New Hubble Space Telescope (HST) images reveal what may be galaxies under construction in the early universe, out of a long sought ancient population of "galactic building blocks."
Hubble's detailed images, taken with the Wide Field Planetary Camera 2, reveal a grouping of 18 gigantic star clusters that appear to be the same distance from Earth, and close enough to each other that they will eventually merge into a few galaxy- sized objects. They are so far away, 11 billion light-years, that they existed during the epoch when it is commonly believed galaxies started to form.
These results add weight to a leading theory that galaxies grew by starting out as clumps of stars, which, through a complex series of encounters, consolidated into larger assemblages that we see as fully-formed galaxies today.
The finding is another step back into the dim past, where astronomers ultimately hope to uncover the earliest seeds of galaxy formation which arose shortly after the birth of the universe, or the Big Bang.
Astronomers at Arizona State University, Tempe, Arizona, (ASU) and the University of Alabama at Tuscaloosa found 18 of these cosmic building blocks packed into an area about two-million light-years across. "It's the first time anyone has seen that many star-forming objects in such a small space. There are not nearly as many such luminous objects in the two-million light-years separating Earth's galaxy, the Milky Way, from the Andromeda Nebula, the nearest major galaxy", says Rogier Windhorst of Arizona State University.
The astronomers will publish their findings in an article, authored by ASU graduate student Sam Pascarelle, in the September 5 issue of the journal Nature. The coauthors are ASU's Rogier Windhorst and Stephen Odewahn, and William Keel of the University of Alabama at Tuscaloosa.
The building blocks seen by Hubble consist of only about a billion young stars each, and Hubble shows star formation is underway through the presence of many blue stars and glowing gases. The objects typically measure only 2,000 light-years across. "That's not very big. Our own galaxy is 100,000 light-years across," Odewahn says. The objects are much smaller than even the central bulge of the Milky Way, which measures about 8,000 light-years in diameter. "We think that by repeated merging, they will grow big enough to become the bulges of nearby galaxies," says Keel, citing other HST studies that have shown that the galaxy merger or collision rate was higher in the past. "In fact, at least four of the objects in this field show double structure in their centers only a few thousand light-years apart, as if we've caught them in the act of falling together."
Hubble shows a new level of detail for determining the true nature of these "pre-galactic blobs." Hubble resolved clumps as small as 2,000 light-years across (1/10th of an arc second). These were seen in a two-day (67-orbit) exposure by Hubble of a small region of sky in the northern part of the Hercules constellation near the border with Draco.
"We've never seen so many of these objects in a single exposure and so small," says Pascarelle. "We are convinced that these objects are not peculiar, but part of the general formation process of galaxies in the early universe."
Astronomers see stars form, because star formation is an ongoing process. However, astronomers have never directly seen galaxies form, because their formation may have happened a long time ago, or because galaxy formation is not as spectacular as once believed, and is therefore much harder to observe.
The idea that galaxies grew from small pieces coming together, rather than through the collapse of a gigantic gas cloud, has been predicted from previous theoretical work and ground-based observations. The Hubble observations offer some of the best direct visual evidence to date, says Pascarelle.
Though many of the objects are isolated in the image, they are close enough together in space that most of them should eventually merge, according to Windhorst. He sketches a scenario where two or more objects will pass through each other, drawing out hydrogen gas to form more stars later. (Although the term "collision" is used, their individual stars don't collide.) They may then evolve to form the numerous faint blue galaxies, a distant population of galaxies seen by Hubble and other telescopes. Later, surrounding hydrogen gas then settles into a disk to form a spiral galaxy.
If this construction plan is correct, our Milky Way galaxy contains all the pieces of the assembly process. The older, redder stars in the Milky Way's central bulge came from the merged clusters, or "sub-galactic units," seen by Pascarelle and collaborators. The spiral arm that our Sun inhabits was made later after hydrogen settled into a disk. Some of the 140 globular star clusters which orbit the Milky Way may be "left over" smaller building blocks which formed before the larger units seen by Pascarelle and collaborators, but were never pulled directly into larger assemblages.
In some of the deepest exposures of the universe (apart from the Hubble Deep Field) yet obtained by the telescope, the astronomers found 18 objects in one image, in the vicinity of a faint radio galaxy they were studying. The researchers used an optical filter precisely tuned to detect the ultraviolet emission from glowing hydrogen gas heated by newborn stars that formed early in the universe, but shifted to longer visual wavelengths by the universal expansion. "This is a case where Hubble is uniquely suited to study sub-galactic objects at these great distances," says Windhorst, "because these objects are so compact that it would be very hard to recognize them from the ground."
Follow-up spectroscopic observations with the Multi-Mirror Telescope at Mt. Hopkins, Arizona (MMT) showed at least five of the clumps are all at the same distance from Earth. The team confirmed that another five objects were at the same distance by imaging another redshifted hydrogen line in the near infrared with NASA's Infrared Telescope Facility, and through spectroscopic follow-up at the 10 meter W.M. Keck Telescope, both on Mauna Kea, Hawaii (the latter by Drs. Nicholas Scoville and Lee Armus of Caltech). The amount of redshift corresponds to a distance of 11 billion light-years far enough to probe the early universe during the period where many of the giant galaxies were being assembled.
In a companion paper in press for the Astrophysical Journal Letters, Stephen Odewahn, Windhorst, Keel, and Simon Driver (from the University of New South Wales in Sydney, Australia) show that the counts of faint blue objects in this field are no different from that in other deep HST fields. Astronomers interpret this to mean that in almost every direction an observer should see similar activity going on at these distances the gradual construction of galaxies from faint blue sub-galactic building blocks.
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Steve Koppes, News Bureau
Arizona State University, Tempe, AZ