The Cosmic Star Formation Rate from the Faintest Galaxies in the Unobservable Universe

TL;DR

This paper investigates the star formation rate during reionization by comparing galaxy surveys and gamma-ray burst data, suggesting a significant amount of unseen star formation at high redshifts.

Contribution

It demonstrates that integrating UV luminosity functions aligns galaxy and GRB-based star formation rates up to z ~ 8, and highlights the potential of GRBs to trace unseen star formation beyond that.

Findings

UV luminosity functions reconcile galaxy and GRB SFR estimates up to z ~ 8

GRBs indicate much higher star formation at z > 9 than observed in surveys

Star formation rate declines slowly up to z ~ 11, consistent with reionization data

Abstract

Observations of high-z galaxies and gamma-ray bursts now allow for empirical studies during reionization. However, even deep surveys see only the brightest galaxies at any epoch and must extrapolate to arbitrary lower limits to estimate the total rate of star formation. We first argue that the galaxy populations seen in LBG surveys yield a GRB rate at z > 8 that is an order of magnitude lower than observed. We find that integrating the inferred UV luminosity functions down to M_UV ~ -10 brings LBG- and GRB-inferred SFR density values into agreement up to z ~ 8. GRBs, however, favor a far larger amount of as yet unseen star formation at z > 9. We suggest that the SFR density may only slowly decline out to z ~ 11, in accord with WMAP and Planck reionization results, and that GRBs may be useful in measuring the scale of this multitude of dwarf galaxies.