NGC 1232 is an

intermediate spiral galaxy An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification system devised by Gerard de Vaucouleur ...

about 60 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 ...

Eridanus. It was discovered by German-British astronomer

William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...

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

spiral arm Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''Open cluster An open cluster is a type of star cluster made of up 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 In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar s ...

, comprising such a high mass that they dominate the dynamics of the inner galaxy. Not visible is matter of unknown form called

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

, needed to explain the motions of the visible material in the outer galaxy. The galaxy is approximately 200,000 light-years across, in between the sizes of 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 ...

and our home galaxy, the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

. 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 The Eridanus Cluster is a galaxy cluster roughly from Earth, containing about 73 main galaxies and about 200 total galaxies. About 30% have Hubble classifications of elliptical or S0 and the remaining 70% are spiral or irregular. These gala ...

of galaxies, along with

NGC 1300 NGC 1300 is a barred spiral galaxy located about 61 million light-years away in the constellation Eridanus. The galaxy is about 110,000 light-years across. It is a member of the Eridanus Cluster, a cluster of 200 galaxies. It was discovered b ...

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 In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges w ...

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 well-defined and flocculent, winding counterclockwise from the galactic center, containing numerous

HII regions An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

. 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 was known for his 1966 ''Atlas of Peculiar Galaxies'', which (it was later theorized) catalogues many examples of interacting and merging galaxie ...

suggested that this is the result of a galaxy interaction. The galaxy contains numerous star-forming regions, making 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 galactocentric distances, 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, an ...

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 luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents are bound ...

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 simultaneous increase in f ...

of the two galaxies do not match, which suggest that they likely never interacted before and are not currently interacting. The discrepant redshifts 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, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

studies suggest that NGC 1232 may have interacted with a dwarf galaxy, 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 1 ...

also suggest that there was an interaction between NGC 1232 and another dwarf galaxy, commenting on a massive cloud of gas only visible in the X-ray superimposed on NGC 1232's images. The paper 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, 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. In 2018, a study of the star 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 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 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 dwarf galaxy and NGC 1232 at all.

References

External links