salpeter is needed at z~6.

Reionization is nearly complete by z=5.8 with <f_HI>_V~10^-4.

Neutral fraction of the IGM rises rapidly at higher redshifts, reaching 22-38% by z=8.8.

Abstract

In the context 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 and a lower mass cutoff of >= 30Msun 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 be fulfilled plausibly by continued existence of some pockets of uncontaminated, metal-free gas for star formation. (2) The volume-weighted neutral fraction of the IGM of <f_HI>_V~ 10^-4 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 un-ionized 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 HII 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.