Galaxy Supercluster Overview

Galaxy Cluster

We know that galaxies are made up of stars and their planets and moons, etc., our Solar System being an example. Our Sun and Solar System is just one member of the Milky Way. The Milky Way is part of a system of galaxies known as the Local Group. The Local Group contains three large spiral galaxies: the Milky Way, Andromeda, and the Triangulum Galaxy plus a few dozen dwarf galaxies. The Local Group is just one member of the Virgo Cluster. The Virgo Cluster is a collection of 1500 to 2000 galaxies that stretch across 15 million light years of space. And then, the Virgo Cluster is just one cluster in the Virgo Supercluster.

The Virgo Supercluster’s shape is like a flattened disk, somewhat like our spiral galaxy. Our Virgo Cluster is actually an outlying group of the Virgo Supercluster. The Virgo Supercluster was given the name "Virgo" because the Virgo Cluster is the largest cluster in the supercluster. The Virgo Supercluster is just one of millions of superclusters across the universe.

Most galaxies are bundled into groups and clusters, although here and there one can find loners. Groups are smaller, usually made up of less than 50 galaxies and have diameters up to 6 million light years. For example, the Local Group, in which our Milky Way is a member, is made up of a little over 40 galaxies.

Generally speaking, clusters are bunches of 50 to 1,000 galaxies that have diameters of up to 10 mega-parsecs. On the largest of scales, clusters are arranged in long filaments surrounding very large voids of space where there are almost no galaxies at all (see the computer simulation image above). One very peculiar property of clusters is that the velocities of their galaxies are too high for gravity alone to keep them bunched together - and yet they are. The concept of dark matter begins at this scale of the universe's structure. Dark matter is believed to provide the gravitational force that keeps clusters grouped together.

Scientists believe that dark matter filaments are a remnant of the initial plasma fluctuations that dominated the universe shortly after the Big Bang. These dark matter filaments are very large and extremely long, like super highways that guide galaxies towards their ultimate destinations. Once a galaxy has joined a filament, it provides a guide path for it to join one of the "galaxy clusters" which are located at the vertices of the filaments.  Top

Great Voids In Space

Void In Space

As one can see from the image in the above section, voids are very common in outer space. Because of the huge filaments that intersect each other, voids are left in between the highways. They are believed to come about over time by the "clumping" of matter in space due to the forces of gravity. A void is almost completely empty of galaxies, loners are found here and there, but there are no galaxy clusters. There is not even dark matter occupying the void space as dark matter occupies the filament highways which channel visible matter into the supercluster centers. The voids are really just empty space!

In 2007 Lawrence Rudnick and colleagues at the University of Minnesota were studying data from the northern hemisphere survey carried out by the Very Large Array (VLA) radio telescope in New Mexico. The photons of the CMB (cosmic microwave background) from a region of the sky in the direction of the constellation Eridanus were much colder than expected. The cold spot was an unexplained anomaly in the map of the CMB created by NASA's WMAP satellite.

They saw almost no radio sources in a volume that is nearly a billion light years in diameter. The lack of radio sources meant that there were no galaxies in that area. The fact that the CMB is cold there suggests the region also lacks dark matter. The void, which is about 6 billion to 10 billion light years away, is considerably larger than any previously known void. The new void, now called the Eridanus Great Void, was 40 times larger than the previous record holder. Rudnick says that the void was probably created billions of years after the Big Bang. "We have taken the problem away from the very early universe and put the problem in the time of structure formation," he says. Computer simulations that recreate the formation of super-clusters have never seen voids of this size.

In 2009 the Six Degree Field Galaxy Survey (6dFGS), led by Heath Jones of the Anglo-Australian Observatory in Australia, scanned 41% of the southern hemisphere sky measuring the position and distance of 110,000 galaxies within 2 billion light years of Earth. The 6dFGS instrument can measure the light spectra of up to 150 galaxies simultaneously in a patch of sky spanning six degrees. The project used the 1.2-meter Schmidt Telescope in Australia and looked only at the sky visible from the southern hemisphere. The survey found some enormous voids including one that is about 3.5 billion light years across, now the record holder for the largest void in space. The cold spot is not a result of the initial CMB, it is a result of radiation passing through a void area and losing energy as it passes through it on its way to earth. The extraordinary size of the new void makes it extremely hard to understand how it developed during the 13.8 billion years since the Big Bang. Astro-physicists have come up with several scenarios to explain this gigantic void including the possibility of the sucking pull of gravity from huge concentrations of mass beyond our visible universe.  Top

The Sloan Great Wall

Sloan Great Wall

The Sloan Great Wall is a massive array of galaxy clusters named after the Sloan Digital Sky Survey (SDSS) data from which they were discovered. The SDSS was an eight year project scanning over half the northern hemisphere sky to generate full 3-D maps of almost a million galaxies. The Great Wall was discovered in 2003 at Princeton University by J. Richard Gott III and colleagues. The Great Wall is located approximately one billion light years from earth.

