Ly α Escape in JWST/NIRCam F430M-selected H α Emitters at z ≃ 5.5

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Abstract We study the Ly α escape fraction ( f esc ) in a H α -selected sample of star-forming galaxies at z ≃ 5.5, identified via JWST/NIRCam F430M excess and covered by archival Very Large Telescope (VLT) MUSE data. By anchoring the intrinsic Ly α production to H α emission, our approach provides a direct and Ly α -unbiased probe of the escape of Ly α photons in galaxies with star formation rate (SFR) ≳0.1 M ⊙ yr −1 at this epoch. Ly α emission is detected in three out of 12 galaxies covered by VLT/MUSE. Combining detections and upper limits, we place a conservative upper bound of 〈 f esc Ly α 〉 < 0.32 on the population-averaged Ly α escape fraction. We find that Ly α detections are preferentially associated with nearly dust-free systems, while there is no clear correlation between SFR and f esc , suggesting a stochastic picture of Ly α escape. Interestingly, three of the four Ly α -detected galaxies reside within a known overdense structure, suggesting that the local environment may further facilitate Ly α photon escape. Our H α -selected approach establishes a general and scalable framework for probing Ly α escape by combining JWST medium- or narrowband imaging with ground-based spectroscopic data, enabling systematic and less biased studies of Ly α visibility in typical star-forming galaxies during the post-reionization era.

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