James Webb Space Telescope Feed Post

Harvard ADS: JWST/NIRSpec spectroscopy of z = 7-9 star-forming galaxies with CEERS: new insight into bright Lyα emitters in ionized bubbles

Paper abstract: We describe new JWST/NIRSpec observations of galaxies at z ? 7 taken from the CEERS survey. Previous observations of this area have revealed associations of Lya emitters at redshifts (z = 7.5, 7.7, and 8.7) where the intergalactic medium (IGM) is thought to be mostly neutral, leading to suggestions that these systems are situated in large ionized bubbles. We identify 21 z ? 7 galaxies with robust redshifts in the CEERS data set, including 10 in the Lya associations. Their spectra are indicative of very highly ionized and metal poor gas, with line ratios (O32 = 17.84 and Ne3O2 = 0.89, linear scale) and metallicity (12+\log {(\rm {O/H})}=7.84) that are rarely seen at lower redshifts. We find that the most extreme spectral properties are found in the six z ? 7 Lya emitters in the sample. Each has a hard ionizing spectrum indicating that their visibility is likely enhanced by efficient ionizing photon production. Lya velocity offsets are found to be very large (?300 km s^-1), likely also contributing to their detectability. We find that Lya in z ? 7 galaxies is 6-12 × weaker than in lower redshift samples with matched rest-optical spectral properties. If the bubbles around the Lya emitters are relatively small (?0.5-1 pMpc), we may expect such significant attenuation of Lya in these ionized regions. We discuss several other effects that may contribute to weaker Lya emission at z ? 7. Deep spectroscopy of fainter galaxies in the vicinity of the Lya emitters will better characterize the physical scale of the ionized bubbles in this field.