A Photometric Redshift of z ~ 9.4 for GRB 090429B

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.

• The paper determines an exceptionally high redshift (z ~ 9.4) for GRB 090429B through detailed photometric analysis.
• The analysis employs spectral energy distributions and dust extinction models to interpret near-infrared detections and reveal a Lyman break.
• The findings imply that GRB 090429B is one of the earliest cosmic objects observed, offering insights into early star formation and cosmic reionization.

Photometric Redshift Determination for GRB 090429B

The paper investigates the photometric redshift of the gamma-ray burst (GRB) 090429B, identified by the Swift satellite, and concludes that GRB 090429B is likely located at an exceptionally high redshift of approximately $z = 9.4$. This conclusion is supported by sophisticated photometric observations compiling data from the Gemini-North, the Very Large Telescope (VLT), and the GRB Optical and Near-infrared Detector (GROND). The redshift estimation employs models of dust extinction and spectral energy distributions which substantiate the claim of one of the highest observed redshift values for a GRB.

Methodology and Observations

The study utilizes a range of ground and space-based observations to characterize the afterglow of GRB 090429B and estimate its redshift. Swift's Burst Alert Telescope (BAT) first detected GRB 090429B, allowing for follow-up observations. Notably, photometric data were collected using Gemini-North's Near-Infrared Imager (NIRI), and spectrophotometry was conducted by the VLT and GROND. Notably, GRB 090429B's afterglow was detected in the near-infrared bands but not in the optical bands, suggesting the presence of a Lyman break, indicative of high redshift.

Data Analysis and Results

The authors implemented photometric redshift analysis using a grid search over possible redshift and extinction parameters. The simple power-law model representing afterglow emission, combined with different dust extinction laws, allowed the construction of spectral energy distributions from which the most probable redshift range was derived. Under the Small Magellanic Cloud (SMC) dust extinction model, the data suggest a redshift range from 9.06 to 9.52 (90% confidence interval) with a most likely value of $z > 7$, confirmed by the non-detection of a host galaxy in deep late-time optical and infrared observations.

Discussion and Implications

The non-detection of a host galaxy in both optical and infrared bands using the Hubble Space Telescope (HST) further supports the extreme redshift hypothesis. At lower redshift scenarios, even considering high dust extinction, a host galaxy should have been observable. Additionally, the GRB emission qualities such as peak energy and isotropic energy release were consistent with expectations for high-redshift GRBs, bolstering the extreme redshift claim.

The extreme redshift of GRB 090429B implies that it is one of the earliest cosmic objects observed, dated to when the universe was relatively young. These results have significant implications for our understanding of early star formation and the conditions of the intergalactic medium during cosmic reionization. The authors highlight that GRB 090429B's properties are not anomalous compared to lower-redshift GRBs, aligning with the hypothesis that its progenitor was a high-mass Population II star, possibly prevalent in early universe environments.

Future Directions and Considerations

The paper concludes with a discussion on the importance of rapid-response infrared imaging for identifying high-redshift GRBs and emphasizes the need for prompt spectroscopic follow-up to confirm such photometric redshifts. As more advanced observational facilities become operational, such as those with highly sensitive near-infrared spectrographs, the identification and study of high-redshift GRBs like GRB 090429B can provide valuable insights into the early universe, aiding in the exploration of star formation and galaxy evolution at these epochs. This study underscores the potential of GRBs as probes of cosmically remote environments and phenomena, paving the way for detailed investigations into primordial star populations and their influence on cosmic structures.