October 21, 1997
"The sheer number of these young star clusters is amazing," says Brad Whitmore of the Space Telescope Science Institute (STScI), Baltimore, Maryland. "The discovery will help us put together a chronological sequence of how colliding galaxies evolve. This will help us address one of the fundamental questions in astronomy: why some galaxies are spirals while others are elliptical in shape."
"These spectacular images are helping us understand how globular star clusters formed from giant hydrogen clouds in space," adds Francois Schweizer of the Carnegie Institution of Washington, Washington, D.C. "This galaxy is an excellent laboratory for studying the formation of stars and star clusters since it is the nearest and youngest example of a pair of colliding galaxies."
By probing the Antennae galaxies (called the Antennae because a pair of long tails of luminous matter formed by the encounter resembles an insect's antennae) and some of the other nearby galactic-scale collisions, Hubble is coming up with a variety of surprises:
* Globular star clusters are not necessarily relics of the earliest generations of stars formed in a galaxy, as once commonly thought, but may also provide fossil records of more recent collisions.
* The "seeds" for star clusters appear to be huge clouds (tens to hundreds of light-years across) of cold hydrogen gas, called giant molecular clouds, which are squeezed by surrounding hot gas heated during the collision and then collapse under their own gravity. Like a string of firecrackers being ignited by the collision, these reservoirs of gas light up in a great burst of star formation.
* The ages of the resulting clusters provide a clock for estimating the age of a collision. This offers an unprecedented opportunity for understanding, step-by-step, the complex sequence of events which take place during a collision, and possibly even the evolution of spiral galaxies into elliptical galaxies.
Earlier Hubble pictures show that nearly a third of very distant galaxies, which existed early in the history of the universe, appear to be interacting galaxies, like the Antennae. In particular, the Hubble Deep Field (a "long-exposure" image from Hubble looking at galaxies far back into time), uncovered a plethora of odd-shaped, disrupted-looking galaxies. They offer direct visual evidence that galaxy collisions were more the rule than the exception in the early days of the universe.
These distant galaxy collisions are too faint and too small to study in much detail. Astronomers say we are fortunate to have such a nearby example as the Antennae to study, since collisions between galaxies are relatively rare today. "The degree of detail in these images is astounding, and represents both a dream come true and a nightmare when it comes to the analysis of such a large amount of data," Whitmore says.
In addition to providing a window into how stars and galaxies formed in the dim past, the Hubble views might also offer a glimpse of the future fate of Earth's home galaxy, the Milky Way, when it either sideswipes or plows head-on into the neighboring Andromeda galaxy billions of years from now.
The Hubble observations of the Antennae galaxies, as well as several other nearby colliding galaxies, were conducted by Whitmore (STScI) and co-investigators Francois Schweizer and Bryan Miller (Department of Terrestrial Magnetism, Carnegie Institution of Washington), and Michael Fall and Claus Leitherer (STScI) over the past several years.
Hubble's resolution and sensitivity allowed the team to uncover over 1,000 exceptionally bright young star clusters, sometimes called super star clusters, within the Antennae -- the prototypical galaxy smashup. Ground-based telescopes were only able to see the brightest of these clusters, and even in these cases were not able to show that the clusters were very compact with the sizes of normal globular clusters.
Observing other galaxy collisions, the Hubble team discovered the presence of young star clusters which were very bright and blue in the case of ongoing collisions, but had faded to become fainter and redder for the older merger remnants. This allowed them to place the snapshots of galaxy collisions into a chronological sequence.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).
A photo and caption are available via the World Wide Web at
and via links in
PHOTO NO.: STScI-PRC97-34a (JPEG)
A ground-based telescopic view of the Antennae galaxies (known formally as NGC 4038/4039) - so named because a pair of long tails of luminous matter, formed by the gravitational tidal forces of their encounter, resembles an insect's antennae. The galaxies are located 63 million light-years away in the southern constellation Corvus.
The respective cores of the twin galaxies are the orange blobs, left and right of image center, crisscrossed by filaments of dark dust. A wide band of chaotic dust, called the overlap region, stretches between the cores of the two galaxies. The sweeping spiral- like patterns, traced by bright blue star clusters, shows the result of a firestorm of star birth activity which was triggered by the collision.
This natural-color image is a composite of four separately filtered images taken with the Wide Field Planetary Camera 2 (WFPC2), on January 20, 1996. Resolution is 15 light-years per pixel (picture element).
