XMM-Newton Discovers
Part Of Missing Matter In The Universe
(6 May 2008) ESA's orbiting X-ray
observatory XMM-Newton has been used by a team of international astronomers to
uncover part of the missing matter in the universe.
10 years
ago, scientists predicted that about half of the missing 'ordinary' or normal
matter made of atoms exists in the form of low-density gas, filling vast spaces
between galaxies.
All the matter in the universe is distributed in a
web-like structure. At dense nodes of the cosmic web are clusters of galaxies,
the largest objects in the universe. Astronomers suspected that the low-density
gas permeates the filaments of the web.
The low density of the gas
hampered many attempts to detect it in the past. With XMM-Newton's high
sensitivity, astronomers have discovered its hottest parts. The discovery will
help them understand the evolution of the cosmic web.
Composite optical and X-ray image of galaxy clusters Abell 222 and Abell 223. The cluster pair is connected by a filament permeated by hot X-ray emitting gas. The optical image was obtained by SuprimeCam at the Subaru telescope, the X-ray image showing the distribution of the diffuse hot gas (yellow to red) was obtained by XMM-Newton. (courtesy: ESA/ XMM-Newton/ EPIC/ ESO (J. Dietrich)/ SRON (N. Werner)/ MPE (A. Finoguenov))
Only about 5% of our universe is made of
normal matter as we know it, consisting of protons and neutrons, or baryons,
which along with electrons, form the building blocks of ordinary matter. The
rest of our universe is composed of elusive dark matter (23%) and dark energy
(72%).
Small as the percentage might be, half of the ordinary baryonic
matter is unaccounted for. All the stars, galaxies and gas observable in the
universe account for less than a half of all the baryons that should be
around.
Scientists predicted that the gas would have a high temperature
and so it would primarily emit low-energy X-rays. But its very low density made
observation difficult.
Astronomers using XMM-Newton were observing a
pair of galaxy clusters, Abell 222 and Abell 223, situated at a distance of
2300 million light-years from Earth, when the images and spectra of the system
revealed a bridge of hot gas connecting the clusters.
"The hot gas that
we see in this bridge or filament is probably the hottest and densest part of
the diffuse gas in the cosmic web, believed to constitute about half the
baryonic matter in the universe," says Norbert Werner from SRON Netherlands
Institute for Space Research, leader of the team reporting the
discovery.
This is a model of the cosmic web. Clusters of galaxies are expected to develop at the intersections of the web. (courtesy: Springel et al., Virgo Consortium)
"The discovery of the warmest of the missing
baryons is important. That's because various models exist and they all predict
that the missing baryons are some form of warm gas, but the models tend to
disagree about the extremes," adds Alexis Finoguenov, a team
member.
Even with XMM-Newton's sensitivity, the discovery was only
possible because the filament is along the line of sight, concentrating the
emission from the entire filament in a small region of the sky. The discovery
of this hot gas will help better understand the evolution of the cosmic
web.
"This is only the beginning. To understand the distribution of the
matter within the cosmic web, we have to see more systems like this one. And
ultimately launch a dedicated space observatory to observe the cosmic web with
a much higher sensitivity than possible with current missions. Our result
allows to set up reliable requirements for those new missions." concludes
Norbert Werner.
ESA's XMM-Newton Project Scientist, Norbert Schartel,
comments on the discovery, "This important breakthrough is great news for the
mission. The gas has been detected after hard work and more importantly, we now
know where to look for it. I expect many follow-up studies with XMM-Newton in
the future targeting such highly promising regions in the sky."
The
article 'Detection of hot gas in the filament connecting the clusters of
galaxies Abell 222 and Abell 223', by N. Werner, A. Finoguenov, J. Kaastra, A.
Simionescu, J. Dietrich, J. Vink and H. Böhringer' has been published in
the Astronomy & Astrophysics Letters on 17 March 2008.
The team of
astronomers that made the discovery includes N. Werner (SRON, Netherlands
Institute for Space Research), A. Finoguenov (MPE, Germany), A. Simionescu, H.
Böhringer (MPE, Germany), J. Kaastra (SRON, Netherlands Institute for
Space Research and Utrecht University, Netherlands), J. Dietrich (ESO, Germany)
and J. Vink (Utrecht University, Netherlands).
(source: ESA)
