Groups & Clusters of Galaxies

Perseus Galaxy Cluster

An innovative interpretation of X-ray data from a galaxy cluster could help scientists understand the nature of dark matter, as described in our latest press release. The finding involves a new explanation for a set of results made with NASA's Chandra X-ray Observatory, ESA's XMM-Newton and Hitomi, a Japanese-led X-ray telescope. If confirmed with future observations, this may represent a major step forward in understanding the nature of the mysterious, invisible substance that makes up about 85% of matter in the Universe.

Four of the 13 galaxies clusters used in the study. The clusters are, starting at the top left
and going clockwise, Abell 262, Abell 383, Abell 1413, and Abell 2390.

Astronomers have used data from NASA's Chandra X-ray Observatory to study the properties of dark matter, the mysterious, invisible substance that makes up a majority of matter in the universe. The study, which involves 13 galaxy clusters, explores the possibility that dark matter may be more "fuzzy" than "cold," perhaps even adding to the complexity surrounding this cosmic conundrum.

For several decades, astronomers have known about dark matter. Although it cannot be observed directly, dark matter does interact via gravity with normal, radiating matter (that is, anything made up of protons, neutrons, and electrons bundled into atoms). Capitalizing on this interaction, astronomers have studied the effects of dark matter using a variety of techniques, including observations of the motion of stars in galaxies, the motion of galaxies in galaxy clusters, and the distribution of X-ray emitting hot gas in galaxy clusters. Dark matter has also left an imprint on the radiation left over from the Big Bang 13.8 billion years ago.

Felipe Andrade-Santos and Reinout van Weeren

For this guest blog post, we are lucky to have not one, but two, contributors. Reinout van Weeren obtained his PhD from Leiden University, The Netherlands, before moving to the Harvard-Smithsonian Center of Astrophysics as an Einstein Postdoctoral Research Fellow. He is currently a Clay Fellow at the same place. He works on merging galaxy clusters, focusing on combined radio and X-ray observations. Felipe Andrade-Santos obtained his PhD from the Universidade de São Paulo, Brazil, before becoming a postdoctoral research fellow at the Harvard-Smithsonian Center of Astrophysics. He works on X-ray observations of galaxy clusters and galaxy cluster samples. Reinout and Felipe recently presented their study on the merging galaxy cluster system Abell 3411 and 3412 at the 229th meeting of the American Astronomical Society meeting in January 2017.

Galaxy clusters are the most massive objects in the Universe bound together by gravity and contain up to a few thousand galaxies. They are also permeated by very thin 100-million-degree gas that is held together by the cluster's strong gravitational pull. This hot gas can be imaged with X-ray satellites such as NASA's Chandra X-ray Observatory. Galaxy clusters form by mergers of smaller clusters and galaxy groups. During a merger event, which typically lasts for about a billion years, the galaxies mostly fly past each other without strongly interacting. In contrast, the diffuse gas in the merging clusters collides, creating giant shock waves, which are cosmic versions of sonic booms generated by supersonic aircraft. Cluster mergers have been of great interest to astronomers and us because of the extreme physical processes that take place during such events.

Radio telescopes have shown that large regions of merging galaxy clusters glow at radio wavelengths. The radio emission is produced by tiny particles, called electrons, which spiral around magnetic field lines and have energies that are a million times higher than the particles making up the hot cluster gas. Astronomers have long been puzzled by how these energetic electrons are produced. One idea is that the energetic particles are accelerated to these extreme energies by shocks created when clusters collide and merge.

Tao Wang

We are pleased to welcome Dr. Tao Wang as a guest blogger. Tao is the first author of a paper that is the subject of our latest press release, about an extremely distant galaxy cluster. Tao is now a postdoc in CEA/Saclay, France, working with Dr. David Elbaz on high-redshift galaxies and galaxy clusters, and received a PhD in astrophysics from Nanjing University, China in 2012. During his PhD, he worked for two years in the Harvard-Smithsonian Center for Astrophysics and then worked as an associate researcher back in Nanjing University for one year before starting his postdoctoral work at CEA/Saclay in 2013.

Galaxy clusters are the largest known gravitationally bound structures in the universe and usually consist of hundreds of galaxies distributed in a relatively small area a few million light-years across. One of the most prominent features of clusters is the presence of a predominant population of massive, elliptical galaxies in the cluster core. These galaxies are among the most massive galaxies in the universe and are believed to have rapidly formed their stars a long time ago. However, how these galaxies formed and why have they stopped forming new stars remain mysteries. Solving these mysteries is essential to our understanding of both galaxy and cluster formation. To answer these questions, the key is to search for and study galaxy clusters (or their progenitors) in the early universe, right when they form.

This image contains the most distant galaxy cluster, a discovery made using data from NASA's Chandra X-ray Observatory and several other telescopes. The galaxy cluster, known as CL J1001+0220, is located about 11.1 billion light years from Earth and may have been caught right after birth, a brief, but important stage of cluster evolution never seen before.

We are happy to welcome Dr. Andrea Morandi as our guest blogger, who is giving us insight into his recent work on using galaxy clusters to investigate the nature of dark energy. Originally from Italy, Dr. Morandi received his Ph.D. in astronomy from the University of Bologna. Prior to his current position as a research assistant at the University of Alabama in Huntsville, Dr. Morandi was a post-doctoral fellow at the DARK Cosmology Center in Copenhagen and Tel Aviv University, followed by time as a research associate at Purdue University.

In 1998 and 1999 astronomers discovered the accelerating expansion of the Universe, providing evidence for the existence of the mysterious dark energy driving this acceleration. The same year I started to study astronomy at the Bologna University, fascinated by this major breakthrough in cosmology. I guess my interest for cosmology started from here.

Galaxy clusters are enormous collections of hundreds or even thousands of galaxies and vast reservoirs of hot gas embedded in massive clouds of dark matter, invisible material that does not emit or absorb light but can be detected through its gravitational effects. These cosmic giants are not merely novelties of size or girth - rather they represent pathways to understanding how our entire universe evolved in the past and where it may be heading in the future.

Astronomers have made the most detailed study yet of an extremely massive young galaxy cluster using three of NASA's Great Observatories, as described in our latest press release [link to PR]. This multi-wavelength image shows this galaxy cluster, called IDCS J1426.5+3508 (IDCS J1426 for short), in X-rays from the Chandra X-ray Observatory in blue, visible light from the Hubble Space Telescope in green, and infrared light from the Spitzer Space Telescope in red.

An extraordinary ribbon of hot gas trailing behind a galaxy like a tail has been discovered using data from NASA's Chandra X-ray Observatory, as described in our latest press release. This ribbon, or X-ray tail, is likely due to gas stripped from the galaxy as it moves through a vast cloud of hot intergalactic gas. With a length of at least 250,000 light years, it is likely the largest such tail ever detected. In this new composite image, X-rays from Chandra (blue) have been combined with data in visible light from the Isaac Newton Group of Telescopes (yellow) in the Canary Islands, Spain.

This month, people around the world are celebrating the hundredth anniversary of Albert Einstein’s Theory of General Relativity (GR). Although this theory can seem esoteric, it has an important practical application: the accuracy of Global Positioning System (GPS) relies on corrections from GR.