Active galactic nuclei flicker: an observational estimate of the duration of black hole growth phases of ~1e5 years

Understand

We present an observational constraint for the typical active galactic nucleus (AGN) phase lifetime.

• The argument is based on the time lag between an AGN central engine switching on and becoming visible in X-rays, and the time the AGN then requires to photoionize a large fraction of the host galaxy.
• Based on the typical light travel time across massive galaxies, and the observed fraction of X-ray selected AGN without AGN-photoionized narrow lines, we estimate that the AGN phase typically lasts $\sim10^{5}$ years.
• This lifetime is short compared to the total growth time of $10^{7}-10^{9}$ years estimated from e.g.