Credit: Brad Whitmore (STScI), and NASA
PHOTO NO.: STScI-PRC97-34b (JPEG)
The collision triggers the birth of new stars in brilliant blue star clusters, the brightest of which contains roughly a million stars. The star clusters are blue because they are very young, the youngest being only a few million years old, a mere blink of the eye on the astronomical time scale.
These close-up views of the cores of each galaxy show entrapped dust and gas funneled into the center. The nucleus of NGC 4038 (lower right) is obscured by dust which dims and reddens starlight by scattering the shorter, bluer wavelengths. This is also the reason the young star clusters in the dusty regions appear red instead of blue.
This natural-color image is a composite of four separately filtered images taken with the Wide Field Planetary Camera 2 (WFPC2), on January 20, 1996. Resolution is 15 light-years per pixel (picture element).
Credit: Brad Whitmore (STScI), and NASA
ASTROFILE (BACKGROUND) #1
It's not certain yet whether we're in store for a head-on collision or a simple sideswiping by the massive galaxy, which is a near twin to the Milky Way. Astronomers will first need to use powerful new telescopes to precisely measure Andromeda's tangential motion across the sky. (Just as a baseball outfielder estimates whether a ball is heading directly toward him or is going to miss him by determining whether the ball is moving sideways.)
A direct collision would lead to a grand merger between the two behemoths, and the Milky Way would no longer be the pinwheel spiral we are familiar with, but would evolve into a huge elliptical galaxy.
It would happen no sooner than five billion years in the future. By then the Sun may have burned out, and the Earth reduced to a frigid, lifeless cinder. It's impossible to predict if there would be any vestige of humanity colonized among the stars, not to mention extraterrestrial civilizations around to witness this great collision.
The collision will take several billion years to fully run its course, so it will be hard for any one civilization, like ours, to fully understand the vast scale - both in time and space -- of the collision.
However, by studying pairs of other colliding galaxies and using computer simulations, astronomers can assemble a series of snapshots of the collision process and get a preview of what might eventually happen to our galaxy.
Here is a scenario of how the Milky Way might change if it were to have a head-on collision with Andromeda.
The Andromeda galaxy appears simply as a spindle-shaped smudge of light in the northern autumn sky. Because it is 2.2 million light-years away -- or roughly 20 times the diameter of our Milky Way galaxy - it only appears four times the width of the full moon. As the two galaxies approach each other, Andromeda will grow ever larger in the sky, resembling an eerie glowing sword of light.
When the Andromeda galaxy and our Milky Way galaxy are close enough, huge clumps of cold, giant molecular clouds, each measuring tens to hundreds of light-years across, will be compressed. Like plugging in a string of Christmas light bulbs, these dark knots will light up as millions of stars burst into life. Most of these stars will be in brilliant blue clusters, many of them 100 times brighter than the original globular star clusters already present in the two galaxies.
The disk of dust and stars that for billions of years marked the lanes of our galaxy and the Andromeda galaxy, will also begin to come apart under the gravitational pull of the two galaxies. As Andromeda swings past our galaxy, the sky will grow increasingly jumbled with tattered lanes of dust, gas, and brilliant young stars and star clusters.
So many new stars will be born that the fraction of massive stars that are present will increase dramatically. These stars will begin popping off like a string of firecrackers as they self-destruct as supernovae.
After swinging by our galaxy, Andromeda will take perhaps 100 million years to make a slow and graceful U-turn, before plunging nearly directly into the Milky Way's core. Another, even more spectacular burst of star formation will then occur, with the winds from the supernovae driving most of the remaining gas and dust out of the galaxy. Soon both the old and new stars of the two galaxies will intermingle to form a single elliptical-shaped galaxy.
As the stars gravitationally settle into their new home, through a dynamic process called "violent relaxation", any hint of the Milky Way and Andromeda as majestic spiral galaxies will be gone. The band known as the Milky Way will be gone, but far in the future some astronomers might gaze out onto a starry sky and look all the way into the core of the new elliptical galaxy. They would have no clue that there were once two majestic spiral galaxies, called the Milky Way and Andromeda by a long forgotten civilization.
ASTROFILE (BACKGROUND) #2
Some parts of space, however, are not wide open frontiers containing a few atoms. These cosmic spaces comprise dense clouds of dust and gas left over from galaxy formation. Since these clouds are cooler than most places, they are perfect breeding grounds for star birth. When the density is 1,000 times greater than what is found in normal interstellar space, many atoms combine into molecules, and the gas cloud becomes a molecular cloud. Like clouds in our sky, these molecular clouds are puffy and lumpy. Molecular clouds in our Milky Way Galaxy have diameters ranging from less than 1 light-year to about 300 light-years and contain enough gas to form from about 10 to 10 million stars like our Sun. Molecular clouds that exceed the mass of 100,000 suns are called Giant Molecular Clouds.
