Astrophysicists consider that around 40% of the ordinary matter that makes up
stars, planets, and galaxies remains undetected, concealed in the form of a
hot gas in the complex cosmic web. Today, scientists at the Institut
d’Astrophysique Spatiale (CNRS/Université Paris-Saclay) may have detected, for
the first time, this hidden matter through an innovative statistical analysis
of 20-year-old data. Their findings are published on November 6, 2020, in
Astronomy & Astrophysics.
Galaxies are distributed throughout the Universe in the form of a complex
network of nodes connected by filaments, which are in turn separated by
voids. This is known as the cosmic web. The filaments are thought to contain
almost all of the ordinary (so-called baryonic) matter of the Universe in
the form of a diffuse, hot gas. However, the signal emitted by this
diffuse gas is so weak that in reality, 40 to 50% of the baryons[1] go
undetected.
These are the missing baryons, hidden in the filamentary structure of the
cosmic web, that Nabila Aghanim, a researcher at the Institut
d’Astrophysique Spatiale (CNRS/Université Paris-Saclay) and Hideki Tanimura,
a post-doctoral researcher, together with their colleagues, are attempting
to detect. In a new study, funded by the ERC ByoPiC project, they present a
statistical analysis that reveals, for the first time, the X-ray emission
from the hot baryons in filaments. This detection is based on the stacked
X-ray signal, in the ROSAT[2] survey data, from approximately 15 000
large-scale cosmic filaments identified in the SDSS[3] galaxy survey. The
team made use of the spatial correlation between the position of the
filaments and the associated X-ray emission to provide evidence of the
presence of hot gas in the cosmic web, and for the first time measure its
temperature.
These findings confirm earlier analyses by the same research team, based on
indirect detection of hot gas in the cosmic web through its effect on the
cosmic microwave background.[4] This paves the way for more detailed
studies, using better quality data, to test the evolution of gas in the
filamentary structure of the cosmic web.
Notes
1: Baryons are particles consisting of three quarks, such as protons and
neutrons. They make up atoms and molecules as well as all the structures
that can be seen in the observable Universe (stars, galaxies, galaxy
clusters, etc). The ‘missing’ baryons, undetected until now, should not be
confused with dark matter, which is made up of non-baryonic matter of
unknown nature.
2: ROSAT was a German space telescope designed for the observation of
X-rays.
3: The Sloan Digital Sky Survey (SDSS) is a program for surveying celestial
objects using a dedicated 2.5-meter optical telescope located at Apache
Point Observatory (New Mexico, USA). It began collecting data in 2000.
4: See the article: Density and temperature of cosmic-web filaments on
scales of tens of megaparsecs, Tanimura, H.; Aghanim, N.; Bonjean, V.;
Malavasi, N.; Douspis, M. Astronomy & Astrophysics, Volume 637, A41
(2020).
Reference:
“First detection of stacked X-ray emission from cosmic web filaments” by H.
Tanimura, N. Aghanim, A. Kolodzig, M. Douspis and N. Malavasi, 6 November
2020, Astronomy & Astrophysics.
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