Groups & Clusters of Galaxies

Witnessing the Formation of One of the Most Massive Objects in the Universe

Image of Gerrit Schellenberger in the desert
Gerrit Schellenberger

We welcome Gerrit Schellenberger as our guest blogger. He received his PhD in Bonn, Germany in 2016, and has been a post-doctoral researcher at the Center for Astrophysics | Harvard & Smithsonian since March 2016. His research includes working on galaxy clusters and groups in large samples for cosmology, but also on individual objects in the X-rays and in the radio regime.

From the beginning of my astronomical career, I was fascinated by studying galaxy clusters, consisting of hundreds, sometimes even thousands of galaxies held together by gravity only. Yet, the galaxies alone do not — by far — sum up to the mass necessary to keep the cluster bound together. Beginning in the 1970s after the birth of X-ray astronomy and the first imaging satellites such as Einstein and ROSAT, scientists discovered that a very hot gas exists between the galaxies of the cluster. The mass of this gas exceeds the mass of all the stars in the galaxies together.

Although this gas is the most dominant, visible structure in galaxy clusters, it is only about 10% of the total mass (while the stars in the galaxies make only about 1%). The rest, roughly 90%, is dark matter, which cannot be observed directly. However, we can see its effect on the hot gas and galaxies in galaxy clusters: the gravity not only keeps the galaxies within the cluster, but also compresses the gas, heating it to the point where it emits X-rays. So we can study dark matter in clusters by measuring the properties (like temperature) of the hot gas from the X-ray emission.

Intrigued by this, I started to analyze a sample comprising 64 clusters during my PhD in Bonn, Germany, with the goal of obtaining total masses (including the dark matter component) for all of them. It turns out that smaller and lighter galaxy clusters, also called galaxy groups, do not follow the expected scaling between X-ray emission and temperature at a given cluster mass, meaning that the X-ray properties of gas in these systems cannot be used to give reliable mass estimates. Therefore, galaxy groups can only be of limited use for cosmological studies, where it is crucial to estimate the amount of matter in objects and how it changes with cosmic time.

Galaxy Clusters Caught in a First Kiss

Image of Cluster Merger
Cluster Merger
Credit: X-ray: NASA/CXC/RIKEN/L. Gu et al; Radio: NCRA/TIFR/GMRT; Optical: SDSS

For the first time, astronomers have found two giant clusters of galaxies that are just about to collide, as reported in a new press release by RIKEN. This observation is important in understanding the formation of structure in the Universe, sincelarge-scale structures—such as galaxies and clusters of galaxies—are thought to grow by collisions and mergers.

The composite image shows the separate galaxy clusters 1E2215 and 1E2216, located about 1.2 billion light years from Earth, captured as they enter a critical phase of merging. Chandra’s X-ray data (blue) have been combined with a radio image from the Giant Metrewave Radio Telescope in India (red). These images were then overlaid on an optical image from the Sloan Digital Sky Survey that shows galaxies and stars in the field of view.

Does the Gas in Galaxy Clusters Flow Like Honey?

Image of Coma Cluster
Coma Cluster
Credit: X-ray: NASA/CXC/Univ. of Chicago, I. Zhuravleva et al, Optical: SDSS

This image represents a deep dataset of the Coma galaxy cluster obtained by NASA's Chandra X-ray Observatory. Researchers have used these data to study how the hot gas in the cluster behaves, as reported in our press release. One intriguing and important aspect to study is how much viscosity, or "stickiness," the hot gas demonstrates in these cosmic giants.

Galaxy clusters are comprised of individual galaxies, hot gas, and dark matter. The hot gas in Coma glows in X-ray light observed by Chandra. Seen as the purple and pink colors in this new composite image, the hot gas contains about six times more mass than all of the combined galaxies in the cluster. The galaxies appear as white in the optical part of the composite image from the Sloan Digital Sky Survey. (The unusual shape of the X-ray emission in the lower right is caused by the edges of the Chandra detectors being visible.)

Chandra Finds Stellar Duos Banished from Galaxies

Image of Fornax Cluster
Fornax Cluster
Credit: X-ray: NASA/CXC/Nanjing University/X. Jin et al.

This image from NASA's Chandra X-ray Observatory shows the region around NGC 1399 and NGC 1404, two of the largest galaxies in the Fornax galaxy cluster. Located at a distance of about 60 million light years, Fornax is one of the closest galaxy clusters to Earth. This relative proximity allows astronomers to study the Fornax cluster in greater detail than most other galaxy clusters.

Chandra Serves up Cosmic Holiday Assortment

This is the season of celebrating, and the Chandra X-ray Center has prepared a platter of cosmic treats from NASA's Chandra X-ray Observatory to enjoy. This selection represents different types of objects — ranging from relatively nearby exploded stars to extremely distant and massive clusters of galaxies — that emit X-rays detected by Chandra. Each image in this collection blends Chandra data with other telescopes, creating a colorful medley of light from our Universe.

