Probing cosmic star formation up to z = 9.4 with GRBs

TL;DR

This paper introduces a PCA-based method to use Gamma-Ray Bursts as probes of the cosmic star formation history up to redshift 9.4, avoiding assumptions about its shape, and suggests star formation was very high early on.

Contribution

The paper presents a novel PCA approach to reconstruct the cosmic star formation history from GRB data without assuming a specific parameterization.

Findings

Star formation at z ≈ 9.4 could be as high as today.

High-redshift star formation may be underestimated by galaxy surveys.

Results could resolve reionization photon shortage issues.

Abstract

We propose a novel approach, based on Principal Components Analysis, to the use of Gamma-Ray Bursts (GRBs) as probes of cosmic star formation history (SFH) up to very high redshifts. The main advantage of such approach is to avoid the necessity of assuming an \textit{ad hoc} parameterization of the SFH. We first validate the method by reconstructing a known SFH from Monte Carlo-generated mock data. We then apply the method to the most recent \textit{Swift} data of GRBs with known redshift and compare it against the SFH obtained by independent methods. The main conclusion is that the level of star formation activity at $z \approx 9.4$ could have been already as high as the present-day one ($\approx 0.01 M_\odot$ yr$^{-1}$ Mpc$^{-3}$). This is a factor 3-5 times higher than deduced from high-$z$ galaxy searches through drop-out techniques. If true, this might alleviate the long-standing problem of a photon-starving reionization; it might also indicate that galaxies accounting for most of the star formation activity at high redshift go undetected by even the most deep searches.