⭐ Why, at a certain mass, do galaxies stop forming stars?

The most active galaxies always eventually stop producing stars, but astronomers struggled to understand why this phenomenon occurs at a very specific mass. An international team has just proposed a clear explanation: the birth of a hot gas halo that cuts off the supply of stellar fuel. This breakthrough is based on one of the largest cosmological simulations ever performed.

To understand what halts galactic growth, the researchers used the Horizon Run 5 simulation. This models a vast virtual cosmic volume, tracking the evolution of dark matter, gas, stars, and black holes from the Big Bang to the present day. The team selected about 20,000 massive galaxies and analyzed their histories over billions of years.


The determining factor is the ratio between the mass of stars and the total mass of the galaxy (stars, gas, dark matter, black hole). This ratio peaks for galaxies with total masses between 10^12.4 and 10^12.7 solar masses. Below that, galaxies efficiently convert gas into stars. Above that, their activity drops by a factor of three. This decisive threshold corresponds to the formation of a hot gas halo in gravitational equilibrium.

Below this mass, the gas falling onto the galaxy cools quickly enough to fuel star formation. Beyond it, the halo becomes so dense and hot that it sustains itself by its own pressure. The gas can no longer cool and fall toward the center, cutting off the supply of fuel. The galaxy continues to attract dark matter and satellite galaxies, but the cold gas needed for new stars no longer arrives.

The study also rules out another explanation: winds produced by supernovae and active galactic nuclei. The team calculated the amount of normal matter lost to these phenomena and found fluctuations of less than 30%, far too small to explain the drop in efficiency. The decisive change therefore occurs on the side of gas inflow, not its expulsion.

Some caveats are in order: the simulations rely on simplified physical models for star formation, supernovae, and black holes. The authors tested the sensitivity of their results and the general trend holds, but the precise value of the determining mass could evolve with more realistic models.

What makes this work interesting is that it links a well-known astronomical observation to a precise physical mechanism: galaxies shut down because their hot gas halo becomes stable. Future studies of galaxy clusters and the hot intergalactic medium will help verify whether this explanation is correct.