The ‘Dark Flow’ Pushes Deeper into the Cosmos

When you’re studying galaxy clusters, it doesn’t pay to be in a hurry. Harald Ebeling (University of Hawaii) is an expert on the matter, working with a catalog of over a thousand such clusters in a new study of the so-called ‘dark flow,’ the apparent motion of galaxy clusters along a path centered on the southern constellations Centaurus and Hydra. Says Ebeling:
“It takes, on average, about an hour of telescope time to measure the distance to each cluster we work with, not to mention the years required to find these systems in the first place. This is a project requiring considerable followthrough.”
The study, led by Alexander Kashlinsky (NASA GSFC), relies on hot X-ray emitting gas within a cluster, which scatters photons from the cosmic microwave background. The wavelength of scattered photons then tells us something about the motion of individual clusters. This tiniest of shifts in the CMB’s background temperature in the cluster’s direction, known as the Sunyaev-Zel’dovich effect, is small enough that it has never been measured in a single galaxy cluster. That’s where the large number of clusters comes in. The researchers used a catalog of 700 clusters from a 2008 study and folded in another ...