Black holes hinder development of galaxies
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).
- Challenge of modelling
Modelling a galaxy with a halo tends to be a challenge for scientists. This is because it is easy for too much matter to enter the digital galaxies. This causes them to grow at such a rapid tempo that the model no longer conforms to what we see. In reality, that is also an outwards stream of matter. This is caused by exploding stars and the strong radiation from around black holes. This force, also known as feedback, must therefore be included in the model, which is something astronomers have been doing for years.
The mass distribution (red) of a halo that results from a calculation. The whole halo can be seen on the left and the central galaxy is visible on the right.
- Lighter galaxies due to feedback
‘But what they took little account of up to now was that this force does not just fling matter out but that it also greatly reduces the matter entering,’ says Van de Voort. ‘This seems to cause galaxies to grow relatively less quickly than the haloes themselves. In our models that include feedback the galaxies were up to no less than ten times lighter than in the models that did not include feedback.’
- Galaxies form more slowly
Alongside the idea of lighter galaxies, Van de Voort came up with an explanation for the fact that the speed at which stars form in the universe began to decrease rapidly three billion years after the big bang. It is because the feedback means that only gas travelling at a high speed can reach the galaxies. If its speed is too low, the feedback prevents the gas from becoming part of the galaxy.
- Does it really work?
Such a model is all well and good, but is this really how it works in real life? ‘We can definitely see indications of this in our observations,’ says Van de Voort. ‘In absorption spectra of areas surrounding galaxies we mainly find cold gas, as the model predicts. But the best thing would be if we could also see this process directly, by broadcasting radiation. That has not yet been achieved but with, for example, an improvement of the Very Large Telescope in Chile it should be perfectly possible within a matter of years.’
PhD ceremony 28 March, Freeke van de Voort
The growth of galaxies and their gaseous haloes