Galaxies grow because their gravity attracts fresh gas from outside. In order to understand this process astronomer, Freeke van de Voort took to the computer and simulated it. She discovered that black holes cause a significant decrease in the fresh gas that is swallowed.
Galaxies are not just a static collection of billions of stars but are continually changing. This occurs as the galaxy’s gravity attracts gas from outside. New stars develop from this matter, causing the galaxy to grow.
However, the gas that enters does not have free passage. On its way it meets what is known as a halo. This is a large collection of visible gas and dark matter that surrounds the galaxy like a mantle. Interaction with the halo is crucial for the growth of the galaxy. Van de Voort and her colleagues created 50 different simulations of this process.
A halo from Van de Voort’s model. This ball of matter with a galaxy at the centre develops in the filament-like structure of the universe (above left). Light takes more than three million years to cover an Mpc (megaparsec = 3,260,000 light years).