The Cartwheel Galaxy (also known as ESO 350-40 or PGC 2248) is a lenticular

ring galaxy A ring galaxy is a galaxy with a circle-like appearance. Hoag's Object, discovered by Art Hoag in 1950, is an example of a ring galaxy. The ring contains many massive, relatively young blue stars, which are extremely bright. The central region co ...

about 500 million

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Sculptor Sculpture is the branch of the visual arts that operates in three dimensions. Sculpture is the three-dimensional art work which is physically presented in the dimensions of height, width and depth. It is one of the plastic arts. Durable sc ...

. It has a D₂₅ isophotal diameter of , and a mass of about

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es; its outer ring has a circular velocity of . It was discovered by

Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and ...

in 1941. Zwicky considered his discovery "one of the most complicated structures awaiting its explanation on the basis of stellar dynamics." An estimation of the galaxy's span resulted in a conclusion of 150,000

light year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s, slightly smaller than the

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gala ...

. The Third Reference Catalogue of Bright Galaxies (RC3) measured a D₂₅ isophotal diameter for the Cartwheel Galaxy at about 60.9 arcseconds, giving it a diameter of based on a redshift-derived distance of . The large Cartwheel Galaxy is the dominant member of the Cartwheel Galaxy group, consisting of four physically associated spiral galaxies. The three companions are referred to in several studies as G1, the smaller irregular blue Magellanic spiral; G2, the yellow compact spiral with a tidal tail; and G3, a more distant spiral often seen in wide field images. SN 2021afdx, a

type II supernova A Type II supernova (plural: ''supernovae'' or ''supernovas'') results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun () to undergo this ...

in the Cartwheel Galaxy, was discovered in November 2021.

Structures

The structure of the Cartwheel Galaxy is noted to be extremely complicated and heavily disturbed. The Cartwheel consists of two rings: the outer ring, the site of massive ongoing star formation due to gas and dust compression; and the inner nucleic ring that surrounds the galactic center. A ring of dark absorbing dust is also present in the nucleic ring. Several optical arms or “spokes” are seen connecting the outer ring to the inner, and are theorized to be reforming spiral arms following the collision. Observations show the presence of both non-thermal radio and optical spokes, but the two do not overlap and have been shown to be unassociated with each other, and thus are different structures.

Evolution

The galaxy was once a normal

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' When the nearby galaxy passed through the Cartwheel Galaxy, the force of the collision caused a powerful gravitational shock wave to expand through the galaxy, like a rock being tossed into a sandbed. Moving at high speed, the shock wave swept up and compressed gas and dust, creating a starburst around the galaxy's center portion that went unscathed as it expanded outwards. This explains the bluish ring around the center, brighter portion. It can be noted that the galaxy is beginning to retake the form of a normal

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Jeans instability In stellar physics, the Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation, named after James Jeans. It occurs when the internal gas pressure is not strong enough to prevent gravitational collapse of a ...

of both axisymmetric (radial) and nonaxisymmetric (spiral) small-amplitude gravity perturbations allows an association between growing clumps of matter and the gravitationally unstable axisymmetric and nonaxisymmetric waves which take on the appearance of a ring and spokes. Based on observational data, however, this theory of ring galaxy evolution does not appear to apply to this specific galaxy. While most images of the Cartwheel display three galaxies close together, a fourth physically associated companion (also known as G3) is known to be associated with the group through an HI tail that connects G3 to the cartwheel. Due to the presence of the HI tail, it is widely believed that G3 is the “bullet” galaxy that plunged through the disk of the cartwheel, creating its current shape, not G1 or G2. This hypothesis makes sense given the size and predicted age of the current structure (~300 million years old as mentioned before). Considering how close G1 and G2 are to the Cartwheel still, it is much more widely believed that the roughly 88 kpc (~287,000 light years) distant G3 is the intruding galaxy. Neutral hydrogen tail mapping is extremely useful in determining “culprit” galaxies in similar cases where the answer is relatively unclear. Hydrogen gas, being the lightest and most abundant gas in galaxies, is easily torn away from parent galaxies through gravitational forces. Evidence of this can be seen in the

Jellyfish Galaxy A jellyfish galaxy is a type of galaxy found in galaxy clusters. They are characterised by ram pressure stripping of gas from the affected galaxy by the intracluster medium, triggering starbursts along a tail of gas. Jellyfish galaxies have be ...

and the Comet Galaxy, which are undergoing a type of gravitational effect called ram pressure stripping, and other galaxies with tidal tails and star forming stellar streams associated with collisions and mergers. Ram pressure stripping will almost always cause ''trailing-dominant'' tails of HI gas as a galaxy infalls into a galaxy cluster, while mergers and collisions like the cartwheel often create ''leading-dominant'' tails as the culprit galaxy’s gravity attracts and pulls on the victim galaxy’s gas in the direction of the culprit's motion. The existing structure of the cartwheel is expected to disintegrate over the next few hundred million years as the remaining gas, dust and stars that haven’t escaped the galaxy begin to infall back towards the center. It is likely that the galaxy will regain a spiral shape after the infall process completes and spiral density waves have a chance to reform. This is only possible if companions G1, G2 and G3 remain distant and do not undergo an additional collision with the cartwheel.

X-ray sources

The unusual shape of the Cartwheel Galaxy may be due to a collision with a smaller galaxy such as one of those in the lower left of the image. The most recent star burst (star formation due to compression waves) has lit up the Cartwheel rim, which has a diameter larger than that of the Milky Way. Star formation via

starburst galaxies A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formatio ...

, such as the Cartwheel Galaxy, results in the formation of large and extremely luminous stars. When massive stars explode as

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

s, they leave behind

neutron star A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...

s and

black hole A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...

s. Some of these neutron stars and black holes have nearby

companion star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

s, and become powerful sources of X-rays as they pull matter off their companions (also known as ultra and hyperluminous X-ray sources). The brightest X-ray sources are likely black holes with companion stars, and appear as the white dots that lie along the rim of the X-ray image. The Cartwheel contains an exceptionally large number of these black hole binary X-ray sources, because many massive stars formed in the ring.

References

External links

*
Galaxy Evolution Simulation:The Cartwheel Galaxy

Cartwheel Galaxy at Constellation Guide

Webb Captures Stellar Gymnastics in The Cartwheel Galaxy
''nasa.gov'' {{Sculptor (constellation) Lenticular galaxies Peculiar galaxies Ring galaxies Sculptor (constellation) 02248 Astronomical objects discovered in 1941