Science —

Locating the biggest fish in the cosmic ocean

Tracking the evolution of galaxy clusters to pin down dark energy.

The dotted line encloses a galaxy cluster identified in a survey of the southern sky. Clusters provide a potentially sensitive test of the properties of dark energy, the unknown influence driving cosmic acceleration.
The dotted line encloses a galaxy cluster identified in a survey of the southern sky. Clusters provide a potentially sensitive test of the properties of dark energy, the unknown influence driving cosmic acceleration.

Matter in the Universe is structured hierarchically: smaller objects like planets and stars group together gravitationally to form larger objects, which also group together to build ever larger structures. This continues until you reach galaxy clusters, the biggest collections of matter bound by mutual attraction.

Because they are sensitive to gravity and the distribution of mass, galaxy clusters potentially provide some sensitive tests of models of cosmology. Particularly, the expansion rate of the Universe contributes to the number of galaxy clusters that can form: if the rate is too fast, then the formation of large clusters would be suppressed. As cosmologists attempt to characterize dark energy, which is causing the expansion of the cosmos to accelerate, the number and size of galaxy clusters may help constrain its properties.

A new survey of galaxy clusters marks the beginning of a promising effort to map the birth and growth of galaxy clusters back to relatively early times. Jeeseon Song and colleagues used optical and infrared telescopes to measure the distances of 158 bright clusters in a large patch of the southern sky, looking back in time to when the Universe was less than one-third its current age. These observations provide the beginnings of a history of galaxy cluster evolution, which should help constrain models of dark energy.

Galaxy clusters may contain hundreds of galaxies and vast amounts of hot, ionized gas, with the gas accounting for more of the total mass of the cluster. Light from the cosmic microwave background (left over from when the Universe became transparent around 380,000 years after the Big Bang) has interacted with this hot gas, which created cold spots in the microwave sky. The Sunyaev-Zel'dovich effect (SZE), as it is known, is independent of the distance of the galaxy cluster from Earth, so it provides a remarkably good way to detect clusters.

However, that same indifference to distance means the SZE itself isn't enough to do cosmology: without the distance to the cluster, there is no way to determine when it formed.

A huge survey using the South Pole Telescope (SPT) identified 224 possible galaxy clusters visible from the Southern Hemisphere using the SZE. However, follow-up observations were required to confirm that these candidates actually are clusters, as well as determine their distance. The current study used the 4-meter Blanco telescope at the Cerro Tololo observatory in Chile for visible-light identification, along with Spitzer and WISE (Wide-field Infrared Survey Explorer) for infrared.

The optical and infrared observations determined distances by measuring the spectrum of light emitted by the brightest galaxy in each cluster. As the Universe expands, this light is redshifted—stretched to longer wavelengths—so that the farther a cluster lies from Earth, the longer the light had to travel to reach us, and the greater its redshift.

Redshift doesn't correlate linearly with age: a redshift of 0.5 isn't half the distance as a redshift of 1. However, it still provides a good measure of how long ago the light from distant objects was emitted. (Technically, this is known as the spectroscopic redshift; the surveys also measured the photometric redshifts of the galaxies, which is a less precise, but much quicker way to measure distances, and correlated the two sets of results.)

The follow-up observations measured the redshifts of 158 of the SPT's 224 candidate galaxy clusters; for the remaining objects, they were only able to determine the lower limits of the redshift. Nevertheless, by plotting the number and size of clusters as a function of redshift, the researchers could begin constructing the detailed history of cluster formation and evolution. The astronomers found galaxy clusters that formed out to a redshift of 1.4—meaning the light has been traveling for more than 2/3 of the current age of the Universe.

As we said above, the mass and number of galaxy clusters depends on the rate of cosmic expansion, which itself depends on the nature of dark energy. By knowing how many clusters formed in a given epoch, and how large they grew, we can constrain various models of dark energy. (N.B. I did related theoretical analysis as part of my Ph.D. thesis.) If dark energy's effects varied in time, then that should be reflected in the cluster counts over billions of years of cosmic evolution.

Right now, we don't have enough of a picture from this data to say much about dark energy, but that's slowly changing. These follow-up observations were performed painstakingly, one galaxy cluster at a time. But the pending Dark Energy Survey (DES) will provide much more detailed and efficient redshift measurements; with the data it will provide, cosmologists will have a much clearer view of galaxy cluster evolution in the relatively near future.

Astrophysical Journal, 2012. DOI: 10.1088/0004-637X/761/1/22  (About DOIs).

Channel Ars Technica