http://www.skyandtelescope.com.au 25ELT: ESO / L. CALÇADA; ALMA: CLEM & ADRI BACRI-NORMIER (WINGSFORSCIENCE
.COM) / ESO
S CAPTURING GALAXIES’ SOULS The Atacama Large Millimeter/
submillimeter Array (ALMA) records ionised gas and dust in galaxies in a
universe as young as 600 million years old. Its observations have revealed
that the irst galaxies’ stars rapidly enriched the universe with ‘metals’.
The gas surrounding galaxies at cosmic dawn is essentially
a huge sea of neutral atomic hydrogen, so it releases these
radio waves. But like all distant sources of light, the 1.4 GHz
signal is redshifted by cosmic expansion. By the time the
radio waves reach us, they are at much lower frequencies,
around 100 MHz or so for gas at the Epoch of Reionisation.
The signal is also extremely faint, drowned out by radio
emission from sources both within our own Milky Way (such
as the Crab Nebula) and in other galaxies. The combination
of low radio frequency and weak signal has made detecting
neutral hydrogen in the early universe challenging with
previous generations of radio telescopes.
But experiments are now under way to record this
primordial radio signal and use it to map out reionisation.
Telescopes such as the Low-Frequency Array (LOFAR), a
network of radio receivers spread across Europe, and the new
Canadian Hydrogen Intensity Mapping Experiment (CHIME)
in British Columbia, are not only trying to detect the radio
signal from distant neutral hydrogen, but they’re also
measuring its fluctuations in brightness, which tell us how
reionisation actually happened.
As the first galaxies radiate out bubbles of ionised
hydrogen, the radio signal from neutral hydrogen should
weaken and ultimately vanish. The various sizes and
distribution of these bubbles will imprint themselves on the
radio signal, enabling us to compare different scenarios for
how reionisation might have proceeded — without having to
detect starlight at all.
It’s been less than 100 years since we realised there were
galaxies outside the Milky Way. The century after that
discovery has seen a journey backward in time as we attemptto understand the origin of galaxies, including our own.
Technological advances have allowed us to read this story in
the increasingly faint and long-wavelength light coming from
the edge of the observable universe. The search for the first
galaxies and the Epoch of Reionisation is the final (or is it
the first?) chapter of that story. What will the next century
bring? Well, we’ve only skimmed the book; now, it’s time to
appreciate it in detail. JAMES GEACH is an astronomer and Royal Society
University Research Fellow at the University of Hertfordshire in
the United Kingdom.OBSERVING EXTREMES This visualisation shows the European Extremely Large Telescope, currently being built
in the Atacama Desert in Chile. Its segmented primary mirror will span 39 metres, making the ELT the largest visible/
infrared telescope on Earth. Its large collecting area will enable it to image aspects of galaxies at cosmic dawn.