Mature before their time: in the youthful universe, some galaxies were already old.
On Jan. 7, another team posted an online report asserting it had found the oldest, and therefore most distant, galaxy known. If confirmed, the study indicates that some galaxies were in place and forming stars at a prolific rate when the universe, now 13.7 billion years old, was just an 800-million-year-old whippersnapper.
The next galaxy-shaking event occurred on Jan. 9, when astronomers reported in Seattle at a meeting of the American Astronomical Society that they had found the farthest known quasar. This quasar is so distant that the light it emitted 13 billion years ago, when the universe was so young that galactic structures were still forming, has only now reached Earth. Quasars shine because they're powered by a supermassive black hole. The light of the newly discovered quasar is so bright that it almost certainly was fueled by a supermassive black hole that already had coalesced and weighed several billion times the mass of the sun.
In the Jan. 23 Nature, other researchers reported evidence that such black holes indeed formed early in the history of the universe and were already devouring matter voraciously a mere billion years after the Big Bang (SN: 1/25/03, p. 31).
Finally, on Feb. 11, cosmologists unveiled at a NASA press briefing in Washington, D.C., what maybe the piece de resistance. When David N. Spergel of Princeton University and his collaborators used a NASA satellite to study the cosmic microwave background, the radiation left over from the Big Bang, the team found something surprising. An analysis of the radiation revealed that the universe had already managed to make a plethora of stars--which had enough collective energy to ionize all the hydrogen in the cosmos--just 200 million years after the Big Bang (SN: 2/15/03, p. 99). That's several hundred million years earlier than many astronomers had estimated.
This early start in stellar mass production--and the formulation of galaxies that housed those stars--may explain why some galaxies appear old and massive when the universe was still quite young, Spergel says. However, astronomers caution, it's still uncertain how much of the chapter on early galaxy formation will need to be rewritten.
SEEING RED Astronomers have known for more than a decade that a few rare galaxies, which arose in unusually dense regions of the universe, managed to acquire a large amount of mass in a short amount of time. "A simple way to understand their early formation is that [these galaxies] are embedded within even bigger overdense regions in the same way that the tallest mountain peaks are usually sitting on the shoulders of a broader mountain range," explains Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "Because their environment is denser than average, they collapse earlier than the collapse time that is typical of objects of the same mass in the rest of the universe," he says. For these galaxies, in other words, the cosmic clock began ticking earlier.
That scenario might seem to jibe with the standard model of galaxy formation. In the model, the vast majority of galaxies are relatively late bloomers, taking many billions of years to pack on mass either by pulling in gas from the surrounding intergalactic medium or merging with neighboring galaxies. In regions of the universe that started out particularly dense, this mass-gathering action could begin sooner than elsewhere. But the standard model still can't easily account for a large number of mature or massive galaxies in the early universe.
More of these galaxies have recently popped into view because astronomers now can peer more easily into the universe at infrared wavelengths, which are invisible to the human eye. Infrared observations are critical because the galaxies that cosmologists typically want to study lie billions of light-years from Earth.
The light from distant galaxies is not only extremely faint. Because of the expansion of the universe, the light from these galaxies is also shifted to substantially longer wavelengths. For extremely distant galaxies, the visible light they originally radiated has shifted into infrared wavelengths. Viewing the visible and infrared light emitted by galaxies is crucial for determining their mass. The old stars that make up the bulk of a galaxy's mass radiate most of their light at visible and infrared wavelengths.
Because the only infrared detectors available until recently had been small, astronomers couldn't conduct vast surveys for mature, distant galaxies. From the few that they detected here or there, scientists had no way of knowing how rare or abundant such galaxies were, notes Richard S. Ellis of the California Institute of Technology in Pasadena.
The recent arrival of larger infrared detectors has lifted this limitation. Now, "we have infrared cameras that are nearly as large as the detectors we've had for a decade in visible light," says Ellis.
BODY OF EVIDENCE Using one of these large-format infrared cameras on a high-precision telescope in Parnal, Chile, astronomers recently examined the Hubble Deep Field South, a patch of sky that previously had been viewed for a solid week in visible light by the Hubble Space Telescope. Viewing the patch of sky for 100 hours with one of the four telescopes collectively known as the Very Large Telescope, Maijn Franx and Ivo Labbe of Leiden Observatory in the Netherlands and their colleagues made some of the farthest-reaching near-infrared observations ever.