Cosmic Fountain Powered by Giant Black Hole

Image of Abell 2597
Abell 2597
Credit: X-ray: NASA/CXC/SAO/G. Tremblay et al; Radio:ALMA: ESO/NAOJ/NRAO/G.Tremblay et al, NRAO/AUI/NSF/B.Saxton; Optical: ESO/VLT

Before electrical power became available, water fountains worked by relying on gravity to channel water from a higher elevation to a lower one. This water could then be redirected to shoot out of the fountain and create a centerpiece for people to admire.

In space, awesome gaseous fountains have been discovered in the centers of galaxy clusters. One such fountain is in the cluster Abell 2597. There, vast amounts of gas fall toward a supermassive black hole, where a combination of gravitational and electromagnetic forces sprays most of the gas away from the black hole in an ongoing cycle lasting tens of millions of years.

Scientists used data from the Atacama Large Millimeter/submillimeter Array (ALMA), the Multi-Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope (VLT) and NASA's Chandra X-ray Observatory to find the first clear evidence for the simultaneous inward and outward flow of gas being driven by a supermassive black hole.

To Boldly Go into Colliding Galaxy Clusters

Abell 1033
Credit: X-ray: NASA/CXC/Leiden Univ./F. de Gasperin et al;
Optical: SDSS; Radio: LOFAR/ASTRON, NCRA/TIFR/GMRT
Enterprise NCC 1701
USS Enterprise NCC 1701
Credit: Smithsonian National
Air & Space Museum

Hidden in a distant galaxy cluster collision are wisps of gas resembling the starship Enterprise — an iconic spaceship from the "Star Trek" franchise.

Galaxy clusters — cosmic structures containing hundreds or even thousands of galaxies — are the largest objects in the Universe held together by gravity. Multi-million-degree gas fills the space in between the individual galaxies. The mass of the hot gas is about six times greater than that of all the galaxies combined. This superheated gas is invisible to optical telescopes, but shines brightly in X-rays, so an X-ray telescope like NASA's Chandra X-ray Observatory is required to study it.

Making Head or Tail of a Galactic Landscape

Abell 2142
Credit:X-ray: NASA/CXC/Univ. of Geneva, D. Eckert. Optical: SDSS provided by CDS through Aladin.

Astronomers have used data from NASA's Chandra X-ray Observatory to capture a dramatic image of an enormous tail of hot gas stretching for more than a million light years behind a group of galaxies that is falling into the depths of an even-larger cluster of galaxies. Discoveries like this help astronomers learn about the environment and conditions under which the Universe's biggest structures evolve.

Galaxy clusters are the largest structures in the Universe held together by gravity. While galaxy clusters can contain hundreds or even thousands of individual galaxies, the lion's share of mass in a galaxy cluster comes from hot gas, which gives off X-rays, and unseen dark matter. How did these cosmic giants get to be so big?

Perseus’s Cosmic Dance Helps Reveal the Secrets of Galaxy Cluster Astrophysics

Stephen A. Walker
Stephen A. Walker

We welcome Stephen A. Walker, first author on our latest results from the Perseus galaxy cluster, as our guest blogger. Originally from the UK, Stephen received his PhD at the University of Cambridge, continuing there as a postdoc, before becoming a NASA Postdoctoral Program Fellow at NASA’s Goddard Space Flight Center.

The story begins in October 2010, when I started my PhD at the University of Cambridge. I was exploring new X-ray observations of the outskirts of galaxy clusters taken with Suzaku. Much like in a city, the outskirts are where clusters continue to grow outwards.

Galaxy clusters are the largest gravitationally bound structures of the universe, consisting of hundreds or thousands of galaxies. The total masses of clusters are colossal, reaching up to and beyond a million billion times the mass of the sun. The vast majority of the ‘normal’ matter in these clusters (like hydrogen and helium) does not actually lie in the galaxies themselves, but rather in an extremely hot and diffuse gas between the galaxies, called the intracluster medium.

As the largest gravitationally bound structures in the universe, galaxy clusters continue to grow and accrete matter from the surrounding cosmic web of gas produced by the Big Bang. When the infalling gas falls into their deep gravitational potential wells, it is shock heated to tens of millions of degrees, and begins to emit prodigiously in the X-ray band.

A Cosmic Cold Front in the Perseus Cluster

A gigantic and resilient "cold front" hurtling through the Perseus galaxy cluster has been studied using data from NASA's Chandra X-ray Observatory. This cosmic weather system spans about two million light years and has been traveling for over 5 billion years, longer than the existence of our Solar System.

This graphic shows the cold front in the Perseus cluster. The image above contains X-ray data from Chandra — for regions close to the center of the cluster —along with data from ESA's XMM-Newton and the now-defunct German Roentgen (ROSAT) satellite for regions farther out. The Chandra data have been specially processed to brighten the contrast of edges to make subtle details more obvious.

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