The Lyα Nondetection by JWST NIRSpec of a Strong Lyα Emitter at z = 5.66 Confirmed by MUSE

Abstract

The detections of Ly α emission in galaxies with redshifts above 5 are of utmost importance for constraining the cosmic reionization timeline, yet such detections are usually based on slit spectroscopy. Here we investigate the significant bias induced by slit placement on the estimate of Ly α escape fraction ( f esc Ly α ) by presenting a galaxy (dubbed A2744-z6Lya) at z = 5.66, where its deep JWST/NIRSpec prism spectroscopy completely misses the strong Ly α emission detected in the MUSE data. A2744-z6Lya exhibits a pronounced UV continuum with an extremely steep spectral slope of β = − 2.640 − 0.008 + 0.008 , and it has a stellar mass of ∼10 8.75 M ⊙ , a star formation rate of ∼5.95 M ⊙ yr −1 , and a gas-phase metallicity of 12 + log ( O / H ) ∼ 7.90 . The observed flux and rest-frame equivalent width of its Ly α from MUSE spectroscopy are 1.2 × 10 −16 erg s −1 cm −2 and 75 Å, equivalent to f esc Ly α = 42 % ± 1 % . However, its Ly α nondetection from JWST NIRSpec gives a 5 σ upper limit of <5%, in stark contrast to that derived from MUSE. To explore the reasons for this bias, we perform spatially resolved stellar population analysis of A2744-z6Lya using the JWST NIRCam and HST imaging data to construct 2D maps of star formation rate, dust extinction, and neutral hydrogen column density. We find that the absence of Ly α in the slit regions probably stems from both the resonance scattering effect of neutral hydrogen and dust extinction. Through analyzing an extreme case in detail, this work highlights the important caveat of inferring f esc Ly α from slit spectroscopy, particularly when using the JWST multiplexed NIRSpec microshutter assembly.