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New 3D map unveiled of the hidden universe

Jenny Winder, News Writer
Oct 21, 2014, 17:08 UTC

Sen—The first three-dimensional map of the “adolescent” Universe, just 3 billion years after the Big Bang, has been created from data collected from the W. M. Keck Observatory at Mauna Kea on Hawaii. The map spans millions of light-years and provides a tantalizing glimpse of the backbone of cosmic structure.

On the largest scales, matter in the Universe is arranged in a network of filamentary structures known as the “cosmic web”, spanning hundreds of millions of light-years. Dark matter forms the backbone of this web, which is also suffused with primordial hydrogen gas left over from the Big Bang. Galaxies like our own Milky Way are embedded inside this web, but fill only a tiny fraction of its volume.

A team of astronomers led by Khee-Gan Lee, a post-doc at the Max Planck Institute for Astronomy, has created a map of hydrogen absorption revealing a three-dimensional section of the universe 11 billion light-years away—the first time the cosmic web has been mapped at such a vast distance. The map reveals the early stages of cosmic structure formation when the Universe was only a quarter of its current age, during an era when the galaxies were undergoing a major spurt in growth.

The map was created by using faint background galaxies as light sources, against which gas could be seen by the characteristic absorption features of hydrogen. The wavelengths of each hydrogen feature showed the presence of gas at a specific distance from us.

Combining all of the measurements across the entire field of view paves the way for more extensive studies that will reveal not only the structure of the cosmic web, but also the ways that pristine gas is funnelled along the web into galaxies, providing the raw material for the formation of galaxies, stars, and planets.


The 3D map of the cosmic web at a distance of 10.8 billion light years from Earth. Image credit: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA)

Lee and his colleagues obtained observing time at Keck Observatory. The team used the Low Resolution Imaging Spectrometer (LRIS) a very versatile visible-wavelength imaging and spectroscopy instrument. Although bad weather limited the astronomers to observing for only 4 hours, the data they collected with the LRIS instrument was completely unprecedented.

“The data were obtained using the LRIS spectrograph on the Keck I telescope,” Lee said. “With its gargantuan 10-metre-diameter mirror, this telescope effectively collected enough light from our targeted galaxies that are more than 15 billion times fainter than the faintest stars visible to the naked eye.

“Since we were measuring the dimming of blue light from these distant galaxies caused by the foreground gas, the thin atmosphere at the summit of Mauna Kea allowed more of this blue light to reach the telescope and be measured by the highly sensitive detectors of the LRIS spectrograph. The data we collected would have taken at least several times longer to obtain on any other telescope.”

Due to the large amount of data, a naïve implementation of the map-making procedure would require an inordinate amount of computing time. Fortunately, team members Casey Stark and Martin White (UC Berkeley and Lawrence Berkeley National Lab) devised a new fast algorithm that could create the map within minutes.

“We realized we could simplify the computations by tailoring it to this particular problem, and thus use much less memory. A calculation that previously required a supercomputer now runs on a laptop”, says Stark.