A typical full-grown spiral galaxy contains about 1,000 to 2,000 Giant Molecular Clouds and many more smaller ones. Such clouds were first discovered in our Milky Way Galaxy with radio telescopes about 25 years ago. Since the molecules in these clouds do not emit optical light, but do release light at radio wavelengths, radio telescopes are necessary to trace the molecular gas and study its physical properties. Most of this gas is very cold (about -440 degrees Fahrenheit) because it's shielded from ultraviolet light. Since gas is more compact in a colder climate, it is easier for gravity to collapse it to form new stars.
Ironically, the same climate that is conducive to star formation also may shut off the star birth process. The problem is heat. Young stars are very hot and can heat the molecular gas to more than 1,000 degrees Fahrenheit, which is an unfavorable climate for star birth. When the temperature exceeds about 3,000 degrees Fahrenheit, the gas molecules break down into atoms.
The density of the gas can increase considerably near the centers of some Giant Molecular Clouds: Gas as dense as 1 billion molecules per cubic inch has been observed. (Though dense by astronomical standards, such gas is still 100 billion times thinner than the air we breathe here on Earth at sea level!) In such dense regions, still denser blobs of gas can condense and create new stars. Although the star formation process is not fully understood, there is observational evidence that most stars are born in the densest parts of molecular clouds.
What happens when stars begin forming in Giant Molecular Clouds depends on the environment. Under normal conditions in the Milky Way and in most other present-day spiral galaxies, star birth will stop after a relatively small number of stars have been born. That's because the stellar nursery is blown away by some of the newly formed stars. The hottest of these heat the surrounding molecular gas, break up its molecules, and drive the gas away. As the celestial smog of gas and dust clears, the previously hidden young stars become visible, and the molecular cloud and its star-birthing capability cease to exist. Two years ago the Hubble Space Telescope revealed such an emerging stellar nursery in the three gaseous pillars of the Eagle Nebula.
Giant Molecular Clouds in colliding galaxies may experience a different fate. As the collision crunches the interstellar gas and stars form at an accelerating rate, the gas pressure around the surviving Giant Molecular Clouds increases one-hundred- to one-thousand-fold. Calculations suggest that the hot surrounding gas can trigger rapid star birth throughout the clouds by driving shock waves into them. The several hundred thousand stars that form from the cold molecular gas in such clouds use up most of the gas before it has time to be heated and dispersed. The result of such violent events is the nearly complete conversion of Giant Molecular Clouds into rich star clusters, each containing up to 1 million stars. Observations by the Hubble telescope suggest that many of these newly born star clusters remain bound by their own gravity and evolve into globular clusters, like those observed in the halo of our Milky Way.
ASTROFILE (BACKGROUND) #3
But there was a group of astronomers who had a less kind view of the universe. They believed that the universe was a violent place, full of collisions, cannibalism, and mergers. Galaxies, they proposed, may not have been created in cookie-cutter fashion early in the universe. Maybe collisions between spirals spawned ellipticals.
Primitive Computer Models
The debate over the role colliding galaxies play in galaxy evolution has continued for decades. In the 1940's, just a few years after American astronomer Edwin Hubble defined galaxy shapes, Swedish astronomer Erik Holmberg wondered what would happen if a couple of galaxies encountered one another. So he constructed an analog computer using about 200 light bulbs to simulate galaxy encounters. Based on this seemingly primitive computer simulation, Holmberg concluded that some galaxies may indeed collide, inducing tides or distortions that rob them of energy, thus causing them to slow down and eventually merge into a single galaxy. The Swedish astronomer's computer simulations also foreshadowed the important role that computers would play in studying galaxy interactions.
Snapping Images of Enigmas
The astronomical community largely ignored Holmberg's work. The snubbing, however, didn't stop some astronomers from pursuing these enigmatic galaxies. Swiss astrophysicist Fritz Zwicky at the California Institute of Technology was the first to systematically photograph interacting galaxies in the 1950's. He noticed wispy tails in these galaxies that were similar to those that Holmberg had discovered in his simulations, and concluded that they must stem from gravitational interaction. Zwicky also guessed that these tails must consist of stars.
