Connecting Lyman-α and ionizing photon escape at cosmic noon with a strongly lensed galaxy arc

Star-forming galaxies were important contributors to the reionization of the universe, but the internal mechanisms that produce ionizing Lyman continuum (LyC) photons and regulate their escape remain unclear. Because Lyman-α (Lyα) emission strongly interacts with the same HI and dust that extincts LyC, its radiative transfer imprints significant knowledge of the underlying HI morphology and kinematics upon the Lyα emission profile. In this talk, I will present results connecting Lyα and LyC escape in the Sunburst Arc, a z=2.37 strongly lensed galaxy with LyC emission resolved to a young super star cluster ≲20 pc across. The strong lensing creates multiple images of the LyC-leaking region, as well as images of different, non-leaking regions of the galaxy. The lensed images are highly magnified and permit studies at much higher SNR and smaller physical distances than otherwise possible. We find great diversity among the Lyα profiles observed along different lines of sight into various locations inside the galaxy, with a differing number of peaks and relative peak strengths. We directly fit the profiles to constrain a large number of common Lyα parameters (such as peak separation, peak FWHM, EW, and more). Briefly, we find the leaking region has a narrow peak separation (~330 km/s), a stronger Lyα EW (~25 Å versus ~10 Å in the non-leaking regions), and other signatures of high LyC escape. We analyze the mutual dependence of the Lyα parameters, as well as their relation to LyC escape. We highlight the correlations and anticorrelations from this analysis, and attempt to offer an explanation for the diversity of observed Lyα profiles. We believe the most likely explanation is a mirror-like HI geometry that effectively reflects Lyα photons into our view, far (≳270 pc) from their suspected origin in the LyC-leaking region, functionally diffusing the central Lyα peak to a much physically larger area. This might explain why we observe central Lyα peaks—closely connected to LyC escape—in areas of the galaxy that do not leak LyC. This would emphasize the stochasticity of Lyα radiative transfer and the highly complex relation between Lyα and LyC escape at subgalactic scales. This will be an important lesson as the number of high-redshift, lensed galaxies grows and JWST and upcoming ELTs target them with unprecedented resolution.