Ly$\alpha$ Escape in JWST/NIRCam F430M-Selected H$\alpha$ Emitters at $z\simeq 5.5$

Abstract

We study the Ly$\alpha$ escape fraction ($f_{\mathrm{esc}}$) in an H$\alpha$-selected sample of star-forming galaxies at $z\simeq 5.5$, identified via JWST/NIRCam F430M excess and covered by archival VLT/MUSE data. By anchoring the intrinsic Ly$\alpha$ production to H$\alpha$ emission, our approach provides a direct and Ly$\alpha$-unbiased probe of the escape of Ly$\alpha$ photons in galaxies with SFR $\gtrsim 0.1M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ at this epoch. Ly$\alpha$ emission is detected in 3 out of 12 galaxies covered by VLT/MUSE. Combining detections and upper limits, we place a conservative upper bound of $⟨f_{\mathrm{esc}}^{\mathrm{Ly}\alpha }⟩<0.32$ on the population-averaged Ly$\alpha$ escape fraction. We find that Ly$\alpha$ detections are preferentially associated with nearly dust-free systems, while no clear correlation between SFR and $f_{\mathrm{esc}}$, suggesting a stochastic picture of Ly$\alpha$ escape. Interestingly, three of the four Ly$\alpha$-detected galaxies reside within a known overdense structure, suggesting that local environment may further facilitate Ly$\alpha$ photons escape. Our H$\alpha$-selected approach establishes a general and scalable framework for probing Ly$\alpha$ escape by combining JWST medium- or narrow-band imaging with ground-based spectroscopic data, enabling systematic and less biased studies of Ly$\alpha$ visibility in typical star-forming galaxies during the post-reionization era.

17 pages, 12 figures, ApJ accepted