In astronomy, dark matter is a hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravitational effects which cannot be explained by general relativity unless more matter is present than can be observed. Such effects occur in the context of formation and evolution of galaxies, gravitational lensing, the observable universe's current structure, mass position in galactic collisions, the motion of galaxies within galaxy clusters, and cosmic microwave background anisotropies.
The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-largest known gravitationally bound structures in the universe after some superclusters (of which only one, the Shapley Supercluster, is known to be bound). They were believed to be the largest known structures in the universe until the 1980s, when superclusters were discovered. One of the key features of clusters is the intracluster medium (ICM). The ICM consists of heated gas between the galaxies and has a peak temperature between 2–15 keV that is dependent on the total mass of the cluster. Galaxy clusters should not be confused with galactic clusters (also known as open clusters), which are star clusters within galaxies, or with globular clusters, which typically orbit galaxies. Small aggregates of galaxies are referred to as galaxy groups rather than clusters of galaxies. The galaxy groups and clusters can themselves cluster together to form superclusters.
A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo:
In modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).
Abell 2029 or A2029 is a large and relaxed cluster of galaxies 315 megaparsecs away in the constellation Virgo. A2029 is a Bautz–Morgan classification type I cluster due to its large central galaxy, IC 1101. Abell 2029 has a diameter of 5.8–8 million light-years. This type of galaxy is called a cD-type brightest cluster galaxy and may have grown to its large size by accreting nearby galaxies. Despite its relaxed state, it is the central member of a large supercluster which shows clear signs of interaction.
In astronomy, the intracluster medium (ICM) is the superheated plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ICM is heated to temperatures on the order of 10 to 100 megakelvins, emitting strong X-ray radiation.
IC 1101 is a class S0 supergiant (cD) lenticular galaxy at the center of the Abell 2029 galaxy cluster. It has an isophotal diameter at about 123.65 to 169.61 kiloparsecs. It possesses a diffuse core which is the largest known core of any galaxy to date, and contains a supermassive black hole, one of the largest discovered. IC 1101 is located at 354.0 megaparsecs from Earth. It was discovered on 19 June 1790, by the British astronomer William Herschel.
Abell 1689 is a galaxy cluster in the constellation Virgo over 2.3 billion light-years away.
The Deep Lens Survey is an ultra-deep multi-band optical survey of seven 4 square degree fields. Mosaic CCD imagers at the National Optical Astronomy Observatory's Blanco and Mayall telescopes are being used. The deep fields took five years to complete (2001–2006), in four bands: B, V, R, and z', to 29/29/29/28 mag per square arcsecond surface brightness. Optical transient events and supernova candidates are released in real time.
The Bullet Cluster consists of two colliding clusters of galaxies. Strictly speaking, the name Bullet Cluster refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of 1.141 Gpc.
The Shapley Supercluster or Shapley Concentration is the largest concentration of galaxies in our nearby universe that forms a gravitationally interacting unit, thereby pulling itself together instead of expanding with the universe. It appears as a striking overdensity in the distribution of galaxies in the constellation of Centaurus. It is 650 million light-years away (z=0.046).
Abell 520 is a galaxy cluster in the Orion constellation, located at a co-moving radial distance of 811 Mpc (2,645 Mly) and subtends 25 arcminutes on the Earth sky.
MACS J0025.4-1222 is a galaxy cluster created by the collision of two galaxy clusters, and is part of the MAssive Cluster Survey (MACS). Like the earlier discovered Bullet Cluster, this cluster shows a clear separation between the centroid of the intergalactic gas and the colliding clusters.
Abell 2142, or A2142, is a huge, X-ray luminous galaxy cluster in the constellation Corona Borealis. It is the result of a still ongoing merger between two galaxy clusters. The combined cluster is six million light years across, contains hundreds of galaxies and enough gas to make a thousand more. It is "one of the most massive objects in the universe."
Abell 222 is a galaxy cluster in the constellation of Cetus. It holds thousands of galaxies together. It is located at a distance of 2.4 billion light-years from Earth.
The Bullet Group is a newly merging group of galaxies, a merger between two galaxy groups to form a new larger one, that recently had a high speed collision between the two component groups. The group exhibits separation between its dark matter and baryonic matter components. The galaxies occur in two clumps, while the gas has expanded into a billowing cloud encompassing all three clumps. As of 2014, it is one of the few galaxy clusters known to show separation between the dark matter and baryonic matter components. The group is named after the Bullet Cluster, a similar merging galaxy cluster, except on a smaller scale, being of groups instead of clusters. The bimodal distribution of galaxies was found at discovery in 2008. The galaxy group is a gravitational lens and strongly lenses a more distant galaxy behind it, at z=~1.2
The Bullet Galaxy (RXC J2359.3-6042 CC) is a galaxy in the galaxy cluster RXC J2359.3-6042 (Abell 4067 or ACO 4067). The Bullet Galaxy is the sole component of one half of a cluster merger between the bulk of the cluster and this galaxy, which is plowing through the cluster, similar to how merging clusters Bullet Cluster and Bullet Group have merged. Unlike those two mergers, the Bullet Galaxy's merger is between one galaxy and a galaxy cluster. The cluster merger is happening at a lower speed than the Bullet Cluster, thus allowing the core of the Bullet Galaxy to retain cool gas and remain relatively undisturbed by its passage through the larger cluster. This cluster merger is the first one observed between a single galaxy and a cluster. The galaxy and cluster lies at redshift z=0.0992, some 1.4×109 ly (4.3×108 pc) away. The galaxy is traveling through the cluster at a speed of 1,310 km/s (2,900,000 mph).
NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of circa 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.
