Sky & Telescope - January 2022 - 37

Absorption of Light by Hydrogen
Emission of Light by Hydrogen
LYMAN ALPHA Left: To move to a
higher energy level in the atom, an electron
must absorb a photon; to move to a lower
level, it must emit a photon. The transition
between the first and second energy
levels involves an ultraviolet photon of
wavelength 121.6 nm, known as Lyman
alpha. Below: Quasars emit strongly at visible
and ultraviolet wavelengths (including
Lyman alpha), and that light is absorbed
by hydrogen clouds between the quasar
and us. But because all the clouds and the
quasar lie very far away, the Lyman-alpha
lines are redshifted to longer wavelengths.
The more distant the absorber or emitter,
the redder the wavelength. The numerous
clouds between us and a quasar create a
comb-like pattern in the quasar's spectrum
called the Lyman-alpha forest.
Wavelength (nanometers)
121.6
121.6
Wavelength (nanometers)
de Weijgaert (University of Groningen, The Netherlands).
The universe makes far more low-energy photons than
high-energy ones, and X-ray photons are rare. " You need an
extremely energetic or very nearby quasar as a background
source, and even then, X-ray astronomy is really about counting
individual photons. "
Alternative Approaches
Little wonder, then, that people have tried other means to
detect the elusive intergalactic filaments. For instance, Jörg
Dietrich (now at University Observatory Munich) and his
colleagues found evidence for an invisible but massive bridge
between galaxy clusters Abell 222 and 223 (some 2½ billion
light-years away), using a technique known as weak gravitational
lensing. In their 2012 Nature paper, they described how
tiny distortions in the shapes of tens of thousands of remote
background galaxies reveal the existence of a 60-millionlight-year-long
filament between the clusters, weighing in
at some 80 trillion solar masses. The find has been hailed as
" the first robust detection of a dark matter filament. "
Later that year, Mathilde Jauzac (now at Durham University,
UK) and her colleagues published a similar result in
Monthly Notices of the Royal Astronomical Society. Through
weak lensing, they were able to make a crude 3D reconstruction
of what appears to be a huge filament, funneling matter
into the massive cluster MACS J0717.5+3745, which is so far
away that its light took more than 5 billion years to reach us.
And weak lensing isn't the only alternative game in town
to track down cosmic-web filaments. In a 2013 paper in
Astronomy & Astrophysics, the Planck Collaboration described
Wavelength
sk yand tele scope .o r g * JANUARY 2022 37
GREGG DINDERMAN / S&T, SOURCES: EDWARD WRIGHT AND NASA / ESA / LEAH HUSTAK (STSCI)
Brightness
Brightness
Brightness
https://skyandtelescope.org/

Sky & Telescope - January 2022

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

Contents
Sky & Telescope - January 2022 - Cover1
Sky & Telescope - January 2022 - Cover2
Sky & Telescope - January 2022 - 1
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Sky & Telescope - January 2022 - Cover3
Sky & Telescope - January 2022 - Cover4
Sky & Telescope - January 2022 - SA1
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