TON 618 (abbreviation of Tonantzintla 618) is a hyperluminous, broad-emission-line, radio-loud quasar, and Lyman-alpha blob located near the border of the constellations Canes Venatici and Coma Berenices, with the projected comoving distance of approximately 18.2 billion light-years from Earth. It contains one of the most massive black holes ever found, at roughly 40.7 billion times the Sun's mass (M☉), though some older, more frequently-cited estimates yield 66 billion M☉.
Observational history As 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 0.7 m (27+1⁄2 in) Schmidt telescope at the Tonantzintla Observatory in Mexico, it appeared as "decidedly violet" or faint blue star and was listed by the Mexican astronomers Braulio Iriarte and Enrique Chavira as entry number 618 in the Tonantzintla Catalogue. Thirteen years later, in 1970, a radio survey at Bologna in Italy discovered radio emissions 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 high redshift of the lines, Ulrich deduced that TON 618 was very distant, and hence was one of the most luminous quasars known.
Components
Supermassive black hole
As a quasar, TON 618 is believed to be the active galactic nucleus at the center of a galaxy, the engine of which is a supermassive black hole feeding on intensely hot gas and matter in an accretion disc. Given its observed redshift of 2.219, the light travel time of TON 618 is estimated to be approximately 10.8 billion years, which is directly proportional to its distance in light-years. Due to the brilliance of the central quasar, the surrounding galaxy is outshone by it and hence is not visible from Earth. With an absolute magnitude of −30.7, it shines with a luminosity of 4×1040 watts, or as brilliantly as 140 trillion times that of our Sun (L☉), making it one of the brightest objects in the known 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 size of the broad-line region can be calculated from the brightness of the quasar radiation that is lighting it up. Shemmer and coauthors used both NV and CIV emission lines in order to calculate the widths of the Hβ spectral line of at least 29 quasars, including TON 618, as a direct measurement of their accretion rates and hence the mass of the central black hole. The emission lines in the spectrum of TON 618 have been found to be unusually wide, indicating that the gas is travelling very fast; the full width half maxima of TON 618 has been the largest of the 29 quasars, with hints of 10,500 km/s speeds of infalling material by a direct measure of the Hβ spectral line, indication of a very strong gravitational force. From this, the mass of the central black hole of TON 618 has been estimated to be at 66 billion M☉. This is considered one of the highest masses ever recorded for such an object; higher than the mass of all the stars in the Milky Way galaxy combined, which is 64 billion M☉, and 15,300 times more massive than Sagittarius A*, the Milky Way's central black hole. With such high mass, TON 618 may fall into a proposed new classification of ultramassive black holes. A black hole of this mass has a Schwarzschild radius of 1,300 astronomical units (194 billion kilometres; 0.0206 light-years) or 283,000 solar radii (R☉), which is more than 60 times the distance from Pluto to the Sun. A more recent measurement in 2019 by Xue Ge et al. which utilizes the C IV emission line, an alternative spectral line to Hβ—using the same data reproduced by the earlier paper by Shemmer—found a lower relative velocity of the surrounding gas of 2,761±423 km/s, which indicates a lower mass for the central black hole at 40.7 billion M☉, consequentially lower than the previous estimate. Due to the Schwarzschild radius growing proportionately to a black hole's mass, this would correspond to an event horizon spanning 800 astronomical units (120 billion kilometres; 0.0127 light-years), about 40 times the distance from Pluto to the Sun. Evolution models based on the revisited mass of TON 618's supermassive black hole predict that it will live for roughly 1.3×1099 years (near the end of the Black Hole Era of the universe, when it is nearly 1089 times its current age), before it dissipates via Hawking radiation.
Lyman-alpha nebula
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