The Ly$\alpha$ non-detection by JWST NIRSpec of a strong Ly$\alpha$ emitter at $z=5.66$ confirmed by MUSE

Abstract

The detections of Lyman-$\alpha$ ($\rm Ly\alpha$) 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 $\rm Ly\alpha$ escape fraction ( $f_{\rm esc}^{\mathrm{Ly\alpha}}$), by presenting a galaxy (dubbed A2744-z6Lya) at $z=5.66$ where its deep JWST NIRSpec prism spectroscopy completely misses the strong $\rm Ly\alpha$ emission detected in the MUSE data. A2744-z6Lya exhibits a pronounced UV continuum with an extremely steep spectral slope of $\beta=-2.574_{-0.008}^{+0.008}$, and it has a stellar mass of $\mathrm{\sim10^{8.82}~M_\odot}$, a star-formation rate of $\mathrm{\sim8.35~M_\odot yr^{-1}}$ and gas-phase metallicity of $\mathrm{12+log\,(O/H)\sim7.88}$. The observed flux and rest-frame equivalent width of its Ly$\alpha$ from MUSE spectroscopy are $1.2\times \rm 10^{-16} erg~s^{-1}cm^{-2}$ and 75\r{A}, equivalent to $f_{\rm esc}^{\mathrm{Ly\alpha}}=78\pm4 \%$. However, its Ly$\alpha$ non-detection from JWST NIRSpec gives a 5-$\sigma$ upper limit of $<13 \%$, 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 imaging data to construct 2-dimensional maps of SFR, dust extinction and neutral hydrogen column density. We find that the absence of Ly$\alpha$ 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 $f_{\rm esc}^{\mathrm{Ly\alpha}}$ from slit spectroscopy, particularly when using the JWST multiplexed NIRSpec microshutter assembly.