A generation's worth of breakthroughs in astronomy have revolutionized humanity's view of cosmic history, transforming our understanding of how the early cosmos evolved into the complex universe of stars and galaxies seen today.
One major breakthrough in the past 10 years has been the use of very large astronomical surveys "of hundreds of thousands or even millions of objects in the sky, which for the first time have provided large enough samples to examine the detailed interconnections between all the properties of galaxies," said astronomer Timothy Heckman of Johns Hopkins University in Baltimore.
"It's really exciting — kind of like doing a census of the entire United States to look at the big picture, rather than just looking at your next-door neighbors," Heckman noted. "It's really based on the rapid acceleration of the power of computers to store and analyze enormous amounts of data, which wasn't feasible in the past." [Stunning Galaxy PhotosFrom NASA's WISE Telescope]
For instance, the Sloan Digital Sky Survey, which has provided key details of approximately one million galaxies, has enabled scientists to explore how galaxy age depends on mass, size and shape, explained astrophysicist James Dunlop at the University of Edinburgh in Scotland.
This data, in combination with computer models of star evolution, clarified that the most massive galaxies are the oldest with generally very little recent star formation, while galaxies with more typical masses are younger with more recent populations. Although scientists had known this before, the lack of substantial present-day star formation in massive galaxies has led theorists to suggest their super-massive central black holes are somehow blocking the birth of new stars.
The past 20 years has also seen an explosion of new telescopes that can peer farther into the universe than ever before. This has essentially provided a way to look into the distant past, as the farther away light started traveling toward us, the longer it needed to get here.
"We're relying on a variety of different techniques to probe deeper into the past," Heckman said. "A brute force approach is just using very large telescopes on Earth and observatories in space to detect extremely faint distant objects. Astronomers are also using fortuitous events such as explosions known as gamma ray bursts — gamma ray telescopes in space can detect these sudden flares from the distant universe and learn more about the objects that caused them. Also, X-ray observatories in space are now used to look at black holes growing at very early times."
Among the more surprising findings that came with this better understanding of history is that about half of all current stars and galaxies were present within 500 million years of the Big Bang. (The cosmos is now about 13.7 billion years old.)
As much as scientists have learned about the origin and evolution of galaxies, many enigmas remain. A major question is "how and when the very first galaxies formed," Dunlop said. Astronomers regard seeing these first galaxies as crucial, much as "paleontologists trying to find the earliest human or biologists trying to find the first life-form on Earth," Heckman said. [Fade from Black - Universe's Evolution Animated]
Another riddle is how the supermassive black holes seen at the center of virtually all galaxies formed in the first place. "When we look back in the early history of the universe, we see black holes are already present, and we don't really understand how such massive objects could build up in the relatively short amount of time they had," Heckman said.
Also puzzling "is how black holes appear to influence the properties of the galaxies they lie in," Heckman added. "It makes sense that the galaxy influences its central black hole, since it provides the gas the black hole swallows to grow, but it's not so clear how the reverse happens, as the black hole is so small relative to its galaxy. We do know that black holes can be extremely powerful sources of energy, and so in principle can influence the galaxies they live in, but it's quite unclear how this happens."
In addition, "if big galaxies are supposed to be formed from numerous mergers of smaller systems, how come the properties of the small galaxies we see today don't match the larger galaxies?" said astrophysicist Eline Tolstoy at the University of Groningen in the Netherlands. "You may start to wonder if there isn't a fundamental flaw in some aspect of our current understanding of how galaxies form and evolve, and exactly how important mergers are in the history of galaxies."
A future both uncertain and bright
A way to answer many of these mysteries would be to gaze at the first stars, galaxies and black holes. However, the key tool astronomers hope to use to peer into the early universe, the James Webb Space Telescope currently under development, is facing considerable trouble, with the proposed 2012 NASA budget bill aiming to cancel the project.
"I think it would be a devastating loss to astronomy and the nation — the James Webb Space Telescope was specifically designed to explore this frontier and find these early ancestors, figure out how we got here," Heckman said. "If it truly got cancelled, there's no other facility on Earth that anyone is building or planning to build that can replace it. It's frustrating to think about, just as we're at this watershed moment where we can get real scientific answers to some of the biggest questions of all time."
Although the James Webb Space Telescope is in peril, scientists could at least see early galaxies shrouded in dust beginning later this year with the Atacama Large Millimeter Array (ALMA) in Chile, Dunlop said.
"The James Webb Space Telescope can look at early stars and galaxies that are not hidden in dust, while ALMA can see ones enshrouded in dust," Dunlop explained. This is due to the different wavelengths they concentrate on — the James Webb Space Telescope can detect the blue or ultraviolet light from un-shrouded stars that have been distorted into infrared wavelengths as the universe has expanded, while ALMA focuses on infrared light from dusty stars that have been distorted into millimeter-wave light.
"The James Webb Space Telescope and ALMA were really meant to complement each other to give a complete picture of what went on in the young universe," Dunlop said. "At least the politics regarding the James Webb Space Telescope have yet to resolve themselves."
The scientists detailed the latest research on galaxies in the July 8 issue of the journal Science.