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| Galaxy clusters connected by gas bridge |
ESA’s Planck space telescope has made the first
conclusive detection of a bridge of hot gas connecting a pair of galaxy
clusters across 10 million light-years of intergalactic space.
Planck’s primary task is to capture the most
ancient light of the cosmos, the Cosmic Microwave Background, or CMB. As this
faint light traverses the Universe, it encounters different types of structure
including galaxies and galaxy clusters – assemblies of hundreds to thousands of
galaxies bound together by gravity.
If
the CMB light interacts with the hot gas permeating these huge cosmic
structures, its energy distribution is modified in a characteristic way, a
phenomenon known as the Sunyaev–Zel’dovich (SZ) effect, after the scientists
who discovered it.
This
effect has already been used by Planck to detect galaxy clusters themselves,
but it also provides a way to detect faint filaments of gas that might connect
one cluster to another.
In
the early Universe, filaments of gaseous matter pervaded the cosmos in a giant
web, with clusters eventually forming in the densest nodes.
Much
of this tenuous, filamentary gas remains undetected, but astronomers expect
that it could most likely be found between interacting galaxy clusters, where
the filaments are compressed and heated up, making them easier to spot.
Planck’s
discovery of a bridge of hot gas connecting the clusters Abell 399 and Abell
401, each containing hundreds of galaxies, represents one such opportunity.
The
presence of hot gas between the billion-light-year-distant clusters was first
hinted at in X-ray data from ESA’s XMM-Newton, and the new Planck data confirm
the observation.
It
also marks Planck’s first detection of inter-cluster gas using the SZ effect
technique.
By
combining the Planck data with archival X-ray observations from the German
satellite Rosat, the temperature of the gas in the bridge is found to be
similar to the temperature of the gas in the two clusters – on the order of 80
million degrees Celsius.
Early
analysis suggests the gas could be mixture of the elusive filaments of the
cosmic web mixed with gas originating from the clusters.
A
more detailed analysis and the possible detection of gas bridges connecting other
clusters will help to provide a more conclusive answer.
The
new finding highlights the ability of Planck to probe galaxy clusters to their
outskirts and beyond, examining their connection with the gas that permeates
the entire Universe and from which all groups of galaxies formed.
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