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surrounding large nearly-empty voids. A new study in 2006 has found that spiral galaxies line up like beads on a string, with their spin axes aligned with the filaments that outline voids (Figure 03-01b). The finding supports current galaxy-formation theory, which derives a galaxy's rotation from the uneven distribution of the visible and dark matter from which it coalesces. It predicts a galaxy's axis should be |
Figure 03-01a The Coma Supercluster [view large image] |
Figure 03-01b Spin of Galaxies [view large image] |
more-or-less perpendicular to the line between the galaxy and the center of the void. |
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are not isolated in space but together with many other smaller concentrations of galaxies, they form parts of extensive walls of galaxies surrounding large voids. Three of the biggest walls as well as several of the largest voids are marked on the map. The Virgo along with the Hydra and other superclusters are streaming at a speed of 6x107 cm/sec toward the "Great Attractor" (Figure 03-02b, 2-D plot on galactic plane with Milky Way at the center, arrows show the directions and magnitude of the motion), which is a gigantic unseen mass located near the A3627 (Norma) cluster (Figure 03-02c) in |
Figure 03-02a Large Scale Structures [view large image] |
Figure 03-02b Attractor A3627 [view large image] |
the Centaurus Wall near the galactic plane. In comparison, the speed of cosmic expansion is about 7x108 cm/sec at a distance of 100 megapc. |
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Figure 03-02d (X-ray: blue; H-alpha: red; Optical: white) is the image of the galaxy ESO 137-001 with a tail that has been created as it plunges toward the center of A3627, shedding material and forming stars behind it. It is estimated that the observed mass of cluster A3627 is not able to account for the huge gravitational attraction exerted on the other clusters. There seems to be something more massive hidden by the dust and gas on the galactic plane represented by the pale blue ribbon of the Milky Way in Figure 03-02c. |
Figure 03-02c Attractor Location [view large image] |
Figure 03-02d Attraction to A3627 |
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patch of sky near A3627 in the direction between the constellations of Centaurus and Vela (the pink area in Figure 03-02e, also see Figure 03-02c). The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase. It is suggested that such motion (now called darkflow) is caused by the gravitational attraction |
Figure 03-02eUnobser -vable Attractor |
Figure 03-02f Darkflow |
of matter that lies beyond the observable universe (Figure 03-02f). Insert in Figure 03-02e shows one of such clusters 1E 0657-56. |
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Figure 03-03 presents a different view of the large scale structure, which covers a region of sky about 100o by 50o around the South Galactic Pole. The APM3 (Automatic Plate Measuring) Galaxy Survey contains positions, magnitudes, sizes and shapes for about 3 million galaxies. The picture shows the galaxy distribution as a density map on the sky. Each pixel covers a small patch of sky 0.1o on a side, and is shaded according to the number of galaxies within the area: where there are more galaxies, the pixels are brighter. Galaxy clusters, containing hundreds of galaxies closely packed together, are seen as small bright patches. The larger elongated bright areas are superclusters and filaments. |
Figure 03-03 APM Galaxy Survey [view large image] |
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more distant galaxies tend to be fainter, and also show less clustering, and so the maps has a generally uniform reddish background. The more nearby galaxies tend to be bright, and are more clustered, so the more prominent clusters of galaxies in the map tend to show up as blue. The small empty patches in the map are regions that have been excluded around bright stars, nearby dwarf galaxies, and globular clusters. Figure 03-04 shows an all sky distribution produced by the Two Micron All Sky Survey4 (2MASS) with more than one million galaxies and similar color codes (as the APM). The Milky Way is at the center of the map. |
Figure 03-04 Galaxy Survey, 2MASS [view large image] |
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The 2dF Galaxy Redshift Survey5 (completed in 2002) provides yet another view of the large-scale structure as shown in Figure 03-05. It used the 3.9 meter Anglo-Australian Telescope to obtain spectra for nearly a quarter million galaxies up to redshift of 0.20. The pattern is remarkably similar to the computer simulation assuming the WMAP dark matter and dark energy composition of 30% and 70% respectively. |
Figure 03-05 2dF Galaxy Survey |
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The Milky Way is a member of the Local Group of galaxies, which in turn is a part of the Virgo supercluster (see Figure 03-06). It is centered on the Virgo cluster and extends some 150 million light years across. The Virgo cluster itself contains thousands of galaxies including M87, which is known to surround a gigantic black hole. Virgo's gravity affects the movement of its neighbors, including the Local Group. The supercluster is the last outpost before a space traveler would enter a nearly galaxy-free region called a cosmic void. Actually, even the supercluster has a mass equaling some thousand trillion suns, virtually all its volume is empty in such a vast space. The Local Group of galaxies extends some 4 million light years across. Most galaxies in the group are considered dwarfs, but the two largest - the Milky Way and the Andromeda galaxy - are giant spirals moving toward each other at a speed of about 100 km/sec. All the galaxies of the Local Group are traveling together through space - indicating a common origin. |
Figure 03-06 Virgo Supercluster[view large image] |
