mountain profile Institute for Astronomy University of Hawaii

Galaxy Clustering

Maintained by LG

Galaxies are not randomly distributed in space. There are major concentrations of galaxies we refer to as clusters, nearly empty areas that we refer to as voids, and more complicated distributions such as filaments and sheets.

The projects on this page show some of the ways IfA astronomers study the universe by focusing their attention on galaxy clusters and voids.


The Local Void

Brent Tully is working to understand the origin of the 600 km/s motion of our Galaxy with respect to the cosmic microwave background. He has found that the reference frame is increasingly understood to be made up of several parts. A significant contributor, at the level of 260 km/s, is a motion away from the Local Void, a nearby region of space that is almost devoid of galaxies. The compelling evidence comes from a discontinuity in velocities just beyond the structure we live in, the Local Sheet. Galaxies within the Local Sheet are moving coherently with a tiny dispersion, while galaxies in the adjacent structures are moving with their own coherent but quite distinct flow. The nature of the motions makes it clear that our Local Sheet is part of the wall of the Local Void and experiencing the expansion of the void. The substantial expansion velocity implies that the Local Void is impressively large and empty.



Each spot in this figure is a galaxy, with the Milky Way at the origin. The arrow shows the motion of the Milky Way away from the Local Void.





Massive Galaxy Clusters

Harald Ebeling has been leading several all-sky searches for the most massive galaxy clusters, among them the MAssive Cluster Survey (MACS) which discovered the majority of the targets studied in depth by the Hubble Frontier Fields initiative and, more recently, the extended MAssive Cluster Survey (eMACS) which focuses on yet more distant systems at redshifts beyond z=0.5. Observing these extreme mass concentrations across the electromagnetic spectrum, from the radio through the optical to the X-ray regime, reveals the physical mechanisms at work in the formation and evolution of structure over a huge range of spatial scales, from galaxies to large-scale filaments. Specifically cluster collisions offer rare opportunities to study and quantify the dynamic properties of both the luminous and dark matter that make up galaxy clusters.

On yet larger scales, Ebeling’s team played a central role in the discovery of the Dark Flow, a large-scale motion of galaxy clusters across the entire observable universe detected via measurements of distortions in the cosmic microwave background data.


The massive galaxy cluster MACS J0717.5+3745 a complex merger of at least four separate galaxy clusters. The diffuse X-ray emission from the hot intra-clusters gas is color-coded to show different gas temperatures.



Gravitational Lensing

Gravitational lensing is the bending of light from a distant background source by a mass concentration between this source and the observer. In the strong-lensing regime (near high mass concentrations) this effect can lead to dramatically magnified and distorted images of faint background objects.

Harald Ebeling uses massive X-ray selected galaxy clusters as gravitational telescopes (a) to constrain the mass distribution within the cluster (most of which consists of dark matter that cannot be detected by other means), and (b) to find and characterize distant background galaxies that would be beyond the reach of even the largest present-day telescopes without amplification by the cluster "lens".


The snake-like feature in this figure is the highly magnified and distorted image of a distant background galaxy, created by the bending of light as it traverses the massive cluster MACSJ1206.2-0847 which acts like a giant lens.



Galaxy Clustering at z = 1.73

Pat Henry, Guenther Hasinger, and others have been studying an X-ray-selected cluster at z = 1.75, one of the most distant ever observed.  The X-ray object lies between two galaxy overdensities at z = 1.688 and 1.751, SE and NW, respectively, with a bridge in-between. This configuration resembles so-called bullet clusters, in which the collision of two clusters separates their collisionless dark matter + galaxies from the dissipative X-ray gas. However, the situation shown in the figure is more like pearls on a string in which clumps of matter slide along large-scale matter filaments eventually merging at filament intersections. Initial analysis of the clusters’ color-magnitude diagrams shows their galaxies formed ~185 Myr before the observation epoch.



The green contours show the X-ray source; the black contours show the galaxies.