Centaurus A Facts

Centaurus A or NGC 5128 is a galaxy in the constellation of Centaurus.

The Centaurus A galaxy might be an ancient cannibalistic galaxy devouring a smaller one. It apparently prefers radio.

Key Facts & Summary

  • The Centaurus A is the largest and nearest radio galaxy, a massive source of radio wavelengths in the constellation Centaurus.
  • It has an active nucleus which means that matter falls into the supermassive black hole in its centre and shoots electrons from its poles at the half the speed of light, creating massive jets that spread thousands light years into space.
  • It is probably a result of two galaxy smashing onto each other, one older elliptical galaxy that merges with a smaller spiral galaxy.
  • The epic radio lobes that spreads millions of light years in the opposite direction from the dust band is probably a result of this collision.
  • It shoots two gigantic jets of high-energy radiation from its centre which is a result of the black hole swallowing the falling matter.
  • There is evidence of enormous starburst, a process of formation of new blue stars in the dust band, as well as the edges of the two massive radio lobes.

Distance: 10-16 million light years (3 -5 megaparsecs)
Diameter: 60 000 light years
Other designations: NGC 5128, PGC 46957, LEDA 46957, ESO 270-9, Arp 153, Caldwell 77, 4U 1322–42
Constellation: Centaurus

Located in the Virgo Supercluster, the deep sky galaxy Centaurus A is the fifth brightest galaxy visible in the sky and the closest active radio galaxy with radio lobes and high-energy X-ray jets shooting millions light years across the Universe. This beautiful deep sky object is particularly interesting for astronomers because it emits strong radiation across all wavelengths of the electromagnetic spectrum.

Discovery

The Centaurus A galaxy was discovered in 1826 by Scottish astronomer James Dunlop who described it as a nebula since the existence of distant galaxies was still a subject of debate at that time. The details of the nebula’s strange structure such as the dust lane and the jet were first described in 1847 by the English astronomer John Herschel.

The galaxy was documented in 1949 as one of the earliest discoveries of radio emission sources outside of the Milky Way. It was the first and the brightest source of radio emissions in the constellation Centaurus, hence the name.

The Centaurus A has been excessively researched since 1940’s using various imaging technics where different wavelengths of light emitted from the galaxy were recorded using delicate and precise instruments. With the help of the Hubble Space Telescope, the Spitzer Space Telescope and various observatories on Earth, we were able to take photographs of the light coming from the Centaurus A galaxy in a broad spectrum ranging from infrared to ultraviolet light, as well as radio and microwave. This revealed the galaxy’s unique features and striking radio lobes that spread around its relativistic jets.

Formation

Scientist are still debating whether to classify the Centaurus A as an elliptical or lenticular galaxy, because of it’s strange appearance. According to the research conducted in visible light, the galaxy seems to have a structure of elliptical galaxies which do not usually have bands of interstellar gas and dust known as a dust lane or dust band. A dust band is the cloud-like smudge that hides the bulge of the Milky Way when we look at it at the night sky. It is a usual feature in spiral galaxies such as our own, but it is also found in Centaurus A.

It was first hypothesized in 1954 that the galaxy’s strange structure and emissions were a result of a collision of two galaxies. Simulation models showed a possibility that a more massive elliptical galaxy merged with a smaller spiral galaxy in a collision that took place somewhere between 200 and 700 million years ago.

Structure

The central bulge of the Centaurus A galaxy is mainly comprised of older red stars. Research conducted in the ultraviolet spectrum showed abundance of blue stars which indicate very high new star formation activity, called starburst. Blue stars are young hot stars usually found in nebular clouds and remnants of galactic collisions.

Images in the infrared spectrum revealed detailed structure of the dusty region probably comprised of multiple rotating disks wrapped around each other. This might be a remnant of the smaller spiral galaxy assimilated by the large elliptical one. The infrared spectrum can also reveal the inner structure of the Centaurus A galaxy and its nucleus which is otherwise obscured under the visible light by dust particles in the disk.

Observation made in the young field of radio astronomy in the 1940’s detected strong emissions in radio wavelengths coming from the constellation Centaurus and the source of the emission was dubbed Centaurus A. It was concluded that the galaxy emits more radiation at radio wavelengths than at visible wavelengths, which is why it is today classified as a radio galaxy. These emissions are the most prominent feature of the Centaurus A as further research revealed two enormous radio lobes perpendicular to the plane of the dust band that measure 650 000 and 1 350 000 light years in diameter. The lobes are depicted in blue colour.

