Sky & Telescope - September 2023 - 8

NEWS NOTES
STELLAR
Fomalhaut's Planet-forming Disk Revealed
Occulted star
Inner disk
Inner gap
Inner belt
Outer gap
Outer belt
" Great Dust Cloud "
(likely background
galaxy)
CONTINUING ITS RUN of groundbreaking
discoveries, the James Webb
Space Telescope (JWST) has snapped the
clearest images yet of the dusty debris
disk around Fomalhaut, a bright, young
star 25 light-years away in Piscis Austrinus.
The images reach a resolution and
sensitivity far beyond the capability of
earlier instruments.
András Gáspár (University of Arizona)
and colleagues presented new
details in this planet-forming disk on
May 8th in Nature Astronomy. The team
also included new Hubble Space Telescope
images in their analysis.
Previously, Hubble, the Spitzer Space
Telescope, and other telescopes had
shown that Fomalhaut hosts a far-out
ring of planet-forming debris that's akin
to the Kuiper Belt in our solar system.
80 au
JWST's image of the dusty disk around
Fomalhaut (blocked out at center) reveals an
inner disk, akin to the solar system's asteroid
belt but more extended; an inner belt; and a
previously imaged outer belt that's analogous
to our Kuiper Belt.
COSMOLOGY
Distorted Galaxy Hints at Nature of Dark Matter
ASTRONOMERS HAVE PROVIDED
the most direct evidence yet that dark
matter does not consist of ultramassive
particles. The work, based on supercomputer
calculations, suggests that dark
matter is instead made up of axions -
hypothetical particles so light that they
travel through space more like waves.
If correct, the researchers' conclusions
would not only help reveal what
85% of matter in the universe is made
of; the results could also lead to new
physics beyond the standard model.
The study was published April 20th in
Nature Astronomy.
Dark matter is tricky to investigate
because it doesn't emit, absorb,
or reflect light. So instead, a team led
by graduate student Alfred Amruth
(University of Hong Kong) looked for
the material's influence on gravitational
lensing. In this effect, a galaxy and its
dark matter halo curve the fabric of
spacetime. Light from a more distant
8
SEPTEMBER 2023 * SKY & TELESCOPE
source follows this curvature and bends
around the galaxy as if it were passing
through a lens.
When the foreground lens and the
distant light source are closely aligned,
astronomers see multiple images of the
same background object. The positions
and brightness of these images depend
on the distribution of dark matter in
the lens, providing a powerful probe of
the mysterious substance.
For the past two decades, astrophysicists
have struggled to reproduce
these multiple images exactly, in part
because they've assumed that dark matter
is made of weakly interacting massive
particles (WIMPs). If WIMPS are dark
matter, then galaxies' densities should
decrease smoothly as you move out
from the center. Except that's not what
astronomers actually infer from the
lensed images.
Amruth's team turned to an alternative
dark matter candidate instead:
axions, ultralight particles first theorized
in the 1970s. Quantum theory
says that due to their small mass, axions
behave more like waves than particles
on large scales, interfering with each
other to produce random density fluctuations.
These fluctuations would make
the distribution of dark matter around
a galaxy bumpy. (It's worth noting that
neither WIMPs nor axions have been
detected directly.)
After assuming that dark matter is
made of axions, Amruth and colleagues
were able to recreate the observed
positions and brightnesses of each
image in the quadruply lensed system
HS 0810+2554, in which a foreground
elliptical galaxy splits the light from a
background quasar into four images.
" We have reached a point where the
existing paradigm of dark matter needs
to be reconsidered, " Amruth says.
However, Edward Hardy (University
of Liverpool, UK), who was not involved
in the research, urges caution. " This
paper is only a first step, and extensive
further analysis is needed before the
NASA / ESA / CSA / A. GÁSPÁR (UNIVERSITY OF ARIZONA),
PROCESSING: A. PAGAN (STSCI)
Sky & Telescope - September 2023

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