Centaurus A Radio Galaxy is one of the most famous objects in Centaurus, and as its name suggests, it is not an ordinary galaxy at all. There is a supermassive black hole in its core emitting million light-years long jets in which matter travels almost at the speed of light.
The galaxy, also known as NGC 5128, was discovered in 1826 by the Scottish astronomer James Dunlop from his home located in New South Wales. There are still uncertainty about numerous properties of the galaxy, including its morphological classification or its distance. Some categorise it as elliptical, while others think it is a lenticular galaxy. The approximations regarding its distance span from 10 to 16 million light-years. The bulge of the galaxy consists of old stars while the disc is a place of star formation at such a high rate that Centaurus A is considered as a star burst galaxy. More than 100 star formation region have been identified in it. The peculiar outlook of the galaxy was first noticed by John Herschel in 1847, and of course Halton Arp has also included it in his Atlas of Peculiar Galaxies.
Like most star burst galaxies, NGC 5128 is a result of collision of two galaxies. To be more precise, a smaller spiral galaxy is being absorbed. The presence of the twisted, inclined dust ring, the remains of the smaller galaxy, is an evidence of the process.
Centaurus A is the fifth brightest galaxy in the sky, and also one of the closest radio galaxy to the Earth. Radio galaxies are galaxies having more intense radiation in radio wavelengths, as it was suggested by its brightness measured in optical range. This anomaly indicates that it is not a regular galaxy, something unusual must be happening there resulting in high emission of radio waves. The process is called the synchrotron radiation or synchrotron process. Charged particles moving almost at the speed of light emit such radiation when their velocity and/or direction of movement is being changed by magnetic field for example.
Most radio galaxies have a super massive black hole at their core. That's the case with Centaurus A as well, the mass of its black hole is about 55 million solar masses. Due to its strong gravitational force an accretion disc is being formed around it, in which the matter falling in the black hole on a spiral orbit. During its fall the matter is being accelerated, reaching very high velocities. Twisted magnetic fields in the disc eject charged particles at relativistic speeds perpendicular to the plane of rotation, along the rotational axes. These high-velocity, focused streams of matter are called relativistic jets, in which all necessary circumstances for the synchrotron process are present: strong magnetic field which accelerates high-speed charged particles.
Although it is not the case with Centaurus A, it worths to be mentioned that an even more interesting phenomenon can be observed when the direction of relativistic jets points directly or close to the observer. In such cases the matter emitting radiation is advancing the observer almost at the speed of light. The light emitted by the approaching particles during a long period of time will arrive in a much shorter period, thus appearing much more intensive for the observer, and it also seems as it travelled faster than the speed of light. It is quite analogue to sonic boom, so it could be called as 'light boom'. This phenomenon could be the explanation how quasars work. Quasars are very distant light sources appearing much brighter than their distance would allow. According to the currently accepted understanding, quasars are very distant galaxies having active galactic nucleus, similar to the one Centaurus A has, and having their jets pointing towards us.
Besides radio wavelengths relativistic jets are emitting at other wavelengths as well, for example they have considerable X-ray output as well. High-energy X-ray radiation is produced by the jets colliding with surrounding gases.
Centaurus A is located at the centre of the galaxy group named after it, close to the M83 group of galaxies. Because of the closeness of the two groups, and because they seem to move at the same speed at the same direction, they often considered as one group, called the Centaurus A/M83 group, having two subgroups, having 44 galaxies altogether.
