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,[1] indicating that the gas is travelling very fast; the hydrogen beta line shows it is moving around at 7000 km/s.[2] Hence the central black hole must be exerting a particularly strong gravitational force.