Astronomy Seminar: A cosmic Sunburst in stunning GravLens-vision

Despite a growing consensus that young galaxies contributed the majority of the
extreme-UV photons that reionized the early Universe, we still do not have a
firm grasp on which mechanisms have allowed these photons to escape through the
abundant galactic neutral medium and into intergalactic space.

Low neutral column densities can arise either where a hot, luminous stellar
population has ionized a large fraction of the galactic gas, or where
turbulence, feedback or similar effects have created pathways through the
neutral medium along certain lines of sight in an otherwise neutral medium.
Realistic systems are generally complicated combinations of these effects; but
in their pure form, they define toy models which are very useful to understand
the underlying physics.

Two years ago, our collaboration took spectra a gravtitationally lensed galaxy
at z=2.4, which showed a theoretically predicted, but not previously observed,
Lyman-alpha profile arising from light escaping through a narrow, channel with
extremely low neutral column density in an otherwise opaque neutral medium, just
like its namesake weather phenomenon gives a direct view of the sun through
perforated clouds. Follow-up UV imaging with Hubble Space Telescope confirmed
that ionizing Lyman-continuum photons in large amounts are pouring out from a
very compact region, exactly where this channel was predicted to be, confirming
that we are looking straight down the barrel of a narrow perforation of galactic

The galaxy is strongly gravitationally lensed by a massive foreground cluster,
which leads to the galaxy being seen in 12 distorted copies, each magnified
10-30 times; adding both extra scientific opportunity and visual spectacle to
the mix.