Kavli Affiliate: Michael D. Gladders
| First 5 Authors: Keunho J. Kim, Matthew B. Bayliss, Jane R. Rigby, Michael D. Gladders, John Chisholm
| Summary:
Extreme, young stellar populations are considered the primary contributor to
cosmic reionization. However, how Lyman-continuum (LyC) escapes these galaxies
remains highly elusive because LyC escape can vary on sub-galactic scales that
are technically challenging to observe in LyC emitters. We investigate the
Sunburst Arc: a strongly lensed, LyC emitter at $z=2.37$. This galaxy reveals
the exceptionally small scale (tens of parsecs) physics of LyC escape thanks to
high magnification from strong lensing. Analyzing HST broadband and narrowband
imaging, we find that the small ($<$100 pc) LyC leaking region shows distinctly
extreme properties: a very blue UV slope ($beta=-2.9pm0.1$), high ionization
state ([OIII]$lambda 5007$/[OII]$lambda 3727=11pm3$ and [OIII]$lambda
5007$/H$beta=6.8pm0.4$), strong oxygen emission (EW([OIII])$=1095pm 40
r{A}$), and high Lyman-$alpha$ escape fraction ($0.3pm 0.03$), none of which
are found in any non-leaking regions of the galaxy. Moreover, a UV slope
comparison with starburst population models indicates that the leaking region’s
UV emission consists of nearly “pure” stellar light with minimal
contamination from surrounding nebular continuum emission and dust extinction.
These results suggest a highly directional LyC escape such that LyC is produced
and escapes from a small, extreme starburst region where the stellar feedback
from an ionizing star cluster may create an anisotropic “pencil beam” viewing
geometry in the surrounding gas. As a result, unabsorbed LyC directly escapes
through these perforated hole(s). Importantly, such anisotropic escape
processes imply that unfavorable sightline effects are a crucial contributor to
the significant scatters between galaxy properties and LyC escape fraction in
observations and that strong lensing uniquely reveals the small-scale physics
that regulates the ionizing budget of galaxies for reionization.
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