Red, Dead, No Redemption

Galaxies may or may not have a social life.  Many, like our Milky Way, live in groups of a few dozen.  Others are loners, and still others live in clusters of hundreds or thousands.  Loners and group members tend to be actively forming stars, and consequently have the large, extremely bright young blue stars that only live for a few millions or tens of millions of years.  By contrast, most of the galaxies in large groups, especially the big elliptical galaxies, aren't forming stars, haven't for  perhaps a few billion years or so, and consequently have only old, red, and small stars - they are red and dead in the professional parlance.

We know why, in a sense.  They lack the cold molecular gas clouds where stars form, and instead, are embedded in hot ionized gas at a million or so kelvins.  However, this presents another puzzle.  Galaxy clusters typically have one or more of the very large cD ellipticals at their center.  Their large gravity draws in the hot ionized gas and increases its density.  Because the cooling rate of hot gas is not only proportional to the square of density but also very effective at the temperatures involved, these flows of hot gas should cool rapidly, and quickly produce enough cool gas to promote vigorous star formation.

So why don't these large cooling flows result in lots of hot young stars, redeeming these otherwise red and dead galaxies?

The answer isn't precisely known, but the most likely theory seems to be that active galactic nebulae, or AGN, powered by supermassive black holes (SBMH) at the hearts of these galaxies,  blast the cooling gas back out into the intergalactic medium.