Hubble Sees Early Building Blocks of Today's Galaxies
New Hubble telescope images unveil what may be galaxies under construction in the early universe.
Hubble's detailed pictures reveal a grouping of 18 gigantic star clusters that appear to be the same distance from Earth, and close enough to each other that they will eventually merge into a few galaxy- sized objects. They are so far away, 11 billion light-years, that they existed during the epoch when it is commonly believed galaxies started to form. These results add weight to a leading theory that galaxies grew by starting out as clumps of stars, which, through a complex series of encounters, consolidated into larger assemblages that we see as fully formed galaxies.
New Hubble Space Telescope (HST) images reveal what may be galaxies under construction in the early universe, out of a long sought ancient population of "galactic building blocks."
Hubble's detailed images, taken with the Wide Field Planetary Camera 2, reveal a grouping of 18 gigantic star clusters that appear to be the same distance from Earth, and close enough to each other that they will eventually merge into a few galaxy- sized objects. They are so far away, 11 billion light-years, that they existed during the epoch when it is commonly believed galaxies started to form.
These results add weight to a leading theory that galaxies grew by starting out as clumps of stars, which, through a complex series of encounters, consolidated into larger assemblages that we see as fully-formed galaxies today.
The finding is another step back into the dim past, where astronomers ultimately hope to uncover the earliest seeds of galaxy formation which arose shortly after the birth of the universe, or the Big Bang.
Astronomers at Arizona State University, Tempe, Arizona, (ASU) and the University of Alabama at Tuscaloosa found 18 of these cosmic building blocks packed into an area about two-million light-years across. "It's the first time anyone has seen that many star-forming objects in such a small space. There are not nearly as many such luminous objects in the two-million light-years separating Earth's galaxy, the Milky Way, from the Andromeda Nebula, the nearest major galaxy", says Rogier Windhorst of Arizona State University.
The astronomers will publish their findings in an article, authored by ASU graduate student Sam Pascarelle, in the September 5 issue of the journal Nature. The coauthors are ASU's Rogier Windhorst and Stephen Odewahn, and William Keel of the University of Alabama at Tuscaloosa.
The building blocks seen by Hubble consist of only about a billion young stars each, and Hubble shows star formation is underway through the presence of many blue stars and glowing gases. The objects typically measure only 2,000 light-years across. "That's not very big. Our own galaxy is 100,000 light-years across," Odewahn says. The objects are much smaller than even the central bulge of the Milky Way, which measures about 8,000 light-years in diameter. "We think that by repeated merging, they will grow big enough to become the bulges of nearby galaxies," says Keel, citing other HST studies that have shown that the galaxy merger or collision rate was higher in the past. "In fact, at least four of the objects in this field show double structure in their centers only a few thousand light-years apart, as if we've caught them in the act of falling together."
Hubble shows a new level of detail for determining the true nature of these "pre-galactic blobs." Hubble resolved clumps as small as 2,000 light-years across (1/10th of an arc second). These were seen in a two-day (67-orbit) exposure by Hubble of a small region of sky in the northern part of the Hercules constellation near the border with Draco.
"We've never seen so many of these objects in a single exposure and so small," says Pascarelle. "We are convinced that these objects are not peculiar, but part of the general formation process of galaxies in the early universe."
Astronomers see stars form, because star formation is an ongoing process. However, astronomers have never directly seen galaxies form, because their formation may have happened a long time ago, or because galaxy formation is not as spectacular as once believed, and is therefore much harder to observe.
The idea that galaxies grew from small pieces coming together, rather than through the collapse of a gigantic gas cloud, has been predicted from previous theoretical work and ground-based observations. The Hubble observations offer some of the best direct visual evidence to date, says Pascarelle.
Though many of the objects are isolated in the image, they are close enough together in space that most of them should eventually merge, according to Windhorst. He sketches a scenario where two or more objects will pass through each other, drawing out hydrogen gas to form more stars later. (Although the term "collision" is used, their individual stars don't collide.) They may then evolve to form the numerous faint blue galaxies, a distant population of galaxies seen by Hubble and other telescopes. Later, surrounding hydrogen gas then settles into a disk to form a spiral galaxy.
If this construction plan is correct, our Milky Way galaxy contains all the pieces of the assembly process. The older, redder stars in the Milky Way's central bulge came from the merged clusters, or "sub-galactic units," seen by Pascarelle and collaborators. The spiral arm that our Sun inhabits was made later after hydrogen settled into a disk. Some of the 140 globular star clusters which orbit the Milky Way may be "left over" smaller building blocks which formed before the larger units seen by Pascarelle and collaborators, but were never pulled directly into larger assemblages.
In some of the deepest exposures of the universe (apart from the Hubble Deep Field) yet obtained by the telescope, the astronomers found 18 objects in one image, in the vicinity of a faint radio galaxy they were studying. The researchers used an optical filter precisely tuned to detect the ultraviolet emission from glowing hydrogen gas heated by newborn stars that formed early in the universe, but shifted to longer visual wavelengths by the universal expansion. "This is a case where Hubble is uniquely suited to study sub-galactic objects at these great distances," says Windhorst, "because these objects are so compact that it would be very hard to recognize them from the ground."
Follow-up spectroscopic observations with the Multi-Mirror Telescope at Mt. Hopkins, Arizona (MMT) showed at least five of the clumps are all at the same distance from Earth. The team confirmed that another five objects were at the same distance by imaging another redshifted hydrogen line in the near infrared with NASA's Infrared Telescope Facility, and through spectroscopic follow-up at the 10 meter W.M. Keck Telescope, both on Mauna Kea, Hawaii (the latter by Drs. Nicholas Scoville and Lee Armus of Caltech). The amount of redshift corresponds to a distance of 11 billion light-years - far enough to probe the early universe during the period where many of the giant galaxies were being assembled.
In a companion paper in press for the Astrophysical Journal Letters, Stephen Odewahn, Windhorst, Keel, and Simon Driver (from the University of New South Wales in Sydney, Australia) show that the counts of faint blue objects in this field are no different from that in other deep HST fields. Astronomers interpret this to mean that in almost every direction an observer should see similar activity going on at these distances - the gradual construction of galaxies from faint blue sub-galactic building blocks.
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.
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.