This idea is strengthened by X-ray observations of the massive cluster RDCS 1252. Chandra and the European Space Agency's XMM-Newton provided astronomers with the most accurate measurements to date of the properties of an enormous cloud of hot gas that pervades the massive cluster. This 160-million-degree Fahrenheit (70-million-degree Celsius) gas is a reservoir of most of the heavy elements in the cluster and an accurate tracer of its total mass. A paper by Piero Rosati of the European Southern Observatory (ESO) and colleagues that presents the X-ray observations of RDCS 1252 will be published in January 2004 in the Astronomical Journal.
"Chandra's sharp vision resolved the shape of the hot gas halo and showed that RDCS 1252 is very mature for its age," said Rosati, who discovered the cluster with the ROSAT X-ray telescope.
RDCS 1252 may contain many thousands of galaxies. Most of these galaxies, however, are too faint to detect. But the powerful "eyes" of the ACS pinpointed several hundred of them. Observations using ESO's Very Large Telescope (VLT) provided a precise measurement of the distance to the cluster. The ACS enabled the researchers to accurately determine the shapes and colors of the 100 galaxies, providing information on the ages of the stars residing in them. The ACS team estimated that most of the stars in the cluster were already formed when the universe was about 2 billion years old. X-ray observations, furthermore, showed that 5 billion years after the big bang the surrounding hot gas had been enriched with heavy elements from these stars and had been swept away from the galaxies.
If most of the galaxies in RDCS 1252 have reached maturity and are settling into a quiet adulthood, the forming galaxies in the distant proto-cluster are in their energetic, unruly youth.
The proto-cluster TN J1338 contains a massive embryonic galaxy surrounded by smaller developing galaxies, which look like dots in the Hubble image.
The dominant galaxy is producing spectacular radio-emitting jets, fueled by a supermassive black hole deep within the galaxy's nucleus. Interaction between these jets and the gas can stimulate a torrent of star birth.
The energetic radio galaxy's discovery by radio telescopes prompted astronomers to hunt for the smaller galaxies that make up the bulk of the cluster.
"Massive clusters are the cities of the universe, and the radio galaxies within them are the smokestacks we can use for finding them when they are just beginning to form," Miley said.
The two findings underscore the power of combining observations from many different telescopes that provided views of the distant universe in a range of wavelengths. Hubble's advanced camera provided critical information on the structure of both distant galaxy clusters. Chandra's and XMM-Newton's X-ray vision furnished the essential measurements of the primordial gas in which the galaxies in RDCS 1252 are embedded, and accurate estimates of the total mass contained within that cluster. Large ground-based telescopes, like the VLT, provided precise measurements of the distance of both clusters as well as the chemical composition of the galaxies in them.
The ACS team is conducting further observations of distant clusters to solidify our understanding of how these young clusters and their galaxies evolve into the shape of things seen today. Their planned observations include using near-infrared observations to analyze the star-formation rates in some of the target clusters, including RDCS 1252, to measure the cosmic history of star formation in these massive structures. The team is also searching the regions around several ultra-distant radio galaxies for additional examples of proto-clusters. The team's ultimate scientific goal is to establish a complete picture of cluster evolution beginning with the formation at the earliest epochs and detailing the evolution up to today.