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The space between superclusters is not absolutely void. Invisible clouds of hydrogen have been detected by their effect on the spectra of distant quasars. As shown in Figure 03-07, when light from the quasars travels through the hydrogen clouds, each cloud imprints an absorption line of Lyman-alpha onto the continuum spectrum. As the clouds are expanding with different rate respect to the quasar, a series of red-shifted lines called Lyman-alpha forest is formed. These features encode information about the distribution and density of cold gas along the line of sight to the quasars. |
Figure 03-07 Lyman-alpha Forest [view large image] |
Incidentally, such pattern constitutes another evidence for cosmic expansion. |
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Figure 03-08 shows two actual quasar spectra. One is the relatively nearby quasar 3C273 at redshift z = 0.158 while Q1422 is a high redshift object at z = 3.62. The actual red shifted Lyman-alpha emission lines for the two quasars seemingly occupy about the same position in the spectrum because the shift becomes in-sensitive for high redshift source. Since there are more clouds in between the high redshift object and us, the Q1422 spectrum displays much more numerous absorption lines earning the name of a "forest". The data also show that there are small number of very big clumps of hydrogen in the distant Universe: the galaxies, but small chunks related to the dwarf galaxies are very much more common. |
Figure 03-08 Lyman-alpha Forest Examples [view large image] |
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Formation of superclusters may be the next stage in a process that is shaping and forming fundamental units in the universe. It is believed that the process began after the Big Bang, when matter in the universe expanded out rapidly. Some matter clumped together to form stars. Then gravity took over and the stars formed galaxies, then groups, then clusters and, now, superclusters. The supercluster formation occurring now is at an early stage. These objects may be at the critical point of overcoming the random motion and are now collapsing under its own gravitation into an increasingly dense superstructure. Figure 03-09 is a computer simulation of the growth of large scale structure as matter is accreted along the filaments. Each square represents a step in the evolution of the universe. The sequence commences at redshift 10.0, less than 500 million years after the Big Bang, and terminates at redshift 0 corresponding to the current epoch. |
Figure 03-09 Com |
puter Simulation of Large Scale Structure Evolution [view large image] |
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The inflation theory attributes the origin of large scale structures to quantum fluctuation (of the scalar field), which occurred near the beginning of Big Bang. The fluctuation is subsequently enlarged by the inflation and served as a blue-print for the large scale structures such as the superclusters. Figure 03-10 depicts the supercluster formation from quantum fluctuations. The dot at the top shows the actual size, just at the end of inflation. An enlargement (about 300X) of a small section of the universe at this time is shown in the middle. Eventually, after about 14 billion years, the imprint has accumulated enough matter and form the Coma supercluster today. In gravitational terms, the superclusters are merely slight irregularities on a basically smooth universe. It requires only one part in 100,000 of its rest-mass energy to pull the structure apart. This is in agreement with the observed fluctuation level in CMBR and is referred to as scale invariance.. There is a problem with the formation of superclusters. Theory associates a characteristic time for the gravitational settling near the center of a clump. For a density fluctuation of 1.7%, it is of the order of 1 billion years; it would be 13 billion years for 0.3% fluctuation, etc. However, CMBR measurements imply a fluctuation of only 0.001%, which requires |
Figure 03-10 Supercluster Formation [view large image] |
a settling time 1000 times longer than the age of the universe. The inconsistency can be resolved only if there is "dark matter" to enhance the fluctuation. |
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Recently in early 2004, several new measurements9 of galaxies and clusters in the early universe indicate that the structures involving galaxies and clusters are larger than expected with the new standard "dark-energy" cosmology. The controversy centers on the inability of a dark-energy dominated universe to create such large structures within such a short time (1/5 of the present age). More researches are required to validate such observations. The next step is to map an area of sky ten times larger, to get a better idea of the large-scale structure. Several such surveys are currently under way. Figure 03-11 is a computer-generated illustration of a universe that shows a string of galaxies of the size measured - 300 million light years. |
Figure 03-11 Galactic String |
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The power spectrum of the cosmic structure is different from the CMBR power spectrum since there is insufficient repulsive force to counteract the gravitational attraction. The plot (see Figure 03-12) does not show the "up and down" variation as in Figure 02-08. It displays a smooth curve for the variation of galaxy counts on different scale. The measurements have been taken by both the 2dF and SDSS10 (Sloan Digital Sky Survey) teams with consistent results. Essentially, the measurements were performed with a series of spheres of a given radius at random in the universe and counting the number of galaxies in each one and compute the average difference. The procedure was repeated with spheres of various radii to produce the plot in Figure 03-12, which is in broad agreement with CDM theory. |
Figure 03-12 Power Spectrum |
