Using the 10-m Keck I Telescope at the W. M. Keck Observatory in Hawaii, an international group of astronomers has discovered a distant quasar illuminating a giant gaseous nebula, about 2 million light-years across, within the network of filaments connecting galaxies in a cosmic web.
This image shows the nebula (cyan) extending across 2 million light-years that was discovered around the quasar UM 287 (center of the image). Image credit: S. Cantalupo / University of California, Santa Cruz.
“This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” said Dr Sebastiano Cantalupo from the University of California, Santa Cruz, who is the lead author of the study published in the journal Nature.
The standard cosmological model of structure formation in the Universe predicts that galaxies are embedded in a cosmic web of matter, most of which is invisible dark matter.
This intergalactic web is seen in the results from computer simulations of the evolution of structure in the Universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of filaments that connect them.
Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionized gas are expected to trace a pattern similar to that seen in dark matter simulations.
Computer simulations suggest that matter in the Universe is distributed in a cosmic web of filaments, as seen in the image above from a large-scale dark-matter simulation. The inset shows a smaller part of the cosmic web, 10 million light-years across, from a simulation that includes gas as well as dark matter). The intense radiation from a quasar can, like a flashlight, illuminate part of the surrounding cosmic web, highlighted in the image, and make a filament of gas glow, as was observed in the case of UM 287. Image credit: Anatoly Klypin / Joel Primack / S. Cantalupo.
Until now, however, these filaments have never been seen. Intergalactic gas has been detected by its absorption of light from bright background sources, but those results don’t reveal how the gas is distributed.
In the current study, the team detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from a distant quasar dubbed UM 287.
The hydrogen gas illuminated by the quasar emits ultraviolet light known as Lyman alpha radiation. UM 287 is located around 10 billion light-years away. The light from the quasar is stretched by the expansion of the Universe from an invisible ultraviolet wavelength to a visible shade of violet by the time it reaches the Keck Telescope.
Knowing the distance to UM 287, the astronomers calculated the wavelength for Lyman alpha radiation from that distance and built a special filter to get an image at that wavelength.
“We have studied other quasars this way without detecting such extended gas. The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow,” Dr Cantalupo said.
“We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar.”
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Sebastiano Cantalupo et al. A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar. Nature, published online January 19, 2014; doi: 10.1038/nature12898
