Sky & Telescope - January 2022 - 39

of hundreds of Lyman-alpha absorbers in the COSMOS
field - an area in the constellation Sextans that many major
telescopes have studied in detail in order to explore galaxy
evolution. Thanks to their instrument's sensitivity, Lee and
his colleagues do not restrict themselves to bright quasars
as background beacons. Instead, they also use fainter starforming
galaxies, which are much more numerous. " This
[provides] an unprecedented view of the cosmic web, which
has never been mapped at such vast distances, " Lee said in a
2014 press statement.
Because of the huge distances and corresponding large redLight
from
background
galaxies
Cosmic web
gas in foreground
shifts of the absorbing systems, the Lyman-alpha absorption
lines are conveniently shifted from the ultraviolet into the
visible part of the spectrum, where astronomers can measure
them with the Low Resolution Imaging Spectrometer (LRIS)
on the 10-meter Keck I telescope at Mauna Kea, Hawai'i.
Using special-purpose computer algorithms, the data are then
converted into 3D maps. Eventually, Lee's CLAMATO survey
(for COSMOS Lyman-Alpha Mapping And Tomography
Observations) is expected to cover a remote region of the universe
some 230 million light-years wide and almost a billion
light-years in depth.
Most recently, in another March 2021 study, scientists
revealed what they described as the first real image of intergalactic
matter in the cosmic web. Not that anyone doubted
its existence anymore, " but really seeing an image is of course
much more convincing, " says study leader Roland Bacon
(University of Lyon, France).
Bacon and his colleagues used the sensitive Multi-Unit
Spectroscopic Explorer (MUSE) spectrograph of the European
Southern Observatory's Very Large Telescope in Chile to map
the incredibly faint Lyman-alpha emission of remote swirls
of intergalactic hydrogen gas in the Hubble Ultra Deep Field.
" This is the most well-known part of the universe, " says
Bacon, " so there's a lot of interesting science you can do here
with long observing times. "
Between August 2018 and January 2019, MUSE was
trained on this small area (in the southern constellation
Fornax, the Furnace) for a total of no less than 140 hours.
According to Bacon, getting awarded so much observing time
was the biggest challenge. " Of all the instruments on the VLT,
MUSE is the one most in demand, " he says.
The faint glow of the cosmic web's hydrogen gas is a form
of fluorescence. As mentioned before, neutral hydrogen atoms
get excited when they absorb ultraviolet photons with a wavelength
of 121.6 nm. But eventually, the electrons fall back to
their lowest energy level, emitting photons of the same wavelength
in the process. Since these are emitted in all possible
directions, the Lyman-alpha emission signal is much fainter
than the absorption signal. However, while studying absorption
lines in the spectrum of a single background source only
gives you information on one tiny part of the cosmic web,
the emission signal can provide you with an image (albeit
extremely faint) of the whole structure.
Bacon and his colleagues were not the first to detect
COSMIC SLICE Astronomers picked up the signal of Lyman-alpha
emission from hydrogen at different times in the first 2 billion years of
cosmic history (red, top). They found 22 notably dense regions (gray rectangles,
center), five of which contained especially prominent filaments
(blue rectangles and bottom row).
Lyman-alpha emission from intergalactic gas. It had already
been observed in 2014, in the proximity of a luminous quasar
that acted as a flashlight. Not much later, the Cosmic Web
Imager - a dedicated instrument pioneered by Christopher
Martin (Caltech) - started to reveal ever-stronger hints of
filamentary structures. And in 2019, a team led by Hideki
Umehata (University of Tokyo) used the MUSE spectrograph
to detect structures between individual galaxies in a remote
protocluster. But the new observations are the first to reveal
cosmic-web filaments in the early universe, 1 to 2 billion
years after the Big Bang. The structures have lengths of up to
15 million light-years.
sk yand tele scope .o r g * JANUARY 2022 39
To Earth
3D MAPPING The CLAMATO project uses distant background galaxies
to look for intervening hydrogen clouds that absorb some of the galaxies'
light. The imprint enables astronomers to reconstruct the cosmic
web that the light traversed.
2
4
6
8
10
12
14
0.8
1.2
1.4
1.6
1.8
2.0
2.2
1.0
Age of universe (billion years)
SLICE OF COSMIC MAP: KHEE-GAN LEE (MPIA) AND CASEY STARK (UNIV. OF
CALIFORNIA, BERKELEY); COSMIC SLICE: ROLAND BACON / DAVID MARY
https://skyandtelescope.org/

Sky & Telescope - January 2022

Table of Contents for the Digital Edition of Sky & Telescope - January 2022

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Sky & Telescope - January 2022 - Cover3
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