Galaxy Clusters Offer Insight On Dark Matter
November 21, 2012

Galaxy Clusters Provide New Insight Into Nature of Dark Energy

April Flowers for - Your Universe Online

Dark Energy is one of the major puzzles of modern astronomy, and one tool that astronomers use to understand this force is encoded in the distribution of clusters of galaxies. A new study by a team of astronomers, led by Dr. Jeeseon Song at the University of Michigan, has yielded exquisitely precise distances of a large sample of clusters. These precise distances may lead to breakthroughs in understanding the expansion history of our universe.

For over 80 years, astronomers have known that our universe was expanding from the Big Bang event. The Nobel Prize in physics was awarded in 2011 for the discovery that the rate of that expansion is increasing rather than slowing down, as had been previously believed. And though dark energy is the cause, it is not well understood.

Dr. Jeeseon Song remarked: “By looking at galaxy clusters at different epochs in cosmic history, astronomers can explore whether dark energy has acted differently at different times in the history of the universe. Galaxies, including our own Milky Way galaxy, are vast assemblages of stars and gas. Clusters of galaxies, conglomerates of tens to hundreds of galaxies, are the largest structures in the universe. They are dynamically changing and aging over time. And that is very crucial in cosmological studies, because that´s where we can see how dark energy is acting on the Universe, pulling the clusters apart.”

In a feat of reverse engineering, the astronomers have been able to gain insight into the nature of dark energy by studying the distribution of clusters at different times in the past and detecting what the dark energy does to the universe

Song and her team have identified an important sample of galaxy clusters whose distances have been determined accurately enough to study how the density of galaxy clusters varies with the age of the universe. The team began their investigation with observations from the South Pole Telescope, a millimeter-wavelength survey telescope, and followed up with work at the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, a division of the National Optical Astronomy Observatory (NOAO). This enabled them to refine cluster distances to within a few percent. Although the Blanco telescope in Chile celebrates its 50th anniversary this year, it still plays a vital role in cutting edge research using modern instrumentation such as the Mosaic camera used in this study.

The farther away an object is, the faster it is receding from us. Scientists measure velocity of an object by observing the color of the light wavelength. As an object moves farther away, its light undergoes a shift to longer, red wavelengths in a process known as redshift. An object moving closer displays a shift to longer, blue wavelengths. This simple color shift, called a Doppler shift, is used by highway patrols to measure the velocity of cars on the highway.

Objects with large redshifts are not only far away, they are also observed as they were a long time in the past because of the expansion of the universe. Astronomers refer to this redshift using the letter Z when measuring distant objects in the universe. The clusters that the team studied had an average redshift, z, of about 0.6, at which point the universe was only half of its present age of 13.7 billion years. The clusters, however, span a range in distance from those close enough to be seen nearly as they are in present time, to some with z as large as 1.4.  This means we see these more distant galaxies as they appeared when the universe was less than a third of its present age.

The team also discovered new information about a phenomenon called Bright Cluster Galaxies, or BCGs. These are the brightest and biggest galaxy in each cluster.

The paper´s second author Alfredo Zenteno of Germany´s Ludwig-Maximilians-University in Munich said: “The position of a BCG within a cluster indicates if the cluster is undergoing some violent internal motion — perhaps because it has suffered a smashup with another cluster. By studying the frequency of such collisions, we learn if these clusters are unique or not. This is crucial to understanding dark energy in clusters.”