In the cluster of galaxies In Abell 98, in which two subclusters are in the process of merging, scientists have discovered a gaseous filament corresponding to the so-called warm-hot intergalactic medium (WHIM).
This plasma haze, believed to float between galaxies, is one of the prime candidates for locating a dearth of visible, diverse particles called “baryonic matter” measured in the local universe.
Previous evidence suggests that WHIM exists, but it has proven difficult to find enough material to argue how it contributes to the absence of baryons.
“Finding these strands of missing matter has proven to be extremely difficult, and only a few examples are known,” says astrophysicist Arnab Sarkar of the Harvard-Smithsonian Center for Astrophysics (CfA). “We’re excited that we’ve likely found another one.”
Missing matter is one of the strangest questions we have about the universe. We more or less know the distribution of matter/energy in space. We can’t detect most of this stuff, and so we don’t even know what it is: 68 percent in the form of dark energy and 27 percent in the form of dark matter.
The other 5 percent or so is baryonic matter. This is what we can detect and what everything we see consists of: stars, planets, dust, galaxies, clouds, black holes, people.
We know how much baryonic matter was around at that time. Big Bang, because we have radiation from that era, the cosmic microwave background (CMB), that scientists have been able to decipher.
When scientists began to take stock of the baryonic matter that is immediately around us today, however, the numbers don’t add up. Much is missing, half to a third of what was predicted based on the CMB.
One possible place for this is WHIM; filaments of gas with temperatures ranging from 10,000 to 10 million kelvins, in which baryons are heated and compressed upon impact. Finding these sparse structures in the space between much brighter galaxies, however, has proved difficult.
Enter Abell 98, a cluster of galaxies about 1.4 billion light-years away. X-ray observations by Abell 98 revealed patterns of hot gas between the two subclusters. Earlier this year, Sarkar and colleagues published an analysis that found that this filament contains a giant shock wave as the subclusters come together.
Their analysis also examined the properties of the gaseous fiber and found two different temperature regimes, one at 20 million kelvins and the other at 10 million kelvins. The researchers say the hotter gas is likely the result of overlapping gas halos around the two subclusters.
On the other hand, colder gas is consistent with the hotter, denser end of the cluster. The group discovered the theoretical range of WHIM.
In the second paper, a team of researchers led by astrophysicist Gabriella Alvarez of the CfA found additional evidence for the existence of WHIM not in the space between the two subclusters, but on the far side of the subcluster, away from the shock wave front. This was also consistent with denser WHIM.
“These measurements,” the researchers write in the paper, “provide tantalizing evidence for the presence of a larger structure, with diffuse WHIM connecting from the edge of the cluster along cosmic filaments.”
We still haven’t identified enough WHIM to explain all the missing baryons. He can also hide in other places; evidence suggests that some of them may be hiding in gas filaments that stretch between galaxies, or hiding as clouds of rarefied gas in intergalactic space.
But our WHIM detection tools are getting more powerful with next-generation X-rays. telescopes go to the sky. As they peer into the voids between the stars, they must uncover even more of the mysteries of deep space and what lurks there.
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