Ton 618 is a hyperluminous, broad-absorption-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 possesses one of the most massive black holes ever found, with a mass of 66 billion solar masses.
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. The light originating from the quasar is estimated to be 10.8 billion years old. Due to the brilliance of the central quasar, the surrounding galaxy is outshined 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 the Sun, 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 7,000 km/s speeds of infalling material by a direct measure of the Hβ line, indication of a very strong gravitational force. From this measure, the mass of the central black hole of Ton 618 is at least 66 billion solar masses. This is considered one of the highest masses ever recorded for such an object; higher than the mass of all stars in the Milky Way galaxy combined, which is 64 billion solar masses, 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 AU (about 390 billion km in diameter) which is more than 40 times the distance from Neptune to the Sun.