Figure 1: 3D map of the
cosmic web at a distance of 10.8 billion light years from Earth. The map
was generated from imprints of hydrogen gas observed in the spectrum of
24 background galaxies, which are located behind the volume being
mapped. This is the first time that large-scale structures in such a
distant part of the Universe have been mapped directly. The coloring
represents the density of hydrogen gas tracing the cosmic web, with
brighter colors representing higher density.
Credit: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA). Larger version for download
Figure 2: Close-up of 3D map
of the distant Universe created by MPIA and UC astronomers. The
filamentary structures seen in this map span distances of millions of
light years, and represent the cosmic web at an earlier stage of cosmic
evolution when the Universe was less than a quarter of its current age.
The region of space seen here is at a distance of 10.8 billion years
from Earth. The coloring represents the density of hydrogen gas tracing
the cosmic web, with brighter colors representing higher density. The
coloring represents the density of hydrogen gas tracing the cosmic web,
with brighter colors representing higher density.
Credit: Casey Stark (UC Berkeley) and Khee-Gan Lee (MPIA). Larger version for download
Figure 3: Artist's
impression illustrating the technique of Lyman-alpha tomography: as
light from distant background galaxies (yellow arrows) travels through
the Universe towards Earth, hydrogen gas in the foreground leaves a
characteristic imprint ("absorption signature"). From this imprint,
astronomers can reconstruct which clouds the light has encountered as it
traverses the "cosmic web" of dark matter and gas that accounts for the
biggest structures in our universe. By observing a number of background
galaxies in a small patch of the sky, astronomers were able to create a
3D map of the cosmic web using a technique similar to medical computer
tomography (CT) scans. The coloring represents the density of hydrogen
gas tracing the cosmic web, with brighter colors representing higher
density. The rendition of the cosmic web in this image is based on a
supercomputer simulation of cosmic structure formation. Credit: Khee-Gan Lee (MPIA) and Casey Stark (UC Berkeley). Larger version for download
On the largest scales, matter in the Universe is arranged in a vast
network of filamentary structures known as the 'cosmic web', its tangled
strands spanning hundreds of millions of light years. Dark matter,
which emits no light, 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.
Now a team of astronomers led by Khee-Gan Lee, a post-doc at the Max Planck Institute for Astronomy, has managed to create a three-dimensional map of a large region of the far-flung cosmic web nearly 11 billion light years away, when the Universe was just a quarter of its current age. Similar to a medical CT scan, which reconstructs a three-dimensional image of the human body from the X-rays passing through a patient, Lee and his colleagues reconstructed their map from the light of distant background galaxies passing through the cosmic web's hydrogen gas.
The use of the combined starlight of background galaxies for this purpose had been thought to be impossible with current telescopes – until Lee carried out calculations that suggested otherwise. Lee says: "I was surprised to find that existing large telescopes should already be able to collect sufficient light from these faint galaxies to map the foreground absorption, albeit at a lower resolution than would be feasible with future telescopes. Still, this would provide an unprecedented view of the cosmic web which has never been mapped at such vast distances."
Lee and his colleagues obtained observing time on one of the largest telescopes in the world: the 10m-diameter Keck I telescope at the W. M. Keck Observatory on Mauna Kea, Hawaii – but were plagued by a problem more terrestrial than cosmic. "We were pretty disappointed as the weather was terrible and we only managed to collect a few hours of good data. But judging by the data quality as it came off the telescope, it was already clear to me that the experiment was going to work," says Joseph Hennawi (MPIA), who was part of the observing team.
Although the astronomers only observed for 4 hours, the data they collected was completely unprecedented. Their absorption measurements using 24 faint background galaxies provided sufficient coverage of a small patch of the sky to be combined into a 3D map of the foreground cosmic web. A crucial element was the computer algorithm used to create the 3D map: due to the large amount of data, a naïve implementation of the map-making procedure would have required 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 them to this particular problem, and thus use much less memory. A calculation that previously required a supercomputer now runs on a laptop", says Stark.
