At the center of nearly every large galaxy sits a supermassive black hole. It doesn't just devour infalling gas; it also blasts some of it back out in powerful jets. These jets act like giant pistons, shoving the surrounding gas and heating it. Astronomers have long suspected that this is how black holes keep galaxies from cooling into cold star factories. But exactly how much energy gets transferred to the gas remained a mystery.

A new study compared two sets of data. On one side are X-ray observations of hot gas in thousands of elliptical galaxies, groups, and clusters. On the other are computer models where gas is heated only by gravity, without any black hole input. The difference between the two is the black holes' contribution. The researchers found that across all systems, from modest ellipticals to giant clusters, black holes transfer only 1 to 3 percent of their total energy to the gas. That's not much, but it's enough to keep the gas hot and diffuse.

Interestingly, in smaller systems the accumulated heat far exceeds radiative losses. The gas there stays hot even billions of years after the black hole stopped actively feeding. In massive clusters, the situation is different: heating and cooling are in balance, like a house with a perfectly tuned thermostat. The black hole acts as a heater that switches on exactly when the gas starts to cool.

This result matters not only for astronomers. It provides precise parameters for computer models of galaxy formation. Knowing that the heating efficiency is 1–3% allows better predictions of how many stars a galaxy will form over its lifetime, and thus what our universe will look like billions of years from now.