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This 10 million light year-wide section of a simulation of the early Universe shows how matter coalesces into galaxies connected by filaments of rarefied gas.
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Light from ancient quasar reveals intergalactic web
Astronomers say it's the first direct imaging of the long-sought gas filaments stretching between galaxies.
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Astronomers have discovered the largest known gas cloud in the Universe. The mammoth nebula may be the first imaged filament of a spidery arrangement of galaxies, gas and dark matter that traces the large-scale structure of the cosmos. The team used a brilliant quasar, seen as it appeared when the Universe was less than 3 billion years old, to illuminate the faint gas in the beacon’s neighbourhood.
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The quasar UM 287 (bright spot at the centre) shines light on the largest gas cloud ever seen in the Universe.
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The flood of light from the quasar (one of a class of intensely bright galaxy cores, thought to be black holes going through a spurt of growth) prompts hydrogen atoms in the gas to emit a characteristic wavelength of ultraviolet radiation. As the Universe expands, the radiation subsequently stretches into longer wavelengths, becoming visible light. Astronomers Sebastiano Cantalupo and Xavier Prochaska of the University of California, Santa Cruz, and their colleagues recorded that light using the Keck Observatory atop Hawaii’s Mauna Kea volcano. The Keck images reveal a gas cloud that is 460,000 parsecs (1.5 million light years) in length — or more than ten times the diameter of our own Milky Way galaxy. It is the first detection of radiation from a cloud “on scales far beyond a galaxy”, Prochaska says. The researchers describe the discovery online today in Nature1.
The paper “reports a spectacular observation — the largest diffuse gas cloud ever found”, comments astronomer Joop Schaye of Leiden University in the Netherlands, who was not part of the study.
Before galaxies formed, the Universe contained a primordial, diffuse gas of matter, consisting of both ordinary atoms and dark matter. According to the leading model of galaxy formation, gravity first concentrated dark matter into halos, and these provided 'gravitational wells' in which galaxies then coalesced. Simulations have repeatedly shown that not all matter would fall into these wells, however, and that instead, some thin bridges would be left over, linking galaxies into a cosmic web (see picture at left).
Researchers have previously found evidence of a filament of the cosmic web that is made of dark matter, inferring the mass and shape of this invisible tendril by the way it gravitationally bends and brightens light from more-distant galaxies that lie behind it as seen from Earth2.
Tentative tendrils
The cloud extends much farther than a halo of dark matter could, suggesting that it must stretch between galaxies. This is best explained if the cloud itself were a visible filament of the cosmic web, Prochaska says.
Other scenarios that might account for the mammoth cloud — such as gas that has been stretched and torn by a galaxy's gravitational pull — don’t work because “the scale and mass of the cloud is far too large” for such an explanation, Prochaska adds.
Although the gas cloud could be the first visible filament of the web that astronomers have detected, Schaye says that the identification remains uncertain. “The authors do make a convincing case that the emitting gas extends beyond the dark halo hosting the bright quasar, but that does not [necessarily] make it a large-scale filament,” he cautions.
Even if it is just a mammoth gas cloud, the discovery is significant, Schaye notes, because it seems that only some galaxies and quasars are surrounded by such clouds. And if the cloud is a filament, its brightness indicates that it contains more than ten times as much cold gas than current simulations of the cosmic web permit.
One explanation for this mismatch in mass, the team notes, is that gas in the cosmic web clumps together more than predicted, on scales of a few thousand parsecs — a length smaller than simulations can clearly resolve. The clumpiness is important to determine, because it may affect the rate at which cold gas, which fuels star formation, falls into galaxies, Prochaska notes.
Two new instruments will soon join the search for more giant gas clouds. At the Very Large Telescope in Paranal, Chile, the Multi Unit Spectroscopic Explorer instrument is expected to begin operation in October, and will hunt for clouds surrounding quasars that are more remote than the one examined by Prochaska’s team. At the Keck Observatory, the Cosmic Web Imager, scheduled for installation next year, will examine clouds in finer detail around quasars that lie closer to Earth.
Quelle: nature
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Astrophile: Giant flashlight illuminates cosmic network
The cobwebby filaments that stretched between galaxies in the early universe have shown themselves for the first time. Light from the activity of a distant supermassive black hole is serving as a giant cosmic flashlight, illuminating an enormous strand of gas held together by invisible dark matter.
The geometry of large-scale cosmic structures helps us piece together the processes that formed the universe. Though this first glimpse mostly fits with existing models, there is one surprise that suggests some processes are currently missing in our understanding.
Theory and simulations of the evolution of the universe suggest that, as the universe cooled after the big bang, dark matter settled into a network of filaments that criss-crossed the cosmos. The greater gravity at points where these strands crossed drew in ordinary matter, which eventually grew dense enough to ignite stars and develop galaxies and galaxy clusters.
The trouble is that most of this network is invisible, says Sebastiano Cantalupo at the University of California, Santa Cruz. "When we look at the night sky, we see galaxies. They look to us like little islands of stars completely unconnected to each other," he says. "Most of the universe is dark."
Previously, researchers had spotted evidence of this web connecting galaxy clusters in the nearby universe in the form of strands of hot gas about a billion light years away from the Milky Way. But the gas tracing the web in the more distant universe is too cold to emit light on its own.
Very lucky night
So Cantalupo and his colleagues used a trick. "If the gas doesn't emit light, we shine a bright light on it, illuminate it with an external source," he says.
They searched for some of the brightest sources of radiation in the universe: discs of hot gas surrounding supermassive black holes at the centres of distant galaxies, called quasars. When the light from these beacons shines on the gas in a cosmic filament, the gas can absorb and re-emit the light in another wavelength.
Detecting such light is still a challenge. The team had to build a custom filter tuned specifically to the wavelength of the light from each quasar, and put it on the camera of the 10-metre Keck telescope in Hawaii, one of the largest telescopes on Earth.
The team also had to be fortunate with their viewing geometry. Quasars emit light in a beam perpendicular to their galactic discs, so they don't illuminate the entire cosmic web around them – only any stray filaments they happen to point towards.
So it was a surprise when the first quasar Cantalupo observed at Keck, called UM 287, shone on a filament right away. "It was a very lucky night," he says. "I thought these things might be extremely rare, and so was very surprised when my first night at Keck we detected this filament." It turned out he was right about their rarity: none of the 10 other quasars they observed revealed anything.
Intergalactic enigma
UM 287 is 10 billion light years away, so its light comes from when the universe was relatively young. The filament it illuminates is 2 million light years across, large enough that it must extend from UM 287's host galaxy into intergalactic space.
In some respects, the filament confirms the current picture of how the universe formed. "We always had this picture of the cosmic web in mind, and we had some ideas of connecting the galaxies," Cantalupo says. "But the way to really see if the gas is in a filament or a different morphology – in clumps, in a ball, in a sphere – is to look in emission. This is in a sense the first image that we have of the cosmic web."
However, the filament does contain one surprise: it is much more massive than simulations predicted, containing gas that weighs the equivalent of a thousand billion suns. "This is probably telling us that we are missing some physical processes in our models of intergalactic gas at large scales," says Cantalupo. "There is still a lot of work to do, but we know at least which direction to go."
"It's a very enigmatic object," says Joop Schaye at Leiden Observatory in the Netherlands, who was not involved in the new discovery. "These things often lead to understanding later on when they're more carefully studied."
Quelle: NewScientist
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