Galactic archaeology

Space telescopes spy ancient galaxy clusters in the young universe, shedding light on the years following the Big Bang.

photo: This galaxy cluster is shown as it existed when the universe was just 5 billion years old. The cluster is as massive as 300 trillion suns and is the most massive known cluster of its epoch. This image, which was taken between May and June 2002 with Hubble's Advanced Camera for Surveys Wide Field Camera, shows just the core of the cluster. Only about 50 galaxies are shown, but the cluster likely contains thousands. Dominating the core are a pair of large, reddish, elliptical galaxies (near the center of the image). Their red color indicates they hold a population of stars that are at least a billion years old. The red galaxies surrounding the central pair are also cluster members. Many of the other galaxies, including several blue galaxies, lie in the foreground. NASA / ESA / J.Blakeslee (JHU) / M. Postman (STScI) / P. Rosati (ESO)

January 5, 2004

In piecing together the story of the universe's history, scientists are hoping to figure out exactly how and when galaxies first formed. It's a crucial question because early structure formation carries the imprint of conditions in the newborn universe, which can help us understand our cosmic beginnings. Now, two key discoveries of early galaxy clusters are helping astronomers see the foundations of the universe's galactic architecture directly.

Using a powerful combination of NASA's Chandra X-ray Observatory and the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope, an international team of astronomers has found and studied two record-breaking ancient galaxy clusters.

The first is a cluster whose light is reaching us from 9 billion years ago, when the universe was a mere 5 billion years old. It's the most massive known cluster of that epoch, which means it must have been growing for quite some time already — a somewhat surprising result for such an early time in the universe's youth.

"We determined that the galaxies in this cluster were already about 3 billion years old," explains astronomer John Blakeslee of Johns Hopkins University, a member of the team. "Thus, these galaxies formed most of their stars about 2 billion years after the Big Bang."

The second finding is a proto-cluster of embryonic galaxies from a time when the universe was only about 1.5 billion years old. This is the most distant, and therefore earliest, proto-cluster ever found.



photo: Taken by Hubble's Advanced Camera for Surveys Wide Field Camera in July 2002, this image shows the embryonic cluster as it was when the universe was just 1.5 billion years old. This is the most distant proto-cluster known. It is dominated by a massive baby galaxy, seen as the green object near the center of this image. The galaxy is producing powerful radio emissions, and it is the brightest galaxy in the proto-cluster. The green color is indicative of glowing hydrogen gas. The galaxy's clumpy appearance suggests it is still developing. Smaller growing galaxies are scattered around the massive galaxy. The bright object in the upper part of the image is a foreground star. NASA / ESA / G. Miley and R. Overzier (Leiden Observatory)

"Given that this is a very dense region of the early universe with so many galaxies, it's quite reasonable to suggest that these galaxies are some of the oldest in the universe," Blakeslee says of this group. "More importantly, though, we are directly observing the galaxy-cluster formation epoch, and it is in clusters that the oldest galaxies tend to reside."



photo: This color composite image of the galaxy cluster RDCS 1252.9-2927 shows the X-ray (purple) light from 70-million-degree Celsius gas in the cluster, and the optical (red, yellow and green) light from the galaxies in the cluster. The X-ray data was taken by the Chandra X-ray Observatory, and the optical data is from European Southern Observatory's Very Large Telescope (VLT) in Chile.
X-ray data indicate that this cluster formed more than 8 billion years ago and has a mass at least 200 trillion times that of the Sun. It is the most massive cluster ever observed at such an early stage in the evolution of the universe. The width of the image spans 2 arcminutes. X-ray: NASA / CXC / ESO / P. Rosati et al.; Optical: ESO / VLT / P. Rosati et al.

Both findings are evidence that galaxies started forming soon after the Big Bang, as slightly denser regions in the primordial universe gravitationally coalesced. That process involved many cosmological factors: the pattern of initial density fluctuations in the universe, the nature of gravity, the expansion rate of the universe, the strength of dark energy, and the abundance of dark matter, to name a few.

Witnessing the sculpting of large-scale structure, then, allows scientists to probe these many mysteries and home in on some details of galaxy formation itself, a complicated process that is still not entirely understood. Having found such ancient clusters at various stages in the process certainly will be helpful in putting all the pieces of the puzzle together.

"It's part of the quest to understand our origins," Blakeslee says.