Over the past decade, the Hubble Space Telescope has literally changed our view of the universe. Much of what we now understand about galaxy formation has been gleaned from Hubble staring for 10 days at a single tiny patch of sky. Within this region, the Earth-orbiting telescope has catalogued the shape, brightness, and color of galaxies that are only 500-millionths as bright as the eye can see. But if one sharp eye on the universe is good, then two, three, four, or more are better.
COSMIC TIMELINE. Observatories are peering far back in time to study the evolution of stars and galaxies. As part of a survey called GOODS, the Hubble Space Telescope’s Advanced Camera for Surveys and the Chandra X-ray Observatory are searching the same part of the sky for some of the first galaxies.
In an ambitious new program, Hubble recently joined forces with NASA’s orbiting Chandra X-ray Observatory and several of the world’s largest ground-based telescopes to expand astronomers’ view to a panoply of wavelengths. These observatories, which are about to be joined by the Space Infrared Telescope Facility, launched late last month, are shedding light on galaxy assembly in a way that no single telescope, looking at the sky at a limited set of wavelengths, could ever manage. It’s the difference between viewing a photograph in color versus black-and-white, except that the range of wavelengths is millions of times larger than those depicted in a color photo.
Known as the Great Observatories Origins Deep Survey (GOODS), the coordinated effort focuses on two regions of the heavens. One swath, known as the Hubble Deep Field North, was originally examined by Hubble’s wide-field and planetary camera in late 1995 and has recently has been scanned by an even more-sensitive instrument, Hubble’s Advanced Camera for Surveys, installed 2 years ago. This camera has more than twice the field of view of the older detector. The other section of sky is known as Chandra Deep Field South. The area encompassed by the two regions is nearly half the size of the full moon as viewed from Earth and 70 times the original Hubble Deep Field North.
Each of the two fields examined by GOODS contains tens of thousands of galaxies, some so distant that they hail from a time when the 13.7-year-old universe was only a billion years old.
“This is the first time that the cosmic tale of how galaxies build themselves has been traced reliably to such early times in the universe’s life,” says GOODS researcher Mauro Giavalisco of the Space Telescope Science Institute (STScI) in Baltimore.
Distant or dusty?
After comparing recent Chandra and Hubble images of the Chandra Deep Field South, astronomers are faced with a mystery. Chandra found seven strong X-ray sources that can’t be seen at all in visible light. Usually when astronomers have detected such intense X rays, the source has been the whirlpool of matter spiraling into a supermassive black hole. However, supermassive black holes lie at the centers of galaxies, where stars produce great amounts of visible and ultraviolet light.
“We should see the optical galaxies in our Hubble images . . . but we don’t see anything,” notes Anton Koekemoer of STScI, who will describe his team’s study in an upcoming Astrophysical Journal devoted to early findings from the GOODS survey.
One explanation is that the X rays that Chandra recorded come from the most distant supermassive black holes known in the universe, dating from a time when the cosmos might have been only about 600 million years old, or just 7 percent of its current age. In that case, Hubble wouldn’t detect the remote galaxy housing the black hole because the expansion of the universe would have shifted all of the galaxy’s ultraviolet and visible light to wavelengths much redder than the telescope can record. Such ancient supermassive black holes are of keen interest because recent evidence suggests that they’re intimately connected to the growth of the galaxies in which they reside (SN: 4/5/03, p. 214: Cosmic Blowout: Black holes spew as much as they consume).
There could be, however, a more mundane explanation for the X-ray findings, cautions Koekemoer. The X-ray sources could be supermassive black holes that lie much closer to Earth but whose small host galaxies are shrouded in so much dust that they can’t be seen in visible light. If so, calculations show that the galaxies would have to be dustier than any other group of galaxies known. In fact, the dust would have to be so implausibly thick that some astronomers favor the first explanation.
Scientists may soon have a solution to this puzzle. “The next steps really will be to see what the Space Infrared Telescope Facility can tell us about these objects,” notes Koekemoer. If the X-ray sources come from remote galaxies, the visible starlight that these galaxies have radiated will have been shifted into the midinfrared, exactly the wavelength range in which the new infrared telescope is sensitive. On the other hand, if the galaxies are nearby, the abundant dust would radiate large amounts of infrared radiation, and the telescope “should be able to measure how much dust they contain,” says Koekemoer. Results from the newly launched observatory are expected within a year.
Combining GOODS observations taken by Hubble with near-infrared and visible-light images taken from the ground has already given astronomers the first evidence that galaxies were forming substantial numbers of stars early in the universe. Casey Papovich of the University of Arizona in Tucson and his colleagues report in the special issue of Astrophysical Journal that when the universe was 2 to 3 billion years old, galaxies bulked up, increasing their stellar mass by about 40 percent.
That number is only a rough estimate, notes Papovich, because it’s not based on a galaxy-by-galaxy measurement. Instead, he and his colleagues added up all the near-infrared and visible light from two sets of galaxies observed in the GOODS survey. In one set, galaxies were observed as they appeared 12 billion years ago; in the other, a billion years later. The increase in light output over this interval indicates that the latter group has more stars.
