GIANT GAS CLOUDS ILLUMINATE UNIVERSE’S LARGEST STRUCTURE
July 26, 2006
A team of astronomers using the Subaru and Keck telescopes on Mauna Kea
has discovered giant, three-dimensional filaments of galaxies extending across
200 million light-years of space. These filaments, which formed a mere 2 billion
years after the birth of the universe, are the largest-known structures ever
discovered. They are studded with more than 30 large concentrations of gas,
each up to ten times as massive as our own galaxy. These giant gas clouds are
probably the progenitors of the most massive galaxies that exist in the universe
This finding is very important because it gives researchers new insight into the large-scale structure of the cosmos. Astronomers expect the universe to look relatively smooth 2 billion years after the birth of the universe. In summarizing the importance of this finding, astronomer Ryosuke Yamauchi from Tohoku University said, "Something this large and this dense would have been rare in the early universe. The structure we discovered and others like it are probably the precursors of the largest structures we see today which contain multiple clusters of galaxies."
Giant 3D Filaments of Galaxies
The research group used the Subaru telescope to make a detailed study of a region of sky 12 billion light-years from Earth that is known to have a large concentration of galaxies. They used Subaru's Suprime-cam camera outfitted with special filters designed to be sensitive to the light from galaxies at that distance. The results showed that this concentration of galaxies is just a small portion of a much larger structure.
The newly found giant structure extends over 200 million light years and has a concentration of galaxies up to four times denser than the universe's average. The only previous known structures with such a high density are much smaller, measuring about 50 million light-years in scale.
Using Subaru's Faint Object Camera and Spectrograph (FOCAS) to study the 3D distribution of galaxies in this filament, the team also discovered at least three overlapping filaments that make up the giant structure.
Large Concentrations of Gas
Astronomers knew this region contained at least two large concentrations of gas. One of them, shown in figure 3, extends across 400,000 light-years. A comparison with the Andromeda Galaxy, thought to be about the same size as the Milky Way Galaxy, shows the relative immensity of this gas structure.
The researchers found that these large concentrations of gas are located near the overlap regions of the filaments.
The Subaru observations were successful in finding much fainter objects than previously discovered in this region. (Figure 4) For example, they found 33 new large concentrations of gas along the filamentary structure extending across 100,000 light-years. This is the first time that so many large concentrations of gas, known to astronomers as Lyman alpha blobs, have been discovered in the distant universe.
Astronomers think that such Lyman alpha blobs, named so since they are seen in the Lyman alpha emission line of hydrogen, are probably related to the births of the largest galaxies. In the "gravitational heating" model, the blobs are regions where gas is collapsing under its own gravity to form a galaxy. The"photoionization" model attributes emission from the gas to ionization by ultraviolet light from newborn stars or a massive black hole. The "shock heating" or "galactic superwind" model hypothesizes that the glow of the gas is caused by the death of many massive stars born early in the history of the universe, living out short lives, and then dying in supernova explosions that blow out surrounding gas. Team members Yoshiaki Taniguchi and Yasuhiro Shioya (Ehime University) have been advocating for the galactic superwind model.
Observations with the DEIMOS spectrograph at the Keck II telescope revealed that the gas inside the blobs move with speeds greater that 500 kilometers per second (300 miles per second). The extent of the gas concentrations and the speed of the material within them suggest that these regions must be up to ten times as massive as the Milky Way Galaxy.
The blobs show a great variety in shape and brightness. For example, some show bubble-like features that match computer simulations of galactic winds such as those by Masao Mori (Senshu University) and Masayuki Umemura (University of Tsukuba). There are also diffuse blobs and those consisting of several galaxies (Figures 3, 5, and 6).
"Galaxies of various sizes surround us," said Yuichi Matsuda of Kyoto University."The large gas concentrations we found may tell us a lot about how the largest of these came to be."
These results were published in a series of research papers in the Astronomical Journal and the Astrophysical Journal. (Papers 6, 7, 8 in the reference list below.)
Team Members: Hayashino, Tomoki (Tohoku University); Yamada, Toru (NAOJ); Matsuda, Yuichi
(Kyoto University); Tamura, Hajime (Tohoku University); Yamauchi, Ryosuke (Tohoku University); Nakamura, Yuki (Tohoku University); Taniguchi, Yoshiaki (Ehime University); Murayama, Takashi (Tohoku University); Shioya, Yasuhiro (Ehime University) ; Nagao, Tohru (Osservatorio Astrofisico di Arcetri); Ajiki, Masaru (Tohoku University); Fujita, Shinobu S. (Tohoku University) ; Okamura, Sadanori (University of Tokyo); Shimasaku, Kazuhiro (University of Tokyo); Ouchi, Masami (Space Telescope Science Institute); Ohta, Kouji (Kyoto University)
Figure 1: The distribution of galaxies in the giant filamentary structure12 billion light years away. Black circles are "emission line" galaxies. Rid circles are "absorption" galaxies. Blue squares are the large gas concentrations called "Lyman Alpha blobs." The region with a high density of galaxies is outlined in green. The region within the orange square is where other astronomers have previously surveyed. (Enlarge)
Figure 2: The filamentary structure in 3D. There are at least three intersecting filaments. (Enlarge)
Figure 3: A comparison of a Lyman alpha blob and theAndromeda Galaxy. Light emitted by the blob is green. In the upper right corner is an image of the Andromeda Galaxy, similar in size to the Milky Way Galaxy, scaled as if were at the same distance as the blob. (Courtesy of the University of Tokyo Kiso Observatory.) The red circle indicates a bubble like structure discovered for the first time with the Subaru telescope observations. (Enlarge)
Figure 4: Images of individual Lyman Alpha blobs. The blobs are green. Each image is 620 thousand light years to the side. (Enlarge)
Figure 5: An example of a diffuse blob. (Enlarge)
Figure 6: An example of a blob composed of several galaxies. (Enlarge)
 Steidel et al. 2000, Astrophysical J., 532, 170-182
 Haiman, Spaans & Quataert 2000, Astrophysical J., 537, L5-L8
 Chapman et al. 2001, Astrophysical J., 548, L17-L21
 Taniguchi & Shioya 2000, Astrophysical J., 532, L13-L16
 Mori & Umemura 2006, Nature, 440, 644-647
 Hayashino et al. 2004, Astronomical J., 128, 2073-2079
 Matsuda et al. 2004, Astronomical J., 128, 569-584
 Matsuda et al. 2005, Astrophys