NGC 6240, also known as the Starfish Galaxy, is a nearby ultraluminous infrared galaxy (ULIRG) 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 ...

Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constellati ...

. It was discovered by French astronomer

Édouard Stephan Édouard Jean-Marie Stephan (31 August 1837 – 31 December 1923) was a French astronomer. His surname is sometimes spelled Stéphan in some literature, but this is apparently erroneous. He was born in Sainte Pezenne (today one of the districts o ...

on 12 July 1871. The galaxy is the remnant of a

merger Mergers and acquisitions (M&A) are business transactions in which the ownership of a company, business organization, or one of their operating units is transferred to or consolidated with another entity. They may happen through direct absorpt ...

between three smaller galaxies. The collision between the three progenitor galaxies has resulted in a single, larger galaxy with three distinct nuclei and a highly disturbed structure, including faint extensions and loops.

Double nuclei

Star formation versus supermassive black holes

The power sources of ULIRGs in general has been greatly debated. Infrared light from galaxies generally originates from dust in the

interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...

. ULIRGs are abnormally bright in the infrared. The infrared dust emission in ULIRGs is over one trillion times more luminous than the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

(i.e. it has an infrared luminosity of ). Astronomers have speculated that either intense

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 ...

regions or

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that this luminosity is not produced by the stars. Such e ...

(which contain

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...

s) may be responsible for the intense dust heating that produces this emission, although the general consensus is that both may be present in most ULIRGs. Studying the exact nature of ULIRGs has been difficult, however, because the dust in the centers of these galaxies obscures both visible and near-infrared starlight and because theoretical models of both starbursts and active galactic nuclei have demonstrated that they may look similar. Because NGC 6240 is a nearby example of such a ULIRG, astronomers have studied it intensively to understand its power source.

X-ray observations

Observations performed by Stefanie Komossa and collaborators with 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 ...

have detected strong hard

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 ...

emission from two of the nuclei. The intensity of this emission and the presence of emission from lowly

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ized or neutral

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

indicate that both of these nuclei are

. Presumably, these are the black holes that were originally at the centers of the merging galaxies. Over the course of millions of years, the black holes are expected to come closer together and form a binary supermassive black hole. Recent studies by Wolfram Kollatschny and collaborators using the MUSE instrument on board the VLT have revealed that there are in fact three, not two, supermassive black holes at the core of this remnant. Their masses are suggested to be 90 million, 710 million, and 360 million solar masses. Two of the three black holes are active. The additional SMBH implies that three original galaxies are merging instead of two.

Supernovae

Four

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

e have been observed in NGC 6240: * SN 2000bg ( typeIIn, mag. 17.4) was discovered by the

Lick Observatory The Lick Observatory is an astronomical observatory owned and operated by the University of California. It is on the summit of Mount Hamilton (California), Mount Hamilton, in the Diablo Range just east of San Jose, California, United States. The ...

Supernova Search (LOSS) on 1 April 2000. * SN 2010gp ( type Ia, mag. 17.5) was discovered by The CHilean Automatic Supernova sEarch (CHASE) on 14 July 2010. * SN 2013dc ( type II-P, mag. 18.7) was discovered by Adam Block on 11 April 2013. * PSN J16525760+0223367 (type Ia, mag. 15.8) was discovered by Robert Gagliano, Jack Newton, and Tim Puckett on 23 April 2014.

Final stages

galaxy merger Galaxy mergers can occur when two (or more) Galaxy, galaxies collide. They are the most violent type of Interacting galaxy, galaxy interaction. The Gravitation, gravitational interactions between galaxies and the friction between the gas and Cosmi ...

is a slow process lasting more than a billion years as two galaxies, under the inexorable pull of gravity, dance toward each other before finally joining together. After research done over the past few years, scientists have concluded that this galaxy has reached its last stages before colliding into one another. Photographic evidence proves that the two nuclei have been growing closer, and in that process, they have been emitting more gasses and

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

s outward. Those winds evict about 100 solar masses in gases from the galaxy every year. This type of winds and growth of the black holes are known to occur during the last 10 to 20 million years of the merger; it is assumed that this is the amount of time left for the galaxy to finish its collision phase. After this phase, the merged galaxies will become one, becoming a post-merger object until if and when any tidal tails and plumes evaporate.

Gallery

File:Hubble and Keck observatories uncover black holes coalescing.tif, Hubble and Keck observatories uncover black holes coalescing. File:Galaxies Gone Wild!.jpg, Arp 148, VV 340, Arp 256, NGC 6670, NGC 6240, ESO 593-8, NGC 454, UGC 8335, NGC 6786, NGC 17, ESO 77-14, Arp 272 File:Iotw2342c.tiff, NGC 6240 imaged by the

Cerro Tololo Inter-American Observatory The Cerro Tololo Inter-American Observatory (CTIO) is an Astronomy, astronomical observatory located on the summit of Mt. Cerro Tololo in the Coquimbo Region of northern Chile, with additional facilities located on Mt. Cerro Pachón about to th ...

References

External links

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NGC 6240: Two Supermassive Black Holes in Same Galaxy
{{DEFAULTSORT:NGC 6240 Irregular galaxies Galaxy mergers Peculiar galaxies Luminous infrared galaxies Ophiuchus 6240 IC objects 10592 59186 Discoveries by Édouard Stephan Astronomical objects discovered in 1871