A Series of Fortunate Events: How the Universe conspired to reveal unique insights about star formation, ISM dynamics and the Epoch of Reionization
How ionizing Lyman Continuum (LyC) radiation escaped the neutral Hydrogen in
the first galaxies to reionize the Universe is still strongly debated.
Extreme-UV 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 angles 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.
In this talk, I present the dramatic observations of Lyman Continuum 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 galaxy cluster, yielding
12 distinct images of the leaking region. Rest-frame UV spectroscopy has
revealed a triple-peaked Lyman-alpha profile teoretically predicted to be a
tell-tale sign of the pure, perforated ionization-bounded scenario; and indeed
the detected Lyman Continuum is compact, unresolved, and has a 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 IGN in a detail not previously seen; and how surprising spectroscopic
observations of Lyman-alpha from distinct regions in the galaxy along multiple
lines of sight, in concert with the multiply-imaged Lyman Continuum data,can
lead to new insights about the spatial and kinematic configuration of neutral
ISM in young starburst galaxies.