Walls are much wider but flatter than filaments. Analysis of the Great Wall images revealed a huge string of galaxies 1.4 billion light years in length, which is approximately 1/60 of the diameter of the observable universe. The Sloan Great Wall is 2.7 times longer than the CfA2 Great Wall (discovered at Harvard University), which is the next largest wall of galaxies. This object is so large that it is extremely hard for us humans to wrap our heads around its humongous size - 1.4 billion years at the speed of light to traverse its length - WOW!

Some parts of the Great Sloan Wall are not gravitationally bound together (and and may never be), so it should not really be considered as a single coherent structure even though it may appear that way. In spite of this, the Sloan Great Wall is a fabulous cosmic object of beauty.  Top

The Great Attractor

Great Attractor


In the 1980s, a group of scientists from Caltech discovered a gigantic magnet in space called The Great Attractor, about 200 million light years from earth. Hundreds of thousands of galaxies are marching towards this Great Attractor from all directions. It is not clear what is attracting them to this destination, as the nearby Centaurus Cluster is nowhere near large enough to be a central attraction.

In the early 1990s, astro-physicists studied the cosmic microwave background (CMB) to determine the velocities of our Local Group of galaxies. The Local Group consists of the Milky Way, Andromeda, and several dwarf galaxies. This group of galaxies tends to travel together because of their mutual gravity attraction. Their speed is roughly two million miles an hour also towards the Great Attractor.

Analysis of of the Great Attractor has been hindered by its location, which is directly behind the center of the Milky Way. The plane of the Milky Way consists of numerous super bright stars which outshine many of the objects behind it. In addition, the center of the Milky Way also contains very heavy Nebula dust clouds which block the visible wavelengths of telescopes like Hubble. There are some tricks for seeing through the dust, for example x-rays, infrared and radio observations, but the region behind the center of the Milky Way, where the dust is thickest, remains an almost complete mystery to astronomers.  Top

The Shapley Supercluster

Shapley Map

In 2005, astronomers conducting an x-ray survey of part of the sky known as the Clusters In the Zone of Avoidance (CIZA, a project to discover objects behind the center of the Milky Way dust) reported that the Great Attractor was actually only one tenth the mass that scientists had originally estimated. The survey also determined that the Milky Way is in fact being pulled towards a much more massive cluster of galaxies, the Shapley Supercluster, which is located in the same general direction, but far beyond the Great Attractor.  The center of the Shapley Supercluster is about 650 million light years from earth, whereas the Great Attractor is about 200 million light years.

The supercluster was named after Harlow Shapley, in recognition of his pioneering survey of galaxies in which this concentration of galaxies was first seen. See the map to the left, each circle is 100 million light years apart.

The Shapley Supercluster consists of the 17 clusters in turquoise on the upper right of the map. It is the most massive known structure in the observable universe topping the list of 220 known superclusters. The Shapley Supercluster is four times as massive as the Great Attractor and more than 40 million billion times the mass of our sun.

The size of the Shapley Supercluster has led many to speculate that this supercluster may be the major ingredient of our galaxy's speed and direction due to its strong pull of gravity. Is it the "Attractor" for all the surrounding galaxies as well? This question has led to a great deal of speculation about the source of the "dark flow" in our local space.

Discovered in 2008 by Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center, "dark flow" is a streak of irregularity in a universe that is otherwise as uniform as a "rising loaf of bread". Kashlinsky and his research team discovered dark flow by cleverly analyzing data collected by the WMAP satellite during the early 2000s. From a survey of more than 1000 clusters, Kashlinsky et. al. provided evidence that dark flow extends out as far as 2.5 billion light years. On such a large scale, this observation incorporates a significant chunk of the observable universe.

