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: S. Cantalupo (UCSC); Joel Primack (UCSC); Anatoly Klypin (NMSU)
This
deep image shows the nebula (cyan) extending across 2 million
light-years that was discovered around the bright quasar UM287 (at the
center of the image). The energetic radiation of the quasar makes the
surrounding intergalactic gas glow, revealing the morphology and
physical properties of a cosmic web filament. The image was obtained at
the W. M. Keck Observatory. Credit: S. Cantalupo, UCSC
MAUNA KEA, HAWAII – Astronomers have
discovered a distant quasar illuminating a vast nebula of diffuse gas,
revealing for the first time part of the network of filaments thought to connect
galaxies in a cosmic web. Researchers at the University of California, Santa
Cruz, led the study, published January 19 in the journal, Nature.
Using the 10-meter Keck I telescope at
the W. M. Keck Observatory in Hawaii, the researchers detected a very large,
luminous nebula of gas extending about 2 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," said Sebastiano
Cantalupo, first author of the paper and a postdoctoral fellow at UC Santa
Cruz.
The standard cosmological model of
structure formation in the universe predicts that galaxies are embedded in a
cosmic web of matter, most of which (about 84 percent) is invisible dark matter.
This 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.
Until now, 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 this
study, the researchers detected the fluorescent glow of hydrogen gas resulting
from its illumination by intense radiation from the quasar.
"This quasar is illuminating
diffuse gas on scales well beyond any we've seen before, giving us the first
picture of extended gas between galaxies," said J. Xavier Prochaska,
coauthor and professor of astronomy and
astrophysics at UC Santa Cruz. “It provides a terrific insight into the overall
structure of our universe.”
The hydrogen gas illuminated by the quasar
emits ultraviolet light known as Lyman alpha radiation. The distance to the
quasar is so great (about 10 billion light-years) that the emitted light 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 and the LRIS (Low Resolution Imaging Spectrometer) used for this discovery. Knowing the
distance to the quasar, the researchers
calculated the wavelength for Lyman alpha radiation from that distance and
built a special filter for LRIS to get an image at that wavelength.
"We have studied other quasars
this way without detecting such extended gas," Cantalupo said. "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."
A quasar is a type of active galactic
nucleus that emits intense radiation powered by a supermassive black hole at
the center of the galaxy. In an earlier survey of distant quasars using the
same technique to look for glowing gas, Cantalupo and others detected so-called
"dark galaxies," the densest knots of gas in the cosmic web. These
dark galaxies are thought to be either 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,"
Cantalupo said. "Some of this gas will fall into galaxies, but most of it
will remain diffuse and never form stars."
The researchers estimated the amount of
gas in the nebula to be at least ten times more than expected from the results
of computer simulations. "We think there may be more 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," Cantalupo said.
In addition to Cantalupo and Prochaska,
the coauthors of the paper include Piero Madau, professor of astronomy and
astrophysics at UC Santa Cruz, and Fabrizio Arrigoni-Battaia and Joseph Hennawi
at the Max Planck Institute for Astronomy in Heidelberg, Germany. This research
was supported by grants from the National Science Foundation (AST-1010004,
OIA-1124453) and NASA (NNX12AF87G).
The W. M. Keck Observatory
operates the largest, most scientifically productive telescopes on Earth. The
two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the
Island of Hawaii feature a suite of advanced instruments including imagers,
multi-object spectrographs, high-resolution spectrographs, integral-field
spectroscopy and world-leading laser guide star adaptive optics systems. The
Observatory is a private 501(c) 3 non-profit organization and a scientific
partnership of the California Institute of Technology, the University of
California and NASA.
Steve Jefferson
Communications Officer, W. M. Keck Observatory
(808) 881-3827
sjefferson@keck.hawaii.edu
Science Contacts:
Sebastiano Cantalupo
(831) 459-5891
cantal@ucolick.org
Xavier Prochaska
(831) 459-2135
xavier@ucolick.org
Source: W. M. Keck Observatory
