|Observation data (Epoch )|
|Right ascension||12h 28m 24.9s|
|Declination||+31° 28′ 38″|
|Distance||3.18 Gpc (1.04×1010 ly)|
|Apparent magnitude (V)||15.9|
|Notable features||Hyperluminous quasar|
|FBQS J122824.9+312837, B2 1225+31, B2 1225+317, 7C 1225+3145|
|See also: Quasar, List of quasars|
TON 618 is a very distant and extremely luminous quasar—technically, a hyperluminous, broad-absorption line, radio-loud quasar—located near the North Galactic Pole in the constellation Canes Venatici. It likely contains one of the most massive known black holes, perhaps weighing in at 66 billion times the mass of the Sun.
Because quasars were not recognized until 1963, the nature of this object was unknown when it was first noted in a 1957 survey of faint blue stars (mainly white dwarfs) that lie away from the plane of the Milky Way. On photographic plates taken with the 0.7m Schmidt telescope at the Tonantzintla Observatory in Mexico, it appeared "decidedly violet" and was listed as number 618 in the Tonantzintla catalogue.
In 1970, a radio survey at Bologna discovered radio emission from TON 618, indicating that it was a quasar. Marie-Helene Ulrich then obtained optical spectra of TON 618 at the McDonald Observatory which showed emission lines typical of a quasar. From the redshift of the lines Ulrich deduced that TON 618 was very distant, and hence was one of the most luminous quasars known.
Ultramassive black hole
As a quasar, TON 618 is believed to be an accretion disc of intensely hot gas swirling around a giant black hole in the centre of a galaxy. The light originating from the quasar is estimated to be 10.4 billion years old. The surrounding galaxy is not visible from Earth, because the quasar itself outshines it. With an absolute magnitude of −30.7, it shines with a luminosity of 4×1040 watts, or as brilliantly as 140 trillion Suns, making it one of the brightest objects in the Universe.
Like other quasars, TON 618 has a spectrum containing emission lines from cooler gas much further out than the accretion disc, in the broad-line region. The emission lines in the spectrum of TON 618 are unusually wide, indicating that the gas is travelling very fast; the hydrogen beta line shows it is moving around at 7000 km/s. Hence the central black hole must be exerting a particularly strong gravitational force.
The size of the broad-line region can be calculated from the brightness of the quasar radiation that is lighting it up. From the size of this region and the speed it is orbiting, the law of gravity reveals that the mass of the black hole in TON 618 is 66 billion solar masses. With a mass this high, TON 618 falls into the new classification of Ultramassive Black Holes.  
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