AGN light echoes and the accretion disc self-gravity limit

Accretion disc theory predicts that an AGN disc becomes self-gravitating and breaks up into stars at an outer radius R-sg similar or equal to 12 light-days, with effectively no free parameter. We present evidence that the longest observed AGN light echoes are all close to 12 d in the AGN rest frames. These observations give a stringent test of the AGN disc theory. Further monitoring should offer insight into the formation angular momentum of the gas forming the disc. For distant AGN, observed lags significantly longer than 12 d give lower limits on their redshifts.