Two findings surprised the astronomers. One was that their data suggest that when the universe was only 2 billion years old, as much as half of its stellar mass resided in galaxies brimming with mature stars. That's in sharp contrast to the surveys of distant galaxies recorded in visible light, which have imaged the relatively small population of stars that were young and hot.
Franx and Labbe also found that some galaxies from this long-ago epoch were already unexpectedly large. Some even show spiral structures similar to those seen in other galaxies, including our own, today.
"These results demonstrate that very deep observations in the near-infrared are essential to obtain a proper census of the earliest phases of the universe," says Franx. "Almost all [the galaxies surveyed] would have been overlooked without the near-infrared data," adds Labbe.
The biggest challenge to the standard model of galaxy formation, Labbe says, could be the number of large galaxies showing the spiral structure that he and his colleagues found in the early universe. Astronomers hold that the formation of spiral galaxies is a simple process, he notes. According to the standard theory of galaxy formation, each galaxy is surrounded by a halo of slow-moving, invisible material dubbed cold dark matter. The size of a spiral galaxy is directly related to the properties of this halo, but the number of large spirals the team found is double that predicted by the standard theory, he says.
One caveat, Labbe notes, is that Hubble Deep Field South is an extremely tiny patch of sky, taking up less than 1 percent of the area of the full moon. There's no consensus on whether the galaxies there are representative of the universe at large, Labbe says. Indeed, near-infrared observations of another tiny patch, known as Hubble Deep Field North, don't show a similar population of old or large galaxies, notes Mark Dickinson of the Space Telescope Science Institute in Baltimore.
"Larger surveys of similar quality are needed to decide if the differences between Hubble Deep Field North and South are normal variations or whether one of the fields is atypical of the rest of the universe," Labbe says.
"Personally, I think that Franx and his collaborators have indeed found some very interesting ... galaxies that may be more massive than most of the ones we find in the Hubble Deep Field North," says Dickinson.
The findings reported by Franx, Labbe, and their collaborators aren't the only evidence for a significant population of rapidly matured galaxies in the early universe. At a galaxy-formation meeting in mid-January in Aspen, Colo., Ellis reported other evidence that the 2-billion-year-old universe was populated with as many galaxies marked by red, senior stars as by blue, more youthful stars. He and his colleagues, Patrick McCarthy of the Carnegie Observatories in Pasadena and Andrew J. Bunker of the University of Cambridge in England, base their findings on a survey of galaxies conducted with a near-infrared camera at Las Campanas in La Serena, Chile. To prove that galaxies in the survey that appeared to be mature, elliptical galaxies really were distant and therefore date from a time when the universe was young, the team took spectra with the Keck telescope atop Hawaii's Mauna Kea. The amount by which light emitted by a galaxy is shifted to longer wavelengths indicates its distance.
"We may have uncovered a population of galaxies that completed star formation in short order, cohabitating with galaxies that are still forming stars," says Ellis.
"This is an important result if true, but it's an extrapolation" from a limited data set, cautions Harry C. Ferguson of the Space Telescope Science Institute. If accurate, this new view of galactic demography might force astronomers to rethink the fundamentals of galaxy formation.
It would also solve a puzzle cited by Ferguson and his collaborators in the April 20, 2002 Astrophysical Journal: Young star-forming galaxies seen in the early universe don't have enough energy to have stripped hydrogen atoms of their lone electrons. If hydrogen atoms in the intergalactic medium had remained unionized, they would have absorbed all the starlight and the universe would have stayed dark. Ferguson speculates that the population of massive galaxies seen only with infrared detectors might be the hidden dynamos responsible for the extensive ionization that must have occurred early in the history of the cosmos.
According to Loeb, these early-to-mature, massive galaxies now reside in galaxy clusters, some of the densest and oldest groupings of galaxies in the universe today. He and Jim Peebles of Princeton University have calculated that such galaxies could indeed have formed quickly in dense regions of space and not changed their basic character for billions of years. The two scientists describe their work in an upcoming Astrophysical Journal.
Although many astronomers still contend that there is too little data to convince them that galaxy-formation models may need revision, that may be about to change. In April, NASA plans to launch the Space Infrared Telescope Facility. The spacecraft will view the universe at infrared wavelengths that can't penetrate Earth's atmosphere and will provide a panorama 30 times larger than both Hubble Deep Fields combined. With this purview, astronomers will have the first dependable census of just how many galaxies in the early universe were old and massive.
With that orbiting telescope, "we'll nail this stuff down in the next year or so, I'm sure," says Dickinson.
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|Date:||Mar 1, 2003|
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