Sky & Telescope - June 2024 - 8

NEWS NOTES
The German consortium has released an
X-ray catalog, tabulating nearly 930,000 distinct
sources within half the sky. The left image shows
extended X-ray emission while the right image
shows point-like X-ray sources.
eROSITA Consortium, which consists
of about 250 scientists. The projects are
wide-ranging and include the effects of
X-ray irradiation on planets, an analysis
of the flickering of supermassive black
holes, and the discovery of a giant filament
of warm gas between two galaxies
as well as two supermassive black holes
showing almost regular eruptions.
All of this comes from eROSITA's
COSMOLOGY
Largest-ever Catalog of X-ray Sources
Tests Cosmology
THE TEAM BEHIND the eROSITA
X-ray observatory has just released
a list of more than 900,000 distinct
high-energy sources - the largest X-ray
catalog ever published. Alongside the
catalog, the consortium also released a
series of scientific papers, including one
study that delves into the evolution of
the largest structures in the universe.
The results relieve tension between
observations and the standard cosmological
model.
In that study, to appear in Astronomy
& Astrophysics, Vittorio Ghirardini
(Max Planck Institute for Extraterrestrial
Physics, Germany) and collaborators
probed the mass of 5,259
galaxy clusters surveyed by eROSITA. By
investigating how those masses change
over time, the researchers pinned down
contested cosmological parameters.
To calculate the mass of a galaxy
cluster, Ghirardini's group measured
the X-rays emitted by hot gas trapped
between the cluster's galaxies. They
then compared those X-ray counts to
those from clusters whose masses were
previously determined by how much the
cluster's gravity bends the light of background
galaxies. In doing so, they determined
how many X-rays they would
detect for different masses. This process
allowed the astronomers to use X-rays
as a proxy for cluster mass, enabling
8
JUNE 2024 * SKY & TELESCOPE
them to tabulate masses for both more
and more distant clusters.
The team then measured the change
in cluster mass over cosmic time to get
at a contested parameter called sigma-8
(S8). Higher values of S8 indicate a universe
in which matter clumps together
more, while lower values correspond to
a smoother distribution of matter. Previous
observations have found today's
universe to be smoother than expected
(S&T: June 2023, p. 11).
But eROSITA doesn't see such a discrepancy.
" Interestingly, we do not see
any hint of S8 tension, " says Esra Bulbul
(also at Max Planck Institute), " or any
departures from the standard model of
cosmology, the cold dark matter model
with a dark-energy component. " So
far, eROSITA is the only probe of the
relatively nearby universe that does not
report that tension.
The study also produced tighter
constraints on other parameters in the
standard model, such as the dark energy
equation of state (w). They found that
dark energy exerts a uniform repulsive
force (with w between -1 and -1.24).
Bulbul says this measured range is
consistent with recent results from the
Dark Energy Survey (S&T: May 2024,
p. 8), within the two probes' accuracy.
This study is one of 50 new scientific
publications released by the German
first six months of data, during which
the telescope conducted its first full
survey of the sky; however, the German
consortium only has access to the
western galactic hemisphere (Russia
has proprietary rights to the eastern
half). While eight all-sky surveys were
initially planned, each one probing the
early universe more deeply, only four
and a half surveys had been completed
when the Max Planck Institute put the
telescope in hibernation after Russia
invaded Ukraine in 2022.
¢ARWEN RIMMER
COSMOLOGY
Astronomers Find
" Big Ring " 1.3 Billion
Light-Years Across
CLOSE TO THE HANDLE of the Big
Dipper, there's a huge ring on the sky
that shouldn't be there. The coil-shape
structure is an apparent overdensity of
distant galaxies with a circumference
of 4 billion light-years. The size is at the
boundary of what the standard model
of cosmology can explain - if the structure
is gravitationally bound.
PhD student Alexia Lopez (University
of Central Lancashire, UK) presented
the result at the meeting of the American
Astronomical Society in January.
Now, the study has been posted to the
arXiv astronomy preprint server.
Two years ago, the same team
published the discovery of a giant arc
spanning 3 million light-years. It's at a
MPE / J. SANDERS (GERMAN EROSITA CONSORTIUM)
Sky & Telescope - June 2024

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