Resolving the mystery of galaxy's colours
Astronomers have shed light on the mystery of why some galaxies look blue while others appear red.
Galaxies exhibit many different features, in terms of size, shape and colour. The most distinguishable separation, known since the time of Edwin Hubble, is into two families: blue star-forming spiral galaxies, and red non-star forming elliptical galaxies. How they end up in this bimodality is, to this day, a major question in galaxy formation.
In general, astronomers believe that red galaxies live in massive dark matter halos, while blue galaxies live in more moderate-mass ones. But recent observations suggest that both types of galaxies can be hosted by even the same halo mass, and blue galaxies have more stars than red ones. Why does this trend exist, and is it telling us something fundamental about the origin of the colour bimodality? Many previous galaxy formation models have failed to reproduce this observed trend. In contrast, the state-of-the-art SIMBA simulation (figure 1), led by the Institute for Astronomy’s Dr Weiguang Cui, is remarkably successful.
Encouraged by this success, Dr Cui and the team of researchers explored how the colour bimodality originates by tracking individual simulated galaxies back in time. It was revealed that this bimodality originates in differences between their host halo mass growth history, called halo assembly bias (left panel on figure 2), and its unexpected interplay with SIMBA’s unique models that quench galaxies using the energy released from black hole growth.
Differences in the way the dark matter halos – a key building block of galaxies – develop over time influence which type of galaxy will form and the number of stars they contain. The team found that halos with the same mass that formed earlier tend to contain blue galaxies while those that formed later usually contain red ones. The team also found that the way energy is released from black holes – which are found at the centre of every galaxy – is crucial. How this happens affects whether a galaxy will be blue or red, researchers say. The findings show that halos formed later will result in their galaxies with less gas, which causes black holes to emit huge amounts of energy that stops the formation of new stars. By contrast, halos that formed earlier contain more gas, which causes black holes to release less energy and does not prevent new stars forming.
SIMBA thus provides a significant step forward in understanding the physical origin of galaxy bimodality, by connecting large-scale structure formation with small-scale processes of black hole growth and feedback.
Prof John A. Peacock said, “We are working with one of the most sophisticated numerical models of galaxy formation, and it’s really encouraging to see that the new details in the simulation produce galaxies that look more and more like those in the real universe”.
Prof Romeel Dave added, "Such increasingly realistic simulations give us confidence that we are moving towards understanding how the billions of galaxies we see in a variety of shapes, sizes, and colours formed out of the primordial soup of the Big Bang."
The research, published in the journal Nature Astronomy, was funded by the European Research Council, Royal Society and Science and Technology Facilities Council.