Sky & Telescope - January 2024 - 10

NEWS NOTES
COSMOLOGY
Data Reveal Dark Matter Clumps Between Galaxies
A TEAM OF JAPANESE astronomers
has mapped the distribution of dark
matter on an unprecedentedly small
scale. Their findings back up the idea
that dark matter particles are " cold, "
that is, slow-moving with respect to the
speed of light.
The team, led by Kaiki Taro Inoue
(Kindai University, Japan), has made
inroads by using the Atacama Large
Millimeter/submillimeter Array
(ALMA) in Chile to observe radio waves
that left a distant quasar 11 billion
years ago. The team's findings are published
in the September 10th Astrophysical
Journal.
The gravity of a massive foreground
galaxy bends the quasar's light as it
passes by, creating four separate images
of the quasar in an effect known as
gravitational lensing. However, the
positions and shapes of the quartet of
STARS
Surf's Up: Waves Might Be
Breaking on This Star
HUMONGOUS WAVES of plasma might
be breaking on the sloshing surface of
a massive star. According to Morgan
MacLeod and Abraham Loeb (both at
Center for Astrophysics, Harvard &
Smithsonian), these waves could reach
heights of more than 4 million kilometers
(2.5 million miles) before crashing
back onto the blue giant star, releasing
incredible amounts of tidal energy in
the process.
The giant star has 35 times the Sun's
mass and is located in the Large Magellanic
Cloud. Identified as MACHO
80.7443.1718, it and its smaller companion
brighten and dim again by 0.4
magnitude every 32.8 days. The pair is
classified as a heartbeat star, an unusual
type of binary system with highly elliptical
orbits that raise enormous tidal
deformations at closest approach.
However, MACHO 80.7443.1718 is
an extreme example of that class. Its
brightness variations are much larger
10 JANUARY 2024 * SKY & TELESCOPE
images don't match up with predictions
based on the gravitational pull of the
foreground galaxy alone.
Based on the precision of the new
ALMA observations, the team concluded
that additional dark matter clumps in
intergalactic space further skew the
quasar's light en route to Earth. The
clumps are about 1.1 arcsecond across;
if the clumps are near the foreground
galaxy, that corresponds to 30,000
light-years across. These observations
mark the first time that dark matter's
distribution has been mapped on such a
small scale.
The gravitational lensing effect due
to the line-of-sight structures is consistent
with cold dark matter models,
the researchers say, indicating that
dark matter particles are heavy and
slow-moving (as opposed to lighter and
speedier particles such as neutrinos).
" The work appears to be very
thorough, " says Kyle Oman (Durham
University, UK), who was not involved
in the research. " It adds a little bit more
weight to an already well-established
cosmological model. "
At the same time, the data are also
consistent with the alternative idea of
modified gravity. In this scenario, rather
than rely on the effects of mysterious
dark matter, gravity itself is variable.
According to Hongsheng Zhao (University
of St. Andrews, UK), who was also
not involved in the research, the mismatched
lensed quasar images " could
also be explained by line-of-sight dwarf
galaxies [and] modified gravity. "
For their part, Inoue and his colleagues
are currently making further
observations in the hope of adding
more data to their gravitational lensing
model. This work - both present
and future - could prove an important
piece of the dark matter puzzle.
¢COLIN STUART
The waves, three times as high as the
Sun's diameter, subsequently break like
ocean waves on a beach before devolving
into a sea foam-like turbulence.
When the waves crash down on the
pThis artist's impression shows a giant blue
star and its smaller companion, whose close
passes create changes in brightness that look
like heartbeats.
than in regular heartbeat systems; it
rotates unusually fast (once every 4.4
days), which should in turn give the
star a highly oblate shape (wider in the
middle); and the orbital period is slowly
decaying by 11 seconds per year.
MacLeod and Loeb carried out
detailed 3D hydrodynamical computer
simulations of the binary, published
August 10th in Nature Astronomy. They
show that during closest approach, the
companion star raises huge tidal waves
on the diluted surface of the blue giant.
bigger star's surface, they accelerate its
rotation, explaining its fast spin rate.
Moreover, part of the foam ends up
in an extended, tenuous atmosphere,
which shows up in the star's spectrum
as bright emission lines. The crashing
waves dissipate energy at a rate 100
times higher than the Sun radiates -
and that neatly matches that of the
observed orbital decay.
Edward Guinan (Villanova University),
who was not involved in what he
calls an " intriguing study, " cautions
that the conclusions are mostly inferred
from calculations and simulations.
" There is really no 'hard' observational
evidence for the proposed huge clashing
waves, " he notes. However, 20 similar
systems offer opportunities for additional
study of what MacLeod calls a
" brief and transformative moment in a
long stellar lifetime. "
¢GOVERT SCHILLING
MELISSA WEISS (CENTER FOR ASTROPHYSICS, HARVARD & SMITHSONIAN)
Sky & Telescope - January 2024

Table of Contents for the Digital Edition of Sky & Telescope - January 2024