The resulting map of hydrogen absorption reveals a three-dimensional section of the universe 11 billions light years away – the first time the cosmic web has been mapped at such a vast distance. Since observing to such immense distances is also looking back in time, 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 'growth spurt'. The map provides a tantalizing glimpse of giant filamentary structures extending across millions of light years, and paves the way for more extensive studies that should reveal not only the structure of the cosmic web, but also details of its function – the ways that pristine gas is funneled along the web into galaxies, providing the raw material for the growth of galaxies through the formation of stars and planets.
Now a team of astronomers led by Khee-Gan Lee, a post-doc at the Max Planck Institute for Astronomy, has managed to create a three-dimensional map of a large region of the far-flung cosmic web nearly 11 billion light years away, when the Universe was just a quarter of its current age. Similar to a medical CT scan, which reconstructs a three-dimensional image of the human body from the X-rays passing through a patient, Lee and his colleagues reconstructed their map from the light of distant background galaxies passing through the cosmic web's hydrogen gas.
The use of the combined starlight of background galaxies for this purpose had been thought to be impossible with current telescopes – until Lee carried out calculations that suggested otherwise. Lee says: "I was surprised to find that existing large telescopes should already be able to collect sufficient light from these faint galaxies to map the foreground absorption, albeit at a lower resolution than would be feasible with future telescopes. Still, this would provide an unprecedented view of the cosmic web which has never been mapped at such vast distances."
Lee and his colleagues obtained observing time on one of the largest telescopes in the world: the 10m-diameter Keck I telescope at the W. M. Keck Observatory on Mauna Kea, Hawaii – but were plagued by a problem more terrestrial than cosmic. "We were pretty disappointed as the weather was terrible and we only managed to collect a few hours of good data. But judging by the data quality as it came off the telescope, it was already clear to me that the experiment was going to work," says Joseph Hennawi (MPIA), who was part of the observing team.
Although the astronomers only observed for 4 hours, the data they collected was completely unprecedented. Their absorption measurements using 24 faint background galaxies provided sufficient coverage of a small patch of the sky to be combined into a 3D map of the foreground cosmic web. A crucial element was the computer algorithm used to create the 3D map: due to the large amount of data, a naïve implementation of the map-making procedure would have required 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 them to this particular problem, and thus use much less memory. A calculation that previously required a supercomputer now runs on a laptop", says Stark.
The resulting map of hydrogen absorption reveals a three-dimensional section of the universe 11 billions light years away – the first time the cosmic web has been mapped at such a vast distance. Since observing to such immense distances is also looking back in time, 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 'growth spurt'. The map provides a tantalizing glimpse of giant filamentary structures extending across millions of light years, and paves the way for more extensive studies that should reveal not only the structure of the cosmic web, but also details of its function – the ways that pristine gas is funneled along the web into galaxies, providing the raw material for the growth of galaxies through the formation of stars and planets.
Background information
The work described here will be published as K.G. Lee et al., "Lyα Forest Tomography from Background Galaxies: The first Megaparsec-Resolution Large-Scale Structure Map at z > 2" in the Astrophysical Journal Letters.
• ADS entry of the article
The team members are Khee-Gan Lee, Joseph F. Hennawi, and Anna-Christina Eilers (Max Planck Institute for Astronomy), Casey Stark and Martin White, (UC Berkeley and Lawrence Berkeley National Laboratory), J. Xavier Prochaska (University of California at Santa Cruz, Lick Observatory), David Schlegel (Lawrence Berkeley National Laboratory), and Andreu Arinyo-i-Prats (Universitat de Barcelona).
This research received financial support from the National Geographic Society/Waitt Grants Program.
Contact
Khee-Gan Lee (first author)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 467
email: lee@mpia.de
Joe Hennawi (co-author)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 263
email: joe@mpia.de
Dr. Markus Pössel (public information officer)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 261
email: pr@mpia.de
Khee-Gan Lee (first author)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 467
email: lee@mpia.de
Joe Hennawi (co-author)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 263
email: joe@mpia.de
Dr. Markus Pössel (public information officer)
Max Planck Institute for Astronomy
Heidelberg, Germany
Phone: (+49|0) 6221 –528 261
email: pr@mpia.de
