I talked about yesterday.
What they found was that there is a critical black hole mass for a galaxy. If the mass of the black hole is above this mass, then its active periods are intense enough to drive gas out of the galaxy and hinder star production. Below that mass it’s activity isn’t sufficient to prevent stars from forming. You can see this in the figure below. The red dots represent galaxies without recent star production, while the blue dots are galaxies with recent star production. The dotted green line is the critical mass.
So it seems that as a galactic black hole increases in mass over time, it eventually becomes a heart of darkness.