January 20, 2014
Filaments Of Cosmic Web Illuminated By Distant Quasar
redOrbit Staff & Wire Reports - Your Universe Online
A distant quasar discovered by astronomers working at Hawaii’s WM Keck Observatory has for the first time revealed part of the filament networks believed to connect galaxies in a cosmic web, according to research published Sunday in the journal Nature.
Using the observatory’s 10-meter Keck I Telescope, a team of researchers including first author Sebastiano Cantalupo, a postdoctoral fellow at the University of California, Santa Cruz, detected an extremely large and luminous nebula of gas extending approximately two million light-years across intergalactic space.
“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,” Cantalupo explained in a statement.
According to the study authors, the standard cosmological model of structure formation in the universe predicts that galaxies are part of a cosmic web of matter, the overwhelming majority of which is dark matter that is invisible to the naked eye. This web can be seen in computer simulations of the evolution of the universe’s structure.
Those models display the distribution of the dark matter that comprised the roughly 84 percent of the universe, including the halos of dark matter in which galaxies form and the cosmic web of filaments connecting them. Regular matter follows the distribution of dark matter due to gravity, so experts anticipate that filaments of diffuse, ionized gases will create a pattern similar to that demonstrated in those dark matter models.
“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,” the university said. In their study, Cantalupo and his colleagues detected the fluorescent glow of hydrogen gas that is created from its illumination from powerful radiation originating from the quasar.
According to study co-author and UC Santa Cruz astronomy and astrophysics professor J. Xavier Prochaska, the quasar is illuminating diffuse gas at a rate far beyond anything he and his colleagues have ever witnessed, and has provided them the first look at extended gas between galaxies.
“It provides a terrific insight into the overall structure of our universe,” he added.
The hydrogen gas that is illuminated by the quasar gives off a type of ultraviolet light known as Lyman alpha radiation. The distance to the quasar is about 10 billion light-years – so far that the light that is emitted is basically “stretched” by the expansion of the universe from an invisible UV wavelength to a visible shade of violet by the time it reached the scientists at the Keck Observatory.
Since the researchers knew the distance to the quasar, they were able to calculate the wavelength for Lyman alpha radiation from that distance. After completing that, they were able to construct a special filter for the Keck telescope’s LRIS spectrometer so that they could capture an image at that wavelength.
“We have studied other quasars this way without detecting such extended gas,” Cantalupo explained. “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. 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.”
Quasars are a form of active galactic nucleus which emits intense radiation that is powered by a supermassive black hole at its center. During a previous survey of distant quasars, Cantalupo and his colleagues detected dense gaseous knots known as “dark galaxies,” which are believed to be too small or too young to have formed stars.
“The dark galaxies are much denser and smaller parts of the cosmic web. In this new image, we also see dark galaxies, in addition to the much more diffuse and extended nebula,” he said. “Some of this gas will fall into galaxies, but most of it will remain diffuse and never form stars.”
Cantalupo said that his team estimated that the amount of gas in the nebula is at least 10 times more than expected based on computer simulations, and that they believe that there may be additional gas “contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models.”
Image 2 (below): 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 (Bolshoi simulation, by Anatoly Klypin and Joel Primack). The inset is a zoomed-in, high-resolution image of a smaller part of the cosmic web, 10 million light-years across, from a simulation that includes gas as well as dark matter (credit: S. Cantalupo). 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 quasar UM287. Credit: Background image: A. Klypin and J. Primack; Inset: S. Cantalupo