HOW CAN MERGER TREES AND TECHNOLOGY BE USED TO MAP DARK MATTER?

Izzi Millar (OHS)

Dark matter is one of the greatest unsolved mysteries of the universe today. In 1980, American astronomer Vera Rubin presented evidence that shows around 83% of the matter in the universe is made up of something that cannot be seen using a telescope. She proved this by showing that the outer regions of galaxies spin a lot faster than expected given the mass that can be calculated using visible light. Dark matter is a form of matter that doesn’t interact with the electromagnetic force, which means it doesn’t absorb heat or light, so the only way astronomers can detect it is through its interaction with gravity, making the outer parts of galaxies spin much faster. Dark matter forms in rings around galaxies and galaxy clusters, called dark matter halos, impacting the way the galaxies evolve and move. Astronomers use n-body simulations to create merger trees to track halos. An n-body simulation is a computer program that simulates a large number of particles and how they interact with forces, usually gravity. In 2005 the Millennium simulation was run. This was an n-body simulation using more than 10 billion particles, showing the galaxy and dark matter distribution in a cubic region of the universe that was over 2 billion light years on each side. It took the principal supercomputer at the Max Planck Society’s Supercomputing Centre over a month to run.

A merger tree is a graph of circles, which represent halos, with the radius indicating mass. These circles are connected using dotted lines, which show the descendants and ascendants of each individual halo. They are constructed by taking information from n-body simulations, such as the Millennium simulation, to predict the masses of previous dark matter halos and the redshifts at which they merge.

An example of a merger tree There are two ways in which scientists can grow a merger tree. The first way is to start at a high red shift (a long time ago) with dark matter particles and run the n-body simulation to see how halos merge and grow, mimicking what happens in the real universe. The other is to start with the halos that we can map in the current day using the gravitational effects on the outer regions of galaxies and work backwards. When astronomers have a full merger tree they can then expand and augment that tree by selecting one of the dark matter halos in it and creating more trees for that halo. If the progenitor halos that have been mapped in the new trees match the masses of the progenitor halos in the original tree then the two trees can then be integrated into a one bigger, more detailed tree.

Because such a large fraction of the matter in the universe is dark matter the way that dark matter halos merge has a large effect on how the galaxy within it forms. A halo of a certain mass could have multiple possible merging histories. Therefore, understanding halo development is important to fully understand galaxy evolution. N-body simulations and merger trees are by far the most accurate way to track dark matter halos but are still very expensive because of the massive number of calculations needed to run them. In the future, more technological advances should lead to the possibility of more detailed simulations, further improving our knowledge of the universe’s development.

Bibliography

https://astronomy.swin.edu.au/cosmos/d/dark+halo http://coewww.rutgers.edu/www2/vizlab/node/84 https://wwwmpa.mpa-garching.mpg.de/galform/ virgo/millennium/ Benson, A.J., Cannella, C. & Cole, S. Achieving convergence in galaxy formation models by augmenting N-body merger trees. Comput. Astrophys. 3, 3 (2016). https://doi.org/10.1186/s40668-016-0016-3 Rachel S. Somerville, Tsafrir S. Kolatt, How to plant a merger tree, Monthly Notices of the Royal Astronomical Society, Volume 305, Issue 1, May 1999, Pages 1–14, https://doi.org/10.1046/j.13658711.1999.02154.x

Mitton, J. (2017). The Astronomy Book. London: Dorling Kindersley Ltd.

Image - https://doi.org/10.1186/s40668-016-0016-3