NGC 1232, also known as the Eye of God Galaxy (not to be confused with the Helix Nebula, also known as Eye of God) is an intermediate spiral galaxy about 50 million

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

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 first constellati ...

Eridanus. It was discovered by German-British astronomer

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

on 20 October 1784. It is dominated by millions of bright stars and dark dust, in

spiral arm Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms i ...

s rotating about the center.

Open cluster An open cluster is a type of star cluster made of tens to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

s containing bright blue stars are sprinkled along these spiral arms, with dark lanes of dense interstellar dust between. Less visible are dimmer stars and interstellar gas, comprising such a high mass that they dominate the dynamics of the inner galaxy. Not visible is matter of unknown form called

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

, needed to explain the motions of the visible material in the outer galaxy. The galaxy is approximately 150,000 light-years across. The galaxy's spiral arms are not smooth and perturbed, leading to some suggesting a collision with a dwarf galaxy. However, some studies doubt this suggestion. NGC 1232 and its apparent companions are possibly part of the Eridanus cluster of galaxies, along with

NGC 1300 NGC 1300 is a barred spiral galaxy located about 1 E23 m, 65 million light-years away in the constellation Eridanus (constellation), Eridanus. The galaxy is about 130,000 light-years across. It is a member of the Eridanus Cluster, a cluster of 2 ...

Description and structure

NGC 1232 is a face-on spiral galaxy. It can be technically considered a grand-design galaxy and is considered a prototype for multi-arm spiral galaxies. Its galactic bulge is small. While NGC 1232 is classified as an intermediate spiral galaxy, the bulge shows hints of a galactic bar.

Spiral arms

NGC 1232's spiral arms are bright and flocculent, winding counterclockwise from the galactic center. They contain numerous HII regions. At large distances from the galaxy's center, the spiral arms either branch out and disperse, producing long spiral arms, or connect with arm segments. However, they are not smooth as expected for a spiral galaxy. Instead, they bend abruptly and show significant deviations from a constant pitch. In fact, the same pitch angle cannot be applied to one arm due to the arms deviating. Astronomer

Halton Arp Halton Christian "Chip" Arp (March 21, 1927 – December 28, 2013) was an American astronomer. He is remembered for his 1966 book ''Atlas of Peculiar Galaxies'', which catalogued unusual-looking galaxies and presented their images. Arp was als ...

suggested that this is the result of a galaxy interaction. That the galaxy contains numerous star-forming regions makes it an excellent laboratory when it comes to studies on star formation. Most of NGC 1232's arms and arm segments widen as they get farther and farther away from the galaxy's center with only one arm or arm segment, designated as arm segment "E", narrowing as it gets further from the center. This is likely due to less and less star formation occurring at farther distances from the galaxy's center, or it may be because the spiral arm reaches a point where the spiral pattern starts to co-rotate orbiting material.

Association with the Eridanus cluster

NGC 1232 and NGC 1232 A may both be associated with the Eridanus cluster of galaxies, but with a distance of around 2.2 million

parsecs The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

from the cluster's center, they may not be bound to the cluster.

Debated collision with another dwarf galaxy and association with NGC 1232A

It was originally thought that one of NGC 1232's companions, NGC 1232A, was interacting with NGC 1232 and causing perturbations within its disk, as NGC 1232A seems to be a

satellite galaxy A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and Luminosity, luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents ...

of NGC 1232 at first sight, but this is likely not the case. In 1988, NGC 1232A was estimated to be 68 million light-years away while NGC 1232 was estimated to be 65 million light-years away. The

redshifts In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

of the two

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar Sys ...

do not match, which suggest that they likely never interacted before and are not currently interacting. The discrepant

for the two galaxies is one of the reasons why the pair is known as one of the most striking examples for discrepant redshifts in galaxy pairs. Instead,

X-Ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

studies suggest that NGC 1232 may have interacted with a

dwarf galaxy A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

, but the dwarf galaxy has apparently left no remnant. X-Ray observations over a three-year period made by the

Chandra X-ray observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources ...

also suggest that there was an interaction between NGC 1232 and another dwarf galaxy, commenting on a massive cloud of

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

only visible in the X-ray superimposed on NGC 1232's images. The

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, Textile, rags, poaceae, grasses, Feces#Other uses, herbivore dung, or other vegetable sources in water. Once the water is dra ...

concludes that this may be the first case of the dwarf galaxy-large galaxy collision whose evidence is only visible in the X-Ray. However, it is noted that this may be due to other causes, such as numerous

supernovae A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original ob ...

, but the study also suggests that there is no other evidence for this. As a result, an interaction with another galaxy, other than NGC 1232A, is sometimes thought to be the cause of unusual bending in the

spiral arms Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms ...

. In 2018, a study of the

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

formation rates in NGC 1232 took images of the galaxy in Hydrogen Alpha, finding over 970 HII regions. It is remarked that there is a concentration of HII regions in one part of the galaxy, but when the star-formation rates are taken into account, the concentration is more diluted. The study found that X-Ray emissions may be quenching

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

in some areas of the galaxy, because there seems to be lower concentration of HII regions there. The amount of HII regions seems to be higher in areas without much X-ray emission. Both of these were suggested to have been caused by a collision with another galaxy. In contrast with previous studies, a recent X-Ray study remarks that the hot gas

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

in NGC 1232 is not very high. According to the study, the cloud claimed to be superimposed on NGC 1232 does not exist, and that peculiarities in NGC 1232's HII regions are just coincidental. The study notes that NGC 1232's disk is not very warped. Thus, the study concludes that there was no interaction between either NGC 1232A or another

and NGC 1232 at all.

References

External links

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NGC 1232

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