Small Region, Big Impact: Highly Anisotropic Lyman-continuum Escape from a Compact Starburst Region with Extreme Physical Properties

TL;DR

This study reveals that a tiny, highly active starburst region in a distant galaxy exhibits extreme physical properties that facilitate anisotropic Lyman-continuum escape, crucial for understanding cosmic re-ionization.

Contribution

It provides detailed observational evidence of a small, extreme starburst region with high LyC escape, highlighting the importance of anisotropic escape mechanisms in galaxy re-ionization.

Findings

LyC escape occurs from a <100 pc starburst region with extreme properties.

The region shows a very blue UV slope and high ionization, indicating minimal dust and nebular contamination.

Lensing enables resolving small-scale structures critical for understanding LyC escape.

Abstract

Extreme, young stellar populations are considered the primary contributor to cosmic re-ionization. How Lyman-continuum (LyC) escapes these galaxies remains highly elusive, and it is challenging to observe this process in actual LyC emitters without resolving the relevant physical scales. We investigate the Sunburst Arc: a strongly lensed, LyC emitter at $z =2.37$ that reveals an exceptionally small scale (tens of parsecs) region of high LyC escape. The small ($<$ 100 pc) LyC leaking region has extreme properties: a very blue UV slope ($\beta = -2.9 \pm 0.1$), high ionization state ([O III]$\lambda 5007$/[O II]$\lambda 3727 $ $= 11 \pm 3$ and [O III]$\lambda 5007$/H$\beta$ $=6.8 \pm 0.4$), strong oxygen emission (EW([O III]) $= 1095 \pm 40 \ \r{A}$), and high Lyman-$\alpha$ escape fraction ($0.3 \pm 0.03$), none of which are found in non-leaking regions of the galaxy. The leaking region's UV slope is consistent with approximately ``pure'' stellar light that is minimally contaminated by surrounding nebular continuum emission or extinguished by dust. These results suggest a highly anisotropic LyC escape process such that LyC is produced and escapes from a small, extreme starburst region where the stellar feedback from an ionizing star cluster creates one or more ``pencil beam'' channels in the surrounding gas through which LyC can directly escape. Such anisotropic escape processes imply that random sightline effects drive the significant scatters between measurements of galaxy properties and LyC escape fraction, and that strong lensing is a critical tool for resolving the processes that regulate the ionizing budget of galaxies for re-ionization.