A Photometric Redshift of z ~ 9.4 for GRB 090429B

TL;DR

This paper reports a photometric redshift of approximately 9.4 for GRB 090429B, using deep multi-telescope observations, supporting its origin in the early Universe and providing insights into high-redshift star formation.

Contribution

It presents the first robust photometric redshift estimate of a gamma-ray burst at z~9.4, utilizing multiple dust laws and deep optical/infrared observations to confirm its high-redshift nature.

Findings

Redshift estimate of z~9.4 with 90% likelihood range 9.06-9.52

Host galaxy undetected at deep limits, supporting high-redshift origin

GRB energetics similar to lower-redshift bursts

Abstract

Gamma-ray bursts (GRBs) serve as powerful probes of the early Universe, with their luminous afterglows revealing the locations and physical properties of star forming galaxies at the highest redshifts, and potentially locating first generation (Population III) stars. Since GRB afterglows have intrinsically very simple spectra, they allow robust redshifts from low signal to noise spectroscopy, or photometry. Here we present a photometric redshift of z~9.4 for the Swift-detected GRB 090429B based on deep observations with Gemini-North, the Very Large Telescope, and the GRB Optical and Near-infrared Detector. Assuming a Small Magellanic Cloud dust law (which has been found in a majority of GRB sight-lines), the 90% likelihood range for the redshift is 9.06 < z < 9.52, although there is a low-probability tail to somewhat lower redshifts. Adopting Milky Way or Large Magellanic Cloud dust laws leads to very similar conclusions, while a Maiolino law does allow somewhat lower redshift solutions, but in all cases the most likely redshift is found to be z>7. The non-detection of the host galaxy to deep limits (Y_AB >~ 28 mag, which would correspond roughly to 0.001 L* at z=1) in our late time optical and infrared observations with the Hubble Space Telescope strongly supports the extreme redshift origin of GRB 090429B, since we would expect to have detected any low-z galaxy, even if it were highly dusty. Finally, the energetics of GRB 090429B are comparable to those of other GRBs, and suggest that its progenitor is not greatly different to those of lower redshift bursts.