“It looks like you are actually seeing galaxies build up their stellar mass, and we hadn’t actually seen that” so early in the universe, says Harry Ferguson of STScI. A large sample of galaxies hailing from 12 billion years back in cosmic history had never been detected before the Advanced Camera for Surveys was installed on Hubble, he notes.
Previous studies had indicated that about 7 billion years ago, star formation dropped to about one-tenth its earlier rate. This indicates that galaxy assembly trailed off when the universe was about half its current age.
The Space Infrared Telescope Facility is likely to refine these numbers, as that observatory will conduct a more representative census of stellar mass among distant galaxies, which date from a time when the universe was young. The bulk of stars radiates at visible and near-infrared wavelengths, and for distant galaxies, this radiation is shifted into the infrared.
In a related result, Ferguson and his colleagues used GOODS data to find that galaxies have continuously increased their size since the time the universe was about 1 billion to 6 billion years old. Both the buildup of stars and galaxy size are consistent with the standard, bottom-up model of galaxy formation, Ferguson notes.
In that model, galaxies start out small and grow by merging with other, similar-size galaxies and capturing smaller, satellite ones (SN: 8/16/03, p. 99: Swallow Thy Neighbor: Strong evidence of galactic cannibalism). Driving that activity is the unseen but ubiquitous material dubbed dark matter. In the bottom-up model, dark matter is the universe’s first stuff to coalesce, and the gravity that results then drives ordinary, visible matter to gather into galaxies.
Astronomers expect GOODS observations to answer another question: Is the mysterious force called dark energy causing the universe to expand at an ever-faster rate? Until 1998, the standard theory of cosmology held that, ever since the Big Bang, gravity’s tug has slowed the expansion of the universe. Then, measurements of the intensity of distant, exploding stars called type 1A supernovas caused a scientific sensation by describing a universe whose expansion is accelerating (SN: 4/7/01, p. 218: A Dark Force in the Universe).
Now, by repeatedly observing the two patches of sky in the GOODS survey, Hubble’s Advance Camera for Surveys has identified 10 extremely distant type 1a supernovas. These remote supernovas should put dark energy to the ultimate test, says Adam Riess of STScI.
Astronomers refer to these supernovas as standard candles because they all have about the same intrinsic brightness, like light bulbs of the same wattage. Most previous observations recorded type 1a supernovas that were several billion light-years from Earth, so astronomers see the supernovas as they appeared when the universe was several billion years younger than it is today.
If gravity had continuously slowed cosmic expansion, the distance between Earth and those supernovas ought to be less than if the expansion rate had remained constant or sped up.
Because the supernovas wouldn’t lie as far away, they should appear brighter.
Yet two teams of astronomers announced in 1998 that they had found just the opposite. Supernovas were 20 percent dimmer than expected if cosmic expansion had remained constant. That suggested that cosmic expansion had in fact sped up and that the space between Earth and those supernovas had stretched out more than anticipated.
Researchers ascribe this strange state of affairs to dark energy, an entity that’s the flip side of gravity. Where ordinary gravity pulls objects together, dark energy pushes them apart.
Not everyone has been convinced by the supernova data. Some astronomers worry that supernovas were intrinsically different in the past or that cosmic dust could make the supernovas appear dimmer than they really are.
But there is a test that could allay such concerns. According to some theorists, dark energy has remained constant in strength throughout the history of the universe. In contrast, the density of matter, which gives rise to gravity’s familiar tug between objects, was much higher in the past, when the universe was smaller.
In fact, more than 5 billion years ago, the density of all the matter in the universe would have been so great that its pull would have overwhelmed dark energy’s push. During that early time, ordinary gravity was at the helm, slowing cosmic expansion.
If all this holds true, then supernovas that are extremely remote and hail from the distant past ought to be slightly closer to Earth and thus appear brighter than they would if the universe has been expanding at either a constant or accelerated rate. That’s not an effect that dust, for example, could mimic.
Astronomers have observed only a few supernovas that are distant enough to qualify for this test, but their brightness supports the hypothesis that dark energy is driving cosmic expansion.
“We definitely have the data in the can, enough to tell us the expansion history [of the universe] and whether there was a deceleration phase prior to an accelerated expansion or whether there’s some strange surprise in store,” Riess says. His team expects to have preliminary results in October.
Next year, the Hubble camera will take an even deeper look at the two sky regions already examined by GOODS. By including the most powerful telescopes on Earth and in space, “the survey is giving us a uniquely comprehensive history of galaxies from early epochs to the relatively recent past,” says Mark Dickinson of STScI.
The findings, he adds, will also serve as a bridge to future explorations with Hubble’s proposed successor, the James Webb Space Telescope. Scheduled to be launched a decade from now, it will have the capability to peer even farther back in time to see the very first galaxies and stars in the universe.
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