Like many galaxies, the Centaurus A hosts a supermassive black hole in its center estimated to be of 55 million solar masses. The black hole is surrounded by a disk of hot gas approximately 130 light years in diameter. It is currently in the process of “feeding” which means that the hot gas is spiraling towards the black hole. As it falls down towards the event horizon, the hot swirling matter forms what is known as accretion disk from which the matter can escape through powerful jets of high-energy particles zooming around the magnetic field lines at half the speed of light.

X-ray imaging revealed two jets shooting symmetrically from the galaxy’s nucleus. Researchers from the Naval Research Laboratory in Washington detected emission of gamma radiation that is more than ten times greater than the radio output of the galaxy. Scientists also found a number of sources of X-ray radiation in the bulge that probably come from neutron stars and active stellar-mass black holes.

Observations conducted in microwave wavelengths can reveal the direction in which the galaxy spins. The green colour is used to show the part of the galaxy that moves towards the Earth and the red part shows objects and gas moving in the opposite direction.

In 1986, a supernova dubbed SN 1986G was discovered within the dust band and was classified as Ia supernova, a type of supernova that is a result of a collision of a binary star system where one of the stars is a white dwarf. I was the first supernova discovered in the Centaurus A galaxy. The second supernova was discovered in 2016 by Backyard Observatory Supernova Search and was dubbed SN2016adj. It was a type IIb supernova, a type of a violent explosion resulting from a rapid collapse of a core of massive stars that are at least eight times larger than our Sun.

There were also found other types of stars within the Centaurus A galaxy, the most interesting are Mira variables and Type II Cepheids. The Mira variables are a type of old red giant stars that undergo helium fusion in the star’s core and shed much of their mass to the surrounding space. They are pulsating stars that change in brightness over time and the loss of the mass will result in becoming with dwarfs over the course of several million years. Cepheid variables are another type of pulsating stars whose brightness varies over time and those types of pulsating stars are often used to determine distances between galaxies.

How to find it in the night sky

Even though it is the fifth brightest galaxy, the Centaurus A is only visible to an unaided eye in exceptional circumstances. It is only visible from the southern hemisphere and low northern latitudes.

It lies near the centre of the constellation Centaurus, approximately four degrees north of the globular cluster Omega Centauri. It is rarely visible to the naked eye, but amateur astronomers are able to observe the details such as the dust band and the central bulge using binoculars or smaller telescopes.

Bibliography

  1. Harris, Gretchen L. H. (2010). “NGC 5128: The Giant Beneath”. Publications of the Astronomical Society of Australia. https://www.cambridge.org/core/services/aop-cambridge-core/content/view/FE04EF2954F65466134985B96AAF6A4F/S1323358000000564a.pdf/ngc_5128_the_giant_beneath.pdf
  2. Quillen, A. C.; Brookes, M. H.; Keene, J.; Stern, D.; Lawrence, C. R.; Werner, M. W. (2006). “Spitzer Observations of the Dusty Warped Disk of Centaurus A”. Astrophysical Journal. https://iopscience.iop.org/article/10.1086/504418/pdf
  3. Majaess, D. (2010). “The Cepheids of Centaurus A (NGC 5128) and Implications for H0”. Acta Astronomicahttps://arxiv.org/abs/1006.2458
  4. Rejkuba, M. (2004). “The distance to the giant elliptical galaxy NGC 5128”. Astronomy and Astrophysics. https://arxiv.org/abs/astro-ph/0310639
  5. Taylor M. A.; Puzia T. H.; Gomez M.; Woodley K. A. (2014). “Observational evidence for a dark side to NGC 5128’s globular cluster system”. The Astrophysical Journal. https://arxiv.org/pdf/1503.04198.pdf 

Image sources

  1. https://upload.wikimedia.org/wikipedia/commons/thumb/d/d2/ESO_Centaurus_A_LABOCA.jpg/569px-ESO_Centaurus_A_LABOCA.jpg
  2. https://upload.wikimedia.org/wikipedia/commons/2/2b/Centaurus_A_EN.webm
  3. https://www.salt.ac.za/news/sn2016adj-supernova-in-centaurus-a/
  4. Stellarium app