JWST/NIRSpec Observations of Lyman α Emission in Star-forming Galaxies at 6.5 ≲ z ≲ 13

Abstract

We present an analysis of JWST Ly α spectroscopy of z ≳ 6.5 galaxies, using observations in the public archive covering galaxies in four independent fields: Great Observatories Origins Deep Survey (GOODS)-N, GOODS-S, A2744, and the Extended Groth Strip (EGS). We measure the Ly α emission line properties for a sample of 210 z ≃ 6.5–13 galaxies, with redshifts confirmed independently of Ly α in all cases. We present three new detections of Ly α emission in JWST spectra, including a large equivalent width (EW; =143 Å) Ly α emitter (LAE) with strong C iv emission (EW = 21 Å) at z = 7.1 in GOODS-N. We measure the redshift-dependent Ly α EW distribution across our sample. We find that strong Ly α emission (EW > 25 Å) becomes increasingly rare at earlier epochs, suggesting that the transmission of Ly α photons decreases by 4× between z ≃ 5 and z ≃ 9. We describe potential implications for the intergalactic medium neutral fraction. There is significant field-to-field variance in the LAE fraction. In contrast to the three other fields, the EGS shows no evidence for reduced transmission of Ly α photons at z ≃ 7–8, suggesting a significantly ionized sight line may be present in the field. We use available NIRCam grism observations from the First Reionization Epoch Spectroscopically Complete Observations survey to characterize overdensities on large scales around known LAEs in the GOODS fields. The strongest overdensities appear linked with extremely strong Ly α detections (EW > 50 Å) in most cases. Future Ly α spectroscopy with JWST has the potential to constrain the size of ionized regions around early galaxy overdensities, providing a new probe of the reionization process.

Keywords

• Reionization
• High-redshift galaxies
• Observational cosmology