Hubble Space Telescope's recent observations identify fully formed elliptical galaxies in a pair of primordial galaxy clusters that have been surveyed by teams lead by Mark Dickinson of the Space Telescope Science Institute and Duccio Macchetto of the European Space Agency and the Space Telescope Science Institute. Although the clusters were first thought to be extremely distant because of independent ground- based observations, the Hubble images provide sharp enough details to confirm what was only suspected previously.
The surprise is that elliptical galaxies appeared remarkably "normal" when the universe was a fraction of its current age, meaning that they must have formed a short time after the Big Bang.
Dickinson, in studying a cluster that existed when the universe was nearly one-third its current age, finds that its red galaxies resemble ordinary elliptical galaxies, the red color coming from a population of older stars.
This has immediate cosmological implications, since the universe must have been old enough to accommodate them. Cosmologies with high values for the rate of expansion of space (called the Hubble Constant, which is needed for calculating the age of the universe) leave little time for these galaxies to form and evolve to the maturity we're seeing in the Hubble image," Dickinson emphasizes.
Macchetto's observation of a galaxy that existed 12 billion years ago, or nearly one-tenth the universe's present age, also finds a light distribution remarkably similar to today's elliptical galaxies. "This seems to show that elliptical galaxies reach their 'mature' shape very quickly, during a robust burst of star formation, and then evolve passively," says Mauro Giavalisco of the Space Telescope Science Institute. "Astronomers suspected that this was the case for at least some ellipticals. Now, Hubble has found direct evidence for it."
To produce such a shape in a galaxy requires one billion years for the gas to settle into the center of the galaxy's gravitational field. Therefore, these galaxies, which we observe as they were less than two billion years after the Big Bang, were beginning to form less than one billion years after the Big Bang! says Macchetto.
"Elliptical galaxies are exceptional laboratories for studying stellar dynamics and evolution," adds Giavalisco, "and the explanation of their origin is still controversial. This new observational evidence is suggesting that at least some ellipticals formed via processes such as 'violent relaxation', where a large grouping of stars will rapidly contract into a dense cluster. Well known from a theoretical point of view, these mechanisms of galaxy formation appear to have been confirmed by the images taken with the Hubble."
A Cosmic Zoo of Bizarre Galaxies
Contrary to the gravitationally "relaxed" and normal looking primordial elliptical galaxies, the same set of Hubble images tells a remarkable story of the creation — and destruction — of spiral galaxies in large clusters.
In one of the longest exposures taken to date with Hubble, representing 18 hours of continuous observing, Dickinson has uncovered a "celestial zoo" of faint, compact objects that might be the primordial building blocks from which spiral galaxies such as our Milky Way formed. These irregular bluish fragments, dating back nine billion years, may ultimately have coalesced into spiral galaxies, he reports.
"We see a bewildering range of galaxy shapes. The Hubble image is like looking at a drop of pond water under a microscope, where we see a menagerie of strange creatures." Though Dickinson does not have a direct measurement of distance, he suspects these objects are also remote cluster members since they group closely around a distant radio galaxy (a class of energetic galaxy with a precisely measured distance) and do not resemble anything seen in the present universe.
Very few of the bluish objects are recognizable as normal spirals, although some elongated members might be edge-on disks, Dickinson concludes. Among this zoo are "tadpole-like'' objects, disturbed and apparently merging systems dubbed "train-wrecks", a multitude of tiny shards and fragments, faint dwarf galaxies or possibly an unknown population of objects.
However, Dickinson cautions that the bright blue light of star formation can dramatically affect apparent galaxy shapes at great distances (where ultraviolet light is redshifted to visible wavelengths due to the uniform expansion of space). "Nevertheless, it is difficult to escape the impression that evolutionary processes are shaping or disrupting disk galaxies."
The Violent History of Spiral Galaxies
While Dickinson sees the birth of spiral galaxies, Alan Dressler's Hubble images of several rich clusters chronicle the demise of spirals inhabiting large clusters. "It seems that almost as soon as nature builds spiral galaxies in clusters, it begins tearing them apart," he says.
"The cause of this disappearance of spirals from clusters, from four billion years ago to the present, is unsettled and vigorously debated. Just the fact that the form of entire galaxies could be altered in so short a time is important in our attempts to find out how galaxies formed in the first place," Dressler concludes.
The evidence provided by Hubble shows that this large-scale galactic "demolition derby" could explain why there were so many more spiral galaxies in rich clusters long ago than there are today. Apparently, many spiral galaxies have since been destroyed or disappeared. Hubble observations also reveal many unusual objects within the clusters that can be considered fragments of galaxies.
"When we look back in time to these clusters, we see many distorted galaxies — they appear to have been disturbed or disrupted in one way or another," says Dressler. "There are so many little shreds of galaxies — it almost looks like galactic debris — flying around in these clusters. Perhaps this is a result of tidal encounters, but at this point we really don't understand what's happening. However, the Hubble pictures make it pretty clear that it had taken a long time for these star systems to organize and that in their younger forms they were still easily perturbed."
