Astronomers Move at Warp Speed to Use ‘Galactic Time Machine’
Until this year, astronomers had to rent “Star Wars” if they wanted to witness events that happened a long time ago in a galaxy far, far away.
But now they can view Luke Skywalker’s neck of the universe with telescopes rather than VCRs. A deceptively simple astronomical technique is pinpointing galaxies so distant that their light has spent most of the universe’s history traveling to Earth.
Using the new technique, astronomers have spent much of this year watching ancient worlds pop out of the sky like fireworks on Independence Day. So far they’ve identified about 100 galaxies so distant that earthbound observers see the star clusters as they were when the universe was less than a quarter of its present age. And there are hundreds more to come.
“It’s as if you could look out and see dinosaurs somehow walking on the face of the Earth,” said Jim Peebles, an astronomer at Princeton University.
Many of the ancient galaxies identified in the last year come from a single astronomical snapshot--the Hubble Deep Field. The Hubble telescope spent 10 days in December trained on one small patch of sky, and brought into focus some of the faintest astronomical objects ever seen.
Reports of distant galaxies are coming in such great numbers that many astronomers believe they soon may be able to address the question of how galaxies are born.
“That’s been the big mystery up to now,” said Robert Williams, director of the Space Telescope Science Institute in Baltimore.
Astronomers have known for about 70 years that the Milky Way, the spiral-shaped swarm of 100 billion stars where we live, is but one galaxy among billions. But how such galaxies came to be is a tough nut to crack, because the galaxies most relevant to that issue are the farthest away.
Far-off objects are important to understanding the universe’s past because, in astronomy, distance equals age. It takes about 50 minutes for light to travel from Jupiter to Earth, for example. So when earthlings observe the solar system’s largest planet, they see it as it was nearly an hour earlier.
Similarly, the nearest star to the sun, Proxima Centauri, is 4.3 light-years away. So people gazing at it tonight will see it as it actually was in July 1992.
When astronomers make a picture like the Hubble Deep Field, what they get is a soup of faint galaxies, most of which are fairly nearby and a precious few that are so distant they appear as they were when the universe was still wet behind the ears. The trick is to tell which galaxies are which.
“There are galaxies everywhere, and the problem is finding the needles in the haystack,” said Harry Ferguson of the space telescope institute.
In the past, figuring out the distance to a galaxy has meant spreading the object’s image out into a spectrum, the way a prism breaks sunlight into the colors of the rainbow. Analyzing the spectrum’s colors reveals how far away the galaxy is. But that’s difficult, if not impossible, for the most distant and faintest objects because there just isn’t enough light to work with.
To overcome that problem, a number of astronomers have spent the last few years developing a shorthand way to analyze the light coming from distant objects. Rather than smear the light out into a rainbow, they take multiple pictures of the same thing in three or four different colors.
The Hubble Deep Field was actually four photographs--one each in red, blue, infrared and ultraviolet. The last two aren’t literally colors, because they’re beyond the range of the human eye, but to telescopes they’re the same thing.
The most distant galaxies simply don’t show up in one or more of the colors. A galaxy that can’t be seen in the ultraviolet, for example, is at least distant enough to appear as it was when the universe was one-fourth its present age. And if a galaxy disappears in both the ultraviolet and the blue pictures, it’s even older than that.
“It’s really not any kind of spectacularly revolutionary idea. It’s just a simple idea that works,” said Charles Steidel, one of the central figures in the effort to develop the new distance-measuring method.
The method works not just with cutting-edge technology, but also with telescopes that don’t often make headlines, such as the four-meter atop Kitt Peak in southern Arizona. Using that telescope, Steidel said, he already has found hundreds of potentially ancient galaxies.
“It’s essentially a 100% successful technique,” said Mark Dickinson of the space telescope institute. “In one year, we’ve suddenly outstripped all the samples known before by huge factors.”
Using the Hubble Deep Field, a trio of astronomers from New York and Spain claims to have found what could be the most distant and oldest galaxy ever seen. That galaxy appears as a barely discernible speck in only one of the four Hubble Deep Field photos--the infrared one.
That suggests that the speck of light was produced less than a billion years after the big bang, when the universe was still in diapers, the astronomers who discovered the galaxy claim. But other astronomers have reacted skeptically, suggesting their colleagues may have pushed the shorthand method too far.
The real power of the new yardstick isn’t in finding galaxies for the record books anyway, astronomers point out. They’re more excited about the possibility of characterizing everyday galaxies at each stage of the universe’s history, the way archeologists study ancient artifacts over successive prehistoric epochs.
With their current collection, astronomers already have confirmed that one major class of galaxies, the ellipticals, formed before spirals, such as the Milky Way. And they’ve found that there was no single dramatic era when all the galaxies simultaneously formed. It seems to have been a gradual process that went on for billions of years.
But the new revelations haven’t settled the most pressing question: How did the galaxies form and how did they get the way they are today?
Two main theories have been proposed, the top-down and the bottom-up. In the top-down scenario, giant clouds of gas are thought to have condensed under the force of gravity until they were dense enough to form stars. In the bottom-up scenario, little clumps of material are thought to have merged together into galaxies.
In the last few months, proponents of both scenarios have used recently discovered galaxies to support their theories.
Support for the top-down approach appears in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society, in which astronomers from the space telescope institute and the California Institute of Technology show that star formation gradually increased until the universe was about a third to half its current age, then fell off and continues to go down today. That pattern is consistent with a universe in which gas clouds slowly condense, the biggest becoming dense enough to form stars fairly early and the rest lighting up in order of size.
But when astronomer Rogier Windhorst of Arizona State University looks deep into the universe, he sees something else. He has studied a group of 18 apparent proto-galaxies that seem to be coming together in a bottom-up fashion, like birds merging into a flock.
He and three colleagues describe the objects, which date to a time when the universe was about 16% its current age, in the Sept. 5 issue of Nature in a paper that argues that the clumps provide direct evidence of bottom-up galaxy formation.
“The only possible way is if these 18 sub-clumps could have gradually merged over time,” Windhorst said. “Repeated corporate mergers, so to speak.”
So who’s right? Did today’s galaxies congeal slowly from widely distributed material, like the dust balls that grow behind living room furniture? Or did they assemble themselves from larger pieces, like chunks of clay pressed together in a ball?
In the spirit of diplomacy, Windhorst noted that both scenarios could be partly correct. The sub-galactic clumps that he’s seeing could have been created by a top-down condensation process and then assembled through bottom-up mergers, for example.
Nobody’s too upset about the confusion, however. A year ago, even discussing the details of galaxy evolution seemed remote.
“We’re no longer just finding a few distant galaxies and saying, ‘Wow, that’s amazing that we found this at all,’ ” said Dickinson, of the telescope institute. “I think it’s just remarkable that we’ve come so far so quickly.”