Still, most astronomers paid little attention to the subject of colliding galaxies, mainly because they believed that the chance of galaxy encounters was relatively small. They didn't understand that galaxies, like stars, often orbit in double and multiple systems, creating a dense environment where collisions are more likely. Some astronomers proposed that the wispy tails were the remnants of gigantic explosions.
Peculiar or Symmetrical?
Many astronomers believed, as Hubble did, that most galaxies were orderly and symmetrical. Astronomer Allan Sandage emphasized those galaxies in his 1961 book "The Hubble Atlas of Galaxies." He also was among a group of astronomers who proposed that the blobby ellipticals were formed before the disk-shaped spirals.
But astronomer Halton Arp believed in a different kind of universe, one filled with violence. In 1966 he published a catalogue of 338 interesting systems called the "Atlas of Peculiar Galaxies." Arp was convinced that colliding galaxies were more than just oddball systems: He was the first to suggest that these galaxies could form stars in bursts.
Faster Computers Equal Better Models
Colliding galaxy research received a boost in the late 1960's when scientists made significant improvements in computer technology. Faster, more powerful computers meant more sophisticated simulations of galaxy interactions, which could furnish astronomers with details about these collisions.
Soon after, several astronomers using computer simulations to study colliding galaxies published scientific papers on their work. The paper with the most-developed theory was written in 1972 by the Toomre brothers, Alar and Juri. Instead of plugging in a couple of generic interacting galaxies into their computer to see the results, they also chose four well-known colliding spiral galaxies, including M51 and the Antennae. They wanted to know whether their computer results would match observational evidence. The brothers discovered that they did. Their models showed that galaxy collisions cause strong gravitational interactions, which produce features similar to the bridges and tails of dust and stars found in many of the galaxies in Arp's atlas of galaxies.
After colliding, these galaxies slow down and are drawn closer together until they eventually merge. The offspring of these mergers are star piles resembling elliptical galaxies. There must have been many more mergers in the past when the universe was younger and denser. Alar Toomre, in his classic 1977 paper, estimated that about 10 percent of all galaxies should be merger remnants, a percentage that roughly matches the number of ellipticals observed in the universe. Their conclusion was a salvo shot at a popular theory that ellipticals came before spirals.
The Toomres also were among the first astronomers to suggest that debris stirred up from galaxy interactions could provide fuel for black holes, which power quasars. They penned the phrases "stoking the furnace" and "feeding the monster," descriptions that are now indelibly linked with black holes and quasars.
A Puzzling Question
Although astronomers debated the Toomres' work, they began to take the study of colliding galaxies more seriously. But they still had objections. Among them was this puzzle. Spirals are full of gas, but contain relatively few globular clusters (dense spherical clusters of about 100,000 stars). Ellipticals, on the other hand, contain very little gas but possess many globular clusters. How, then, can two merging spiral galaxies produce an elliptical? It's almost like saying 2 plus 2 equals 8. Our Milky Way galaxy, a spiral, has about 150 globular clusters while an elliptical with the same brightness would contain about 600 globulars.
These dissenting astronomers did not consider the important role that gas plays in mergers. Most mergers involve gas being compressed, which triggers intense star formation. Perhaps this burst of star formation could produce new globular clusters?
A Burst of Infant Stars
Astronomers studying colliding galaxies hoped that a new infrared satellite would provide some clues. They weren't disappointed. The satellite, called the Infrared Astronomical Satellite (IRAS), was launched in 1983 to take an infrared survey of the sky. This survey revealed that the most luminous galaxies in the infrared part of the spectrum were always colliding galaxies, illuminated by dust surrounding a burst of infant stars. The images provided evidence that interacting galaxies showed signs of unusually vigorous star formation, a theory that originally had been proposed by Zwicky and Arp.
When two galaxies collide, their interstellar gas is compressed into thick clouds. These clouds of gas collapse even more under gravity's intense force to form new stars. The resulting star burst uses up nearly all of the interstellar gas and expels most of the remaining gas through supernovae explosions, leaving a gas-poor system similar to that of an elliptical galaxy.