Hubble shows that spiral galaxies could not easily survive in the dynamic environment of a dense galaxy cluster. Detailed Hubble images show that these "fragile" disk galaxies were prone to being warped from their pancake shape.
Analysis of the pictures has inspired several alternate mechanisms for explaining the galaxy distortion. One possibility is that the galaxies were disrupted by mergers and tidal interaction caused by close encounters between galaxies in the dense cluster. Also, there is evidence from nearby clusters of galaxies that the hot, high pressure gas residing in a cluster can work to remove the gas in the disks of individual spiral galaxies.
Finally, disk galaxies might have been stripped of their mantles of "dark matter" (unknown material that is probably not made up of stars but accounts for a significant fraction of a Galaxy's mass) as they plunge through the cluster. Dressler points out that computer models of galaxies show that a spherical halo of material is important to stabilizing a thin disk, so loss of this material could result in the disk warping or fracturing, diminishing the galaxy's chance of survival as a spiral.
Thankfully, galaxy "bumper cars" took place only in large clusters, containing hundreds or even thousands of galaxies. Our Milky Way, one of the largest members of a Local Group of nearly two dozen galaxies, presumably evolved in a far less crowded region of the universe.
Finding Primeval Galaxy Clusters
"We have very likely identified the long-sought population of primeval galaxies," Macchetto reports. Until the Hubble results, astronomers had searched unsuccessfully for several decades for truly primeval galaxies, which are hard to find when they are in their very early phase of existence.
"If you can find the primeval galaxies at the cosmic epoch when they started to form and understand their shape, mass, color and brightness, then chances are that you will develop a better understanding of cosmology," comments Giavalisco.
Macchetto and his team used quasars (bright cores of distant active galaxies) as beacons to look for the "shadowing effect" of galaxies between Earth and the quasar. Their search strategy is based on the theory that the first galaxies to appear in the universe were highly clumped in space. Therefore, if a quasar's light is modified by an intervening galaxy, it more than likely belongs to a primeval cluster.
"All you have to do is to look around the quasar using a specially developed optical filter, fine-tuned to observe galaxies at the distance suggested by the change in the quasar's light," Macchetto says.
Using this novel technique with ground-based telescopes, the team looked at the field around quasar Q0000-263 in the constellation Sculptor and found the farthest "normal" galaxy ever observed, at a distance of 12 billion years.
This observation led Macchetto and Giavalisco to identify a whole cluster of primeval galaxies in that region of the sky. Remarkably, the Hubble has shown that the cluster members are characterized by a compact shape, supporting the idea that they all underwent a similar mechanism of formation.
"The very presence of the cluster shows that these large structures already existed two billion years after the Big Bang. This is unexpected and counter to many theories of cluster and galaxy formation," says Macchetto. "Although nothing conclusive can be stated with only one cluster, now that we know how to search for them we will be able to strongly constrain these theories."
Dickinson selected a candidate cluster for Hubble's sharp vision as a result of a ground-based infrared survey of the environments of distant radio galaxies. Based on the color and the statistical distribution of the galaxies, Dickinson concluded that a cluster is at the same distance as the radio galaxy 3C 324, located nine billion light-years away in the constellation Serpens. The cluster appeared to have a population of very red galaxies similar in color to present-day elliptical galaxies.
Hubble's 18-hour long exposure reveals thousands of faint galaxies near the limit of what Hubble can detect (29th magnitude). "Though many are presumably closer or farther than the cluster, since Hubble is peering across a tremendous volume of the universe to reach 3C 324, the galaxies concentrated around 3C 324 are most likely cluster members, he reports.
The Birth of Galaxies
Island cities of hundreds of billions of stars each, galaxies allow astronomers to trace the evolution of matter and structure since the beginning of the universe in the Big Bang. Scientists have sought to understand this evolution ever since American astronomer Edwin Hubble sorted nearby galaxies into three fundamental shapes: spiral or disk-shaped, elliptical, and irregular.
As the Big Bang theory gained acceptance in the 1950s, astronomers realized that galaxies simply weren't made the way they appear today but must evolve over time. This notion was reinforced by two dramatic discoveries in the 1960s: the confirmation of the Big Bang by detection of the cosmic microwave background and the discovery of quasars. Quasars are theorized to be the active cores of extremely distant galaxies. Their abundance at great distances clearly shows that galaxies were at a different evolutionary stage billions of years ago.
However, the fainter "normal" population of early galaxies has been elusive, because the tiny images of distant galaxies smear into faint blurs when viewed through Earth's atmosphere. In the late 1970s, astronomers found the first evidence that the stellar populations of galaxies had changed dramatically, even over a relatively small fraction of the time back toward the Big Bang. Astronomers also were puzzled by a specie of blue galaxies in distant clusters, which disappeared in our current epoch.
Now, Hubble Space Telescope's sharp view at last provides for detailed studies of the properties of early galaxies. Hubble's initial results show that the mysterious blue cluster galaxies are mostly spirals, often with signs of disturbance that may provide clues about their disappearance by the present epoch. Paradoxically, elliptical galaxies appear normal throughout most of the history of the universe, with little evidence for dramatic changes in their stellar population or shape.
