The Enigma of Ancient Galaxy Clusters -“Like finding modern skyscrapers in ancient Rome”


Astronomers using the South Pole Telescope reported that they discovered the most massive galaxy cluster yet seen at a distance of 7 billion light-years. The cluster (designated SPT-CL J0546-5345) weighs in at around 800 trillion Suns, and holds hundreds of galaxies. The infrared/optical representative-color image above shows galaxies with "old" stellar populations, like modern-day ellipticals, circled in yellow; galaxies with "young" stellar populations, like modern-day spirals, are circled in blue.

"This galaxy cluster wins the heavyweight title. It's among the most massive clusters ever found at this distance," said Mark Brodwin, a Smithsonian astronomer at the Harvard-Smithsonian Center for Astrophysics. Brodwin is first author on the paper announcing the discovery, which appeared in the Astrophysical Journal.

Redshift measures how light from a distant object has been stretched by the universe's expansion. Located in the southern constellation Pictor (the Painter), the cluster has a redshift of z=1.07. This puts it at a distance of about 7 billion light-years, meaning we see it as it appeared 7 billion years ago, when the universe was half as old as now and our solar system didn't exist yet.

Even at that young age, the cluster was almost as massive as the nearby Coma cluster. Since then, it should have grown about four times larger. If we could see it as it appears today, it would be one of the most massive galaxy clusters in the universe.

"This cluster is full of 'old' galaxies, meaning that it had to come together very early in the universe's history – within the first two billion years," stated Brodwin.

Galaxy clusters like this can be used to study how dark matter and dark energy influenced the growth of cosmic structures. Long ago, the universe was smaller and more compact, so gravity had a greater influence. It was easier for galaxy clusters to grow, especially in areas that already were denser than their surroundings.

"You could say that the rich get richer, and the dense get denser," quipped Harvard astronomer Robert Kirshner, commenting on the study.

As the universe expanded at an accelerating rate due to dark energy, it grew more diffuse. Dark energy now dominates over the pull of gravity and chokes off the formation of new galaxy clusters.

400px-South_Pole_Telescope_(2008) Brodwin and his colleagues spotted their quarry in the first 200 square degrees of data collected from the new South Pole Telescope. The SPT is currently completing its pioneering millimeter-wave survey of a huge swath of sky covering 2,500 square degrees. 

They're hunting for giant galaxy clusters using the Sunyaev-Zel'dovich effect – a small distortion of the cosmic microwave background (a pervasive all-sky glow left over from the Big Bang). Such distortions are created as background radiation passes through a large galaxy cluster.

Surveying for this effect has significant advantages over other search techniques. It works just as well for very distant clusters as for nearby clusters, which allows astronomers to find very rare, distant, massive clusters. Further, it provides accurate measurements of the masses of these clusters, which are crucial to unraveling the nature of dark energy.

The main goal of the SPT survey is to find a large sample of massive galaxy clusters in order to measure the equation of state of the dark energy, which characterizes cosmic inflation and the accelerated expansion of the universe. Additional goals include understanding the evolution of hot gas within galaxy clusters, studying the evolution of massive galaxies in clusters, and identifying distant, gravitationally lensed, rapidly star-forming galaxies.

Once this distant cluster was found, the team studied it with the Infrared Array Camera on the Spitzer Space Telescope to pinpoint galaxies within the cluster. Detailed observations of the galaxies' speeds with the Magellan telescopes in Chile proved that the galaxy cluster was a heavyweight.

The team expects to find many more giant galaxy clusters lurking in the distance once the South Pole Telescope survey is completed.

In a twist, another recently discovered galaxy cluster looks shockingly modern. Called CLG J02182-05102, the ancient cluster is dominated by old, red and massive galaxies, typical of present-day clusters. For example, it is similar to a young version of the Coma Cluster of today, which has had billions of more years to develop. Most of the galaxies that inhabit the central portion of the Coma Cluster are ellipticals. Both dwarf, as well as giant ellipticals, are there in abundance.

"We are seeing something already aged and red like a younger version of the Coma Cluster from a distant, bygone era," said Casey Papovich, lead author of a new study and an assistant professor of physics and astronomy at Texas A&M University in College Station. Papovich added, "it is as though we dug an archeological site in Rome and found pieces of modern Rome in amongst the ruins."

At its center regions loom red, monster galaxies containing about 10 times as many stars as our Milky Way galaxy. This puts them on par with the most mammoth galaxies in the nearby universe, which have grown fat through repeated mergers with other galaxies.

These big galaxies are so uncharacteristic of those in the early universe that in some sense it is like finding modern skyscrapers in ancient Rome.

ClG J02182-05102 might have indeed been ahead of its time. Just as Rome was the world's biggest city more than 2,000 years ago with a population of about a million residents, observed Papovich – a figure not again matched until the early 1800s in London – so too was this galactic grouping an advanced civilization for so early an era in the developing universe.

Galaxy clusters are the largest gravitationally bound structures in the universe and are thought to have formed piecemeal over cosmic time. For now, ClG J02182-05102 is the only known galactic grouping so far away in the past, and studying it will help researchers understand the overall history of how galaxies congregate and evolve.

Casey Kazan via Harvard-Smithsonian Center for Astrophysics

Image credit: NASA/JPL-Caltech/M. Brodwin (Harvard-Smithsonian CfA) Optical Image: CTIO Blanco 4-m telescope/J. Mohr (LMU Munich)

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