MATs (Monday Afternoon Talks)

3:00pm – 3:30pm, Susan Redmond
Quantifying the escape of Lya at z=5-6: a census of Lya escape fraction with Ha emitting galaxies spectroscopically confirmed by JWST and VLT/MUSE

JWST provides an unprecedented opportunity for unbiased surveys of Hα-emitting galaxies at z>4 with the NIRCam wide-field slitless spectroscopy (WFSS). In this talk, I will present a census of Lyα escape fraction (fesc,Lyα) of 165 star-forming galaxies at z=4.9−6.3 using their Hα emission directly measured from FRESCO NIRCam/WFSS data. We search for Lyα emission of each Hα-emitting galaxy in VLT/MUSE data. The overall fesc,Lyα measured by stacking is fesc,Lyα is 0.090±0.006. We find that fesc,Lyα displays a strong dependence on the observed UV slope (βobs) and E(B-V), such that the bluest galaxies (βobs∼−2.5) have the largest escape fractions (fesc,Lyα≈0.6), indicative of the crucial role of dust and gas in modulating the escape of Lyα photons. fesc,Lyα is less well related to other parameters, including the UV luminosity and stellar mass, and the variation in fesc,Lyα with them can be explained by their underlying coupling with E(B-V) or βobs. Our results suggest a tentative decline in fesc,Lyα at z≳5, implying increasing intergalactic medium attenuation towards higher redshift. Furthermore, the dependence of fesc,Lyα on βobs is proportional to that of the ionizing photon escape fraction (fesc,LyC), indicating the escape of Lyα and ionizing photon may be regulated by similar physical processes. With fesc,Lyα as a proxy to fesc,LyC, we infer that UV-faint (MUV>−16) galaxies contribute >70% of the total ionizing emissivity at z=5−6. If these relations hold during the epoch of reionization, UV-faint galaxies can contribute the majority of UV photon budget to reionize the Universe.

Paper