Young Blue Star Clusters
Some astronomers believed this intense star formation might produce globular clusters. These clusters would shine with the blue light of hot stars. Other astronomers, however, argued that there was no such evidence for young globular clusters. They contended that globular clusters, such as the ones in our Milky Way, are old. But astronomer Francois Schweizer of the Carnegie Institution of Washington disagreed. Schweizer had teamed up with Alar Toomre to probe several interacting galaxies. In 1982 Schweizer studied the interacting galaxy NGC 7252 (the Atoms for Peace galaxy) using ground-based telescopes and observed six bluish knots of light near the galactic nucleus. He interpreted these knots as young star clusters formed during the merger. He and other astronomers (e.g. Keith Ashman at the Space Telescope Science Institute and Steve Zepf at Johns Hopkins University) suggested that the formation of young globular clusters by colliding spirals might explain why ellipticals have so many globular clusters.
But Schweizer and other astronomers couldn't provide solid evidence for the existence of new star clusters in interacting galaxies. Ground-based telescopes didn't have the resolution to completely define these clusters. Enter the Hubble Space Telescope, with its high-resolution capabilities and its great location above the Earth's atmosphere. In the Antennae, for example, one giant star- forming knot from a ground-based telescope often turns into 10 to 12 star clusters through the eyes of the Hubble telescope, each with the size of a normal globular cluster.
Even a Hubble telescope without corrective vision found plenty of young star clusters. Peering into the core of the interacting galaxy NGC 1275, the Hubble telescope's Wide Field and Planetary Camera found in 1992 what astronomer Jon Holtzman of Lowell Observatory described as 50 young clusters less than several hundred million years old. He concluded that the clusters were spawned by a merger.
In 1993 a team of astronomers, including Schweizer of Carnegie and led by Brad Whitmore of the Space Telescope Science Institute, provided conclusive evidence that mergers produce new star clusters. Using the Hubble telescope, the team identified 40 young clusters, mostly between 50 and 500 million years old, near the center of NGC 7252.
No Longer Oddball Galaxies
Since then, Whitmore and Schweizer and their collaborators, Miller of the Carnegie Institution of Washington, and Fall and Leitherer of the Space Telescope Science Institute, have continued to probe colliding galaxies. The Wide Field and Planetary Camera 2 with its corrective vision has penetrated more than 10 times deeper into the heart of colliding galaxies than earlier observations. Recent observations of NGC 7252, for example, have revealed more than 500 star clusters, compared with only 40 in 1993.
Whitmore now believes he can tell how long ago these collisions occurred by measuring the colors and brightness of young globular clusters. These clusters, many astronomers agree, may play an essential role in understanding how galaxies evolve.
From oddball galaxies to galaxy building blocks: The part colliding galaxies play in galaxy evolution has changed dramatically over the decades.
ASTROFILE (BACKGROUND) #4
In the field of archaeology, for example, the classic problem is the search for the "missing link" between primates and humans. In a similar way, the search for the missing link between elliptical galaxies and colliding spiral galaxies has become one of the primary questions of extragalactic astronomy.
Since astronomers cannot watch an individual galaxy evolve in real time, they rely on a set of galaxy snapshots to tell about its life. The trouble is, astronomers don't know in which order they belong. What is needed is a tool to place them in the correct chronological sequence so that we can study their evolution.
Young star clusters formed in merging galaxies provide such a tool. When star clusters are born they are very blue and bright, because the stars forming in this group are extremely luminous and hot (blue stars mean hot stars).
During this stage the cluster may be more than 100 times brighter than it will appear in old age. After about 10 million years these super bright, blue stars die out, and the luster fades in brightness and becomes redder. Astronomers can use this fading and reddening to age-date the star clusters.
With this technique in mind, Hubble Space Telescope observations were obtained of a variety of merger remnants. The Hubble telescope photo collection ranges from ongoing interactions between two disk galaxies (i.e., NGC 4038/4039 - the Antennae galaxies - which are roughly 50 million years old), to recent merger remnants with characteristics of both mergers and elliptical galaxies (NGC 7252 - the Atoms for Peace galaxy - and NGC 3921, both of which are about 500 million years old), and on to dynamically young ellipticals, such as NGC 3610, where only the faint loops and shells reveal their merger history.
Scientists hope to use the information gleaned from these Hubble telescope observations to determine whether these galaxies can be linked together into an evolutionary sequence. The Hubble telescope observations, for example, provide evidence for the hypothesis that elliptical galaxies can be created by the merger of two spiral galaxies, as described in the attached chart.
Based on the results from observing a small sample of galaxies to date, the technique of using the colors of the star clusters to age-date merger remnants looks promising. However, astronomers must obtain observations from a much larger sample before making any firm conclusions about whether elliptical galaxies can be created by the merger of two spiral galaxies.