Astronomers Find Elusive Primeval Galaxies
"Galaxies are the fundamental structures into which matter is organized in the universe, yet we don't have a clear understanding of how they formed or how they have evolved into the varieties we see today," explained Chuck Steidel, assistant professor of astronomy at Caltech. Steidel's colleagues in the research, which is in press at Astrophysical Journal Letters, are Mauro Giavalisco of the Carnegie Observatories of Washington; Max Pettini of the Royal Greenwich Observatory in Cambridge, England; Mark Dickinson of the Space Telescope Science Institute in Baltimore; and Kurt Adelberger, a graduate student in astronomy at Caltech.
Astronomers know that there was an epoch soon after the Big Bang when there were no galaxies in the universe, and a later epoch, after the universe had slowed its expansion and cooled sufficiently, when galaxies began to form. But just when and how this formation process got started is still unknown. The discovery reported here opens a direct observational window into galaxies' mysterious pasts.
One of the most important aspects of the new finding is that the team of astronomers now know how to systematically observe distant, young galaxies. For more than two decades astronomers have been searching, with little success, for the earliest-forming galaxies, often called primeval galaxies because they existed long ago in the early universe.
The apparent general absence of very distant galaxies, despite intensive searches, had supported some theoretical ideas in which matter from the Big Bang did not begin to clump into galaxies like ours until the universe was relatively old, or that the incipient galaxies are hidden from view by large amounts of dust. The discovery of Steidel and his colleagues shows that bright galaxies had in fact formed quite early in the universe's history, and that it is now quite straightforward to observe them.
Astronomers have had difficulty finding these young galaxies for several reasons, including their extreme distances, which makes them exceptionally faint as seen from Earth. In addition, astronomers did not know exactly what these early galaxies would look like. Having almost no examples, the scientists had based previous searches on uncertain theoretical expectations about the appearance of galaxies soon after their formation. The new technique does not rely on any particular theory, but is designed to detect anything harboring ongoing star formation, which any young galaxy must, by definition, contain.
The method involves taking images of the sky using three custom-made color filters, allowing light of only red, green, or ultraviolet (UV) wavelengths to be seen. Young galaxies have a strong blue or UV tint, but when they are very distant, the UV wavelengths are strongly absorbed by hydrogen atoms both in the galaxy itself and in any gas that might be present between the galaxy and us. If a galaxy is within a particular range of high redshifts (corresponding to large distances from us), its UV light will be completely absorbed by the intervening hydrogen. By screening digital images of the sky through these filters, and watching for objects that are present in both red and green but vanish in UV, the astronomers have located many objects that are likely to be distant galaxies.
One of the advantages of this technique over previous methods is that astronomers can look at a much larger volume of space in a single viewing, up to 100 times larger, so that in one or two nights of observing on a large ground-based telescope they can find as many as 30 or 40 extremely distant galaxies based on their unique color signatures. The team has taken images over the last five years at the 200-inch Hale Telescope at Palomar Mountain, California; the 4.2-meter William Herschel Telescope in La Palma, the Canary Islands; and at the 3.5-meter New Technology Telescope at the European Southern Observatory and the 4-meter telescope at the Cerro Tololo Inter-American Observatories, both in Chile.
To confirm that the objects they found are distant galaxies, the team had to see their spectra. A galaxy's spectrum contains information on its redshift, or distance, but these objects are so faint that, until recently, seeing their spectra was impossible. The recently commissioned W. M. Keck Telescope on Mauna Kea, a nearly 14,000-foot extinct volcano on the Big Island of Hawaii, has made getting spectra from extremely faint objects—and thereby confirming their distance—almost routine.
In late September and October 1995, Steidel and his team went to the Keck Observatory to measure the distances to about 25 of the candidates that they had chosen from previous deep images of the sky. At least 18 of them turned out to be at the distances predicted on the basis of the galaxies' colors, placing them at redshifts far beyond where any sample of galaxies had previously been studied.
Several pieces of evidence lead Steidel and his colleagues to believe that they have identified the long-sought progenitors of today's luminous galaxies—the elliptical and spiral galaxies, like our Milky Way. First, the number of distant galaxies found in the new survey is large: they appear to be roughly as common in the early universe as bright galaxies are in the present-day universe.
A second important clue came from the sharpest-ever images of the distant galaxies taken with the Hubble Space Telescope. These images show that most of the infant galaxies are relatively round and compact, with intense star formation occurring in relatively small regions. The astronomers believe these regions of new stars in young, distant galaxies correspond to the conglomerations of much older stars seen nearby in the cores of elliptical galaxies and central bulges of spiral galaxies like our own.
Third, the astronomers can estimate masses for the galaxies by examining the width of absorption lines in the spectra obtained with the 10-meter Keck Telescope in Hawaii, and the inferred masses are in line with those of bright galaxies in the nearby universe. Detailed analysis of the distant galaxies' colors shows that this population of objects was quite "new" at the time being observed, and probably had formed no longer than about one billion years previously—a relatively short time by cosmological standards. The existence of these objects at such early times fairly accurately pinpoints when the bright galaxies like ours were first coming together.
The color technique and the power of the Keck Telescope turn out to be a winning combination for picking out these extremely distant galaxies from among the myriad dimmer objects that are much nearer. The astronomers have identified more than 100 additional objects as possible distant galaxies, and estimate 90 percent of them will turn out to be at high redshifts.
The astronomers will continue to add to their sample of young galaxies, with the ultimate goals of gaining a reasonably complete picture of conditions in the universe when the galaxies seen today were forming, and of understanding how the early structure of the universe led to its present state. Now that the technique of finding these extremely distant objects has been shown to work, it will be possible to learn a great deal about how far the process of galaxy formation had progressed during the early years of the universe.
This research has been supported by grants from the National Science Foundation's Young Investigator program, from the Alfred P. Sloan Foundation, and from NASA by way of the Associated Universities for Research in Astronomy and the Space Telescope Science Institute. The research also would not have been possible without the help of the W. M. Keck Foundation, whose generous gift allowed construction of the 10-meter telescope on Mauna Kea.
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