According to several theoretical versions of the early universe, the observable universe grew from a fluctuation in a primordial energy field. Beyond our “bubble universe" could be countless other universes that grew from other fluctuations in the great cosmic bath called the "multiverse". Perhaps the "dark flow" of our galaxies towards the Shapley Supercluster represents the result of a gravitational tug from some mass from "another universe", or at least a region beyond the "observable universe". This interaction would have occurred very early in cosmic history, long before the universe grew to its present day size. The answers to these very fundamental questions remain a mystery to be solved by future scientists studying our remarkable universe.  Top

Lyman-alpha blobs (LABs)

Lab Blob

Immense clouds of ionized plasma as large as galaxies have been observed in deep space. Located in the constellation Aquarius, the “blob” to the left is known as LAB-1, or the “Lyman-Alpha Blob 1.” It is the largest known blob and has several primordial galaxies inside it, including an active galaxy (WOW!). A press release from the European Southern Observatory (ESO) discusses LAB-1 which has been measured to be about 300,000 light years in diameter. LAB-1 was discovered in 2000 and it is so far away that its light has taken approximately 11.5 billion years to reach us (redshift 3.1).

Lyman-alpha blobs (LABs) are some of the biggest objects in the universe - gigantic clouds of hydrogen gas and dust that can reach diameters of several hundred thousand light years (three times larger than the Milky Way). These gigantic blobs emit radiation as powerful as the brightest galaxies. They are typically found at extreme distances, so we see them as they were when the universe was only a few billion years old.

Where does the name Lyman-alpha come from? As an electron drops down from a higher energy orbit in an atom to a lower one, it emits light in the ultraviolet range. The photo emissions from electrons at the two level of hydrogen dropping to the one level measure 122 nanometers. This frequency line found in the gas spectrum is known as the “Lyman-alpha” radiation line, named after its discoverer, Theodore Lyman. A LAB is a huge concentration of mainly hydrogen gas emitting the Lyman-alpha emission line.

By studying how cosmic light is polarized, astronomers can determiner the physical processes that produced the light or what happened to it between its origin and its arrival on earth. If it is reflected or scattered, it becomes polarized and this subtle effect can be detected by very sensitive instruments. To measure the polarization of light from a Lyman-alpha blob, however, is very challenging because of the great distances involved.

By observing their target for about 15 hours with ESO’s Very Large Telescope (VLT) the ESO team found that the light from LAB-1 was polarized in a ring around the central region but that there was no polarization in the center. This effect is almost impossible to produce if light comes from gas falling into the blob under gravity. However, it is just what is expected if the light comes from galaxies embedded in the central region before being scattered by the gas. The galaxies within the LAB-1 structure are packed together four times closer than the universe's average.

Another Lyman-Alpha blob discovered by Masami Ouchi, a Carnegie Institution Fellow, is an object that existed at a time when the universe was only about 800 million years old. This blob was named "Himiko" after a legendary, mysterious Japanese queen. It stretches for 55 thousand light years, a record for a blob that early in time. Large blobs discovered so far have mostly been from when the universe was 2 to 3 billion years old. No extended blobs had previously been found before Himiko when the universe was only 800 million years old.  Top

Largest Quasar Group Discovered

Huge LQC

Since the early 1980s it has been known that quasars tend to group together in clumps forming large quasar groups ( LQGs). In January, 2013 astronomers at the University of Central Lancashire in the UK announced the discovery of the Huge-LQG (pictured at the left). The Huge-LQG, comprised of 73 quasars, is the largest known structure in the universe by far. This particular LQG stretches 4 billion light years at its largest dimension and 1.4 billion light years at its shortest. Such an enormous object is extremely hard for us humans to comprehend.

A LQG is a collection of extremely luminous quasars powered by supermassive central black holes. LQG quasars are the nuclei of galaxies from the early days of the universe that undergo brief periods of extreme  brightness. These periods are "brief" in astro-physics terms but actually last 10 to 100 million years, a long time in human terms.

The largest known structure in the universe, the Huge-LQG, challenges the Cosmological Principal, the assumption that the universe, when viewed at very large scales, looks the same no matter the direction from which it is observed.

Modern Theory of Cosmology is based on the mathematics of Einstein and deeply depends on the assumption of the Cosmological Principle. The Principle is assumed but has never been demonstrated "beyond reasonable doubt". When one runs into the very large structures of the universe, such as the Huge-LQG, the Great Wall, the Great Voids, and superclusters like Shapley, the validity of the Cosmological assumption is very questionable. However, without this assumption, the mathematical equations describing the universe become just about unsolvable, so astro-physicists continue to make the assumption in order to make general predictions about our universe.