Not All Is Flawed in Hubble Images
Despite a defect in its mirror, the space telescope is taking useful photographs of remote galaxies
HUBBLE Space Telescope is performing better than astronomers expected when they first learned that its mirror is flawed.
Recently reported observations by the telescope show an ability to resolve detail and to image faint distant galaxies that astronomers had thought only a near-perfect mirror could achieve.
For example, the image reproduced here represents what astronomer Daniel W. Weedman of Pennsylvania State University at University Park calls "one of the most dramatic pictures" Hubble has taken. The photograph shows what astronomers call an accretion disk of swirling dust and gas circling the core of an energetically active galaxy, just as theory says it should.
Earlier this month, it was announced that Carnegie Institution astronomer Alan Dres- sler and colleagues have resolved images of individual galaxies that are members of remote galaxy clusters. Dr. Dressler, who works at the Carnegie Observatories offices in Pasadena, Calif., said, "The results have important implications for theories of how galaxies have evolved since the beginning of the universe 15 billion years ago."
This kind of study is a primary goal of the Hubble-telescope program. But astronomers had thought that the mirror's flaw foreclosed that possibility.
Ray Villard, spokesman for the Space Telescope Science Institute in Baltimore, notes that "it's fair to say" that Hubble astronomers now are getting some of the research opportunities they thought they had lost.
Hubble's mirror has an optical flaw called spherical aberration. This means the 2.4-meter (7.9-foot) primary mirror can't bring enough of the light it gathers into a single focus. It was supposed to concentrate 70 percent of a star's light. Instead, only 15 percent of that light forms a sharp image. With the help of computer processing, astronomers have been able to use that 15 percent to study a wide range of relatively nearby objects and distant bright objects. But it seemed much too skimpy to allow th em to resolve some of the finer details or to image the faint distant galaxies that are one of their prime targets.
Dr. Villard recalls that many astronomers simply gave up thinking about such goals. Dressler says, "I almost gave up, too." He explains, however, that he and his colleagues - Augustus Oemier of Yale University in New Haven, Conn., James E. Gunn of Princeton University in New Jersey, and Harvey Butcher of the Netherlands Foundation for Research in Astronomy - decided to give distant galaxy study a try. And, he adds, "It worked."
Making the most of Hubble's largely blurry images is like working with a partly rotten apple. Dressler explains, "We just cut away the bad part and used the rest." This left enough good data to resolve images of galaxies that form two galaxy clusters, each of which is 4 billion light-years away.
This shows the galaxies as they were 4 billion years ago, since it has taken their light that long to reach us. There seem to be relatively more spiral galaxies in those clusters than in the relatively nearby galaxy clusters astronomers have been able to study up to now.
Galaxies with spiral arms like our own Milky Way galaxy are fairly rare in the nearby clusters. These are seen at a later stage in their development.
Dressler speculates that some of the spirals may have pulled each other apart in a violent gravitational interaction or may have merged and lost their spiral form. Some spirals in one of the distant clusters seem to be undergoing such interaction.
Dressler calls the new data "the first unambiguous sign of the influence of environment on the form of a galaxy."
His group also has imaged a galaxy cluster that seems even more distant. It may be 10 billion light-years away. More study is needed to confirm that it is remote. Dressler says, however, that with many distant galaxy clusters clearly within Hubble's range, "It seems likely that among them are the ancestors of common galaxies like our own Milky Way."
The image reproduced on this page represents a different kind of research opportunity. It represents galaxy NGC 4261 in the nearby Virgo galaxy cluster about 45 million light-years away. This is one of the many so-called active galaxies that emit strong radiation and eject jets of material, indicating the presence of a powerful energy generator at the galaxy core.
Many theorists suggest that this generator is a black hole. A black hole is a mass so highly condensed that its gravity is too strong for anything, including light, to escape. Such an object - if it existed - would draw dust and gas into a swirling disk around itself. Matter in this disk would emit vast amounts of energy as it spirals into the black hole.
The picture shown here on the right is the first image ever taken of such a disk surrounding the core of an active galaxy. Holland Ford at Johns Hopkins University in Baltimore and Walter Jaffe of the Leiden Observatory in the Netherlands released the picture last month. Dr. Jaffe calls it "our best view to date of the immediate surrounding of the nucleus of an active galaxy." He adds, speculatively, that "the nucleus is probably the home of a black hole with a mass of 10 million times that of our sun."
Penn State astronomer Weedman says he is "not all that enamored about the black hole [explanation] ... which is only a theoretical concept." But he acknowledges, "There's no question that we [astronomers] are excited by this observation." He explains that "it's pretty definitive evidence" that the cold disk of dust and gas controls the direction of the jets of material shooting out of the galaxy.
Villard notes that observations such as this disk and Dressler's galaxy clusters have "sort of teased astronomers." By making the most of what Hubble can do now, they show what can be achieved when its full capabilities are restored. A repair mission using the space shuttle now is scheduled for next December (see story at left).
One thing the restored telescope could do is measure velocities of material in the NGC 4261 disk to give a measure of the mass in the galaxy core. If it's big enough, Weedman believes there would be no other explanation for it but a black hole. "That probably would convince even skeptics like me," he says.