REIONISATION: M. ALVAREZ (HTTP://WWW.CITA.UTORONTO.CA/~MALVAREZ), R. KAEHLER & T. ABEL; RED GRADIENT: HARDIK PETHANI / GETTY IMAGESWSLICES OF REIONISATION The irst galaxies and their stars lit
up the early universe, kicking electrons away from hydrogen nuclei
in what is known as the Epoch of Reionisation, between 400 million
and 900 million years after the Big Bang. What was once a neutral
medium between the stars gave way once more to ionised plasma.
(The universe remains ionised today, but its lower density enables
photons to still travel freely.) These slices come from a Swiss cheese-
like simulation of the process.the collecting area that will enable astronomers to detect
starlight and ionised gas in galaxies at cosmic dawn.
JWST and ELT will provide an important next step, but
there is a facility online that is already revolutionising our
view of the early universe: the Atacama Large Millimeter/
submillimeter Array (ALMA). Sited two kilometres higher
than ELT, on the Chajnantor Plateau in the Atacama Desert,
ALMA is a collection of 66 radio dishes that act as one
telescope to deliver unprecedented sensitivity and exquisite
resolution at wavelengths between about 300 microns and
nearly 4 millimetres. ALMA can detect the thermal glow of
interstellar dust heated by starlight, as well as emission from
cooler gas clouds, even in the most distant galaxies.
One of ALMA’s breakthroughs has been to show that
the first generation of stars quickly enriched the universe
with heavy elements, a development that surprised some
astronomers. ALMA has detected light emitted by ionised
oxygen and interstellar dust granules in galaxies as far back
as a redshift of 8.4, just 600 million years after the Big Bang.
These elements can only have been produced in and dispersed
by stars, yet this was a time when reionisation was still in
progress — the lights hadn’t all been turned on yet. ALMA has
also observed the telltale glow of a unusually massive amount
of starlight-heated dust — 2.5 billion Suns’ worth — in a
galaxy at redshift 7, just 200 million years later.
How heavy-element enrichment happened so quickly
is one of the key questions for studies of the first galaxies.
The answer underpins much of our understanding of the
subsequent 13 billion years of galaxy evolution.Forget starlight
There’s a new way of studying the Epoch of Reionisation
that’s exciting astronomers. Instead of looking at the galaxies,
we’re trying to detect the signature of the first stars’ ignition
in the stuff between the stars: the intergalactic medium.
A gas of neutral atomic hydrogen will emit radio waves due
to a quantum effect called hyperfine splitting. According to
the laws of quantum mechanics, the electron and proton in
a hydrogen atom can either have the same spins or opposing
ones. Atoms with electrons in aligned spins will occupy
a state with marginally higher energy than atoms in the
opposite, anti-aligned state. Occasionally, an electron’s spin
can flip, and the atom will release the energy as a photon.
Because the energy transition is so small, the photon has a
very low energy: We detect it at a frequency of 1.4 gigahertz,
corresponding to a wavelength of 21 centimetres.12738495 106COSMIC DAWN24 AUSTRALIAN SKY & TELESCOPE May | June 2018