Special TAPIR Seminar

How ionizing Lyman Continuum (LyC) radiation escaped the neutral Hydrogen in the first galaxies to reionize the Universe is still actively debated. Strong radiation can ionize the surrounding gas to a low enough neutral fraction to allow escape, leading in its pure form to the idealized, isotropic "density bounded scenario", in which the escape fraction is regulated by the residual neutral column density in the gas. Additionally, radiative or mechanical feedback can carve holes in a neutral, optically thick medium, through which photons can escape freely, in its pure form leading to a highly anisotropic "perforated ionization bounded scenario", in which the ionizing escape fraction is regulated by the combined opening angle of the channels. Both mechanisms contribute to any configuration we can study "in the wild", but studying and understanding the edge cases where one of them is dominating provides us important benchmarks for our models.

Here, I present dramatic observations of LyC from a galaxy in which the perforated, ionization-bounded escape scenario is dominating. Last year, Hubble Space Telescope observed bright, compact Lyman-continuum escape from the Sunburst Arc, a young Green Pea-analog at z=2.4 which is gravitationally lensed and distorted by a foreground cluster, yielding 12 distinct images of the leaking region. Rest-frame UV spectroscopy has revealed a triple-peaked Lyman-α profile theoretically predicted to be a tell-tale sign of the pure, perforated ionization-bounded escape scenario; and indeed the detected LyC is compact, unresolved, and has high line-of-sight escape fraction, supporting this interpretation. In addition, I will discuss how the ionizing photons traversing multiple lines of sight from the same source traces variations in the neutral column density in the CGM and IGM in a detail not previously seen, and how surprising spectroscopic observations of Lyman-α from distinct regions in the galaxy along multiple lines of sight, in concert with the multiply-imaged LyC data, have led to new insights about the spatial and kinematic configuration of the neutral gas like e.g. the size of the neutral outflows ubiquitous in Lyα emitting galaxies.