Most active galactic nuclei are at large distances from us; relatively few nearby galaxies harbor active galactic nuclei. What does this imply?

Question

Quizplus · Accepted Answer

The fact that most active galactic nuclei are at large distances from us implies that they tend to become less active as they age. This may be due to a depletion of gas and dust that fuels the activity in the nucleus, or the merger of the black hole with another black hole, which can extinguish the activity.

Most Active Galactic Nuclei Are at Large Distances from Us;