The Hubble images of sub-galactic objects which may be merging show exactly what the so-called "cold dark matter" theory for the evolution of cosmic structures predicts, say the researchers. The dark matter theory tries to explain why 90 percent of the matter in the universe is invisible to telescopes. Astronomers theorize the existence of dark matter from the gravitational effects it exerts on the galaxies it surrounds.
Cold dark matter could be as simple as rocks, or as exotic as cosmic particles such as neutrinos, or some other unknown particle. Whatever it is, there is a lot of it. "Since dark matter has mass, it has gravity, and therefore it affects this entire scenario of forming clumps and forming bigger clumps," Windhorst says. "These halos of cold dark matter help form galaxies."
If the dark matter is "cold," meaning that its random motions are much slower than the speed of light, it tends to form structure from the bottom up. That is, the smallest clumps of stars (star clusters and small galaxies) formed first, then merged together to form larger galaxies like our own Milky Way. These galaxies, in turn, then grouped together into clusters or superclusters giving us the highly fragmented and filamentary structure that we see in the universe today.
In contrast, if the dark matter is "hot," i.e. made up of particles, such as neutrinos, that move at nearly the speed of light, then only the largest structures can condense in the early universe. Smaller structures such as galaxies and star clusters must have formed later, from fragments within these larger structures.
"Neither scenario can be completely correct, strictly speaking, because we know that these sub-galactic objects already existed a very long time ago. They must have formed shortly after the Big Bang," Windhorst says. "But also some structure on the scales of superclusters may have condensed out of the primeval soup as shortly as one million years after the Big Bang, leading to the seeds of large scale structure in the universe."
He concludes: "Perhaps a hybrid model is necessary, but mostly leaning toward the cold dark matter. This idea that small clumps grow into bigger ones is very effective. It explains a lot of things, but it doesn't quite explain the existence of large scale structures early on."
In fact, Pascarelle and his colleagues suggest that their tightly packed group of faint blue sub-galactic building blocks are themselves part of such an early, large-scale structure. "One wonders if we simply got lucky, that we looked at an unusual patch of sky, and that these objects are not a regular part of the large-scale structure of the universe," says Pascarelle. But this concern would be relieved if the Hubble Telescope reveals a similar crowding of objects in other parts of the sky at similar large distances. Some hints for that are now being seen in other areas of the sky. The team has further Hubble observations scheduled for other sky fields, to see whether in fact these small objects at such large distances are the rule, rather than the exception. "That would show that this kind of structure existed at some level throughout the universe, and confirm our hypothesis that most galaxies may have formed through the mergers of smaller clumps," Pascarelle says.
Galaxies are the largest assemblages of stars in the universe. In a galaxy, billions of stars are bound together by the mutual pull of gravity. The Sun resides in the Milky Way galaxy.
Galaxies come in different sizes: dwarf galaxies, average galaxies, and massive galaxies. The Milky Way is an average spiral galaxy. It has two satellite galaxies orbiting it. These dwarf irregular galaxies are the Small and Large Magellanic Clouds discovered by the explorer Magellan.
The simplest galaxy classification system, invented by Edwin P. Hubble, classifies galaxies as either spiral, elliptical, or irregular in shape.
Spiral galaxies have unmistakable characteristic features. The arms of the spiral define a plane. A large concentration of stars at the center of the galaxy makes a bulge there. Spiral galaxies are rich in the gas and dust needed to form new stars. Their blue color tells astronomers that star formation is indeed ongoing in these galaxies. Our solar system lies about two-thirds of the distance from the nucleus in the Milky Way's spiral arm, called the Sagittarius Arm. The stars of the constellation Sagittarius all lie in this spiral arm of the Milky Way.
Elliptical galaxies also have characteristic structure, but are quite different from spirals. These galaxies can range in shape from nearly spherical to cigar shaped. Unlike spirals, there is not much gas and dust in ellipticals from which new stars can be made. The red color of elliptical galaxies tells astronomers that star formation has finished in these galaxies, and the stars in them are old stars.
Irregular galaxies do not to have definite structure. Often, irregular galaxies are small satellites of larger galaxies. The Large and Small Magellanic Clouds, satellites of the Milky Way, are irregular galaxies.
Galaxies themselves are also subject to the universal power of gravity. The Milky Way is in a group of galaxies loosely bound together, appropriately called the Local Group. Along with the Milky Way, the Local Group contains the giant spiral galaxy Andromeda and some small elliptical galaxies.
In larger groups of galaxies, called clusters, galaxies are so densely packed that they are gravitationally interacting with each other. The nearest cluster to the Local Group is called the Virgo Cluster, because from our vantage on Earth it appears to lie inside the constellation Virgo. Clusters and smaller groups of galaxies often are bound together in even larger structures, forming superclusters. The supercluster in which our Local Group resides contains the Virgo Cluster and other smaller clusters.
Studying galaxies falls into the realm of cosmology, the study of the evolution of the universe on the largest scale. By looking at the distribution of galaxies in space, Edwin P. Hubble discovered that the universe is expanding. Hubble found that all galaxies in all directions are receding from us, and the ones farther away are receding the fastest.
Investigations since Hubble's time have increased the types of galaxies known. Strange, unusually active galaxies and faint, blue, odd-shaped galaxies have been discovered. The active galaxies are thought to be powered by black holes in their nuclei.