Although astronomers have uncovered many of the details revealing the life cycles of individual stars, they still do not completely understand how galaxies, like our Milky Way, begin and end their lives.
The problem is that, although stars within the Milky Way may be seen in a variety of evolutionary stages, few examples of young galaxies are known, and their images come to us from the most distant edge of our visible universe. At these vast, multi-billion-light-year distances, it becomes increasingly difficult to determine what role environment plays in the formation of a galaxy.
Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas — the primordial elements forged in the Big Bang. They also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies.
Matter either was clumped into vast collections more than a million times the mass of the Milky Way, or into small clumps one million times smaller than the mass of our Milky Way. Superclusters of galaxies may have evolved from the former. Globular clusters — spherical collections of very old, densely packed stars usually found in orbit around galaxies, like the Milky Way — may have evolved from the latter.
Could these globular clusters be the meager leftovers of an ancient, once-common population of small clumps as predicted by theory? This possibility now seems increasingly more likely. So the question then arises: what formed the vast majority of the galaxies?
Detailed ground-based and space-based images of distant galaxies are beginning to turn up some interesting insights into galaxy formation.
First, as we look deeper into the universe, galaxies appear to emit more of their light in the blue part of the visible spectrum. From studies of nearby galaxies, blue light is a sign that very young, massive and luminous stars are forming. Since we see these galaxies as they were between 5 and 10 billion years ago, we appear to be witnessing events that occurred within a few billion years after these galaxies were formed.
Astronomers also have noticed that as they examine the images of these distant blue galaxies, the images are frequently distorted or contain what appear to be multiple nuclei. The Milky Way seen at a similar great distance would look like a uniformed flattened disk, with a single bright nucleus — the galactic center. Nearby "multiple-nuclei" galaxies that have been studied show the cores of individual galaxies colliding and merging into one single system of stars and gas. These collisions are violent, and take millions of years to play out. But in at least some instances, such as NGC 1275, recently observed with the Hubble Space Telescope, galaxy collisions can actually trigger the formation of massive stars.
In the depths of space, we may be witnessing collisions between smaller galaxies triggering the formation of massive luminous stars. The images, rich in blue light, gives tantalizing evidence that "environment" may have been more important than cosmic "genetics."
Galactic cannibalism was far more common in the ancient past. Galaxies may have grown to their current size by consuming their neighbors. The ultimate building blocks may indeed have been the paltry million-solar-mass clumps that theoreticians believe were abundant before the universe was a few million years old.
Galaxies come in three major classes distinguished by their appearance: spirals, like the Milky Way, are shaped like pinwheels; irregulars have no discernible shape at all; and ellipticals are round- or oval-shaped objects.
Spirals and irregulars are typically sites of ongoing star-formation and therefore contain young stars. Ellipticals, having finished their supply of fresh gas, cannot form stars any more and contain mostly very old stars.
Spiral galaxies are a composite of stars and gas in a disk surrounding a central bulge, which is rather similar to an elliptical galaxy, just smaller. Waves in the disk form the spiral arms and cause the gas to collapse and form new stars. Therefore, the disk is rich in young stars. Older stars are typically found in the bulge.
Elliptical galaxies and the bulges of spirals have been the subject of several decades of observational and theoretical work. For decades, astronomers thought that the rotation rate of these spherical star systems determined whether they would be round or oval shaped, with the more rapidly rotating ellipticals being the flattest.
Detailed studies of thousands of ellipticals over the years now suggest an entirely different picture. Ellipticals and bulges are supported against their self-gravity, which would cause them to shrink, by the random velocities of the stars, pretty much like the motion of molecules in a hot gas. The distribution of stellar motion determines the final shape of the galaxy, that is whether it is spherical, oblate, or very flattened.
In recent years, astronomers also have discovered that apparently simple galaxy shapes hide the complex, violent events that occurred in these galaxies long ago.
Some contain dense cores in which millions of stars move in orbits completely different than stars farther out from the galaxy's center. In many ways, the cores of some resemble isolated populations transplanted from outside the galaxy. Astronomers are beginning to believe that these cores are the remains of companion galaxies that were consumed when they wandered too close to these elliptical galaxies many millions of years ago. When galaxies collide, the rapidly changing gravitational fields also can synchronize the stellar orbits, creating great rings of stars which surround some ellipticals like haloes.
Elliptical galaxies also contain some of the oldest stars in the universe. While spirals and irregulars continue to produce new stars even to the present day, most ellipticals stopped forming stars more than 10 billion years ago in what must have been one great star-forming epoch.
Ellipticals contain little or no gas and dust of their own, apparently having consumed what they had when their stars were born long ago. Those ellipticals that contain higher concentrations of gas and dust apparently accumulated the material because they cannibalized their companion galaxies.
The material accumulated from these cannibalizations collides as it sinks farther and farther into the galaxy's core, and in many instances, creates new generations of massive, luminous stars. Eventually over the course of millions of years, the gas reaches the center of the galaxy where supermassive black holes may lie in wait for a new supply of fuel.