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Massive Galaxy Formation

Researchers create Universe Machine to understand the formation and evolution of galaxies

The science behind the formation of galaxies and their evolution has remained a puzzle for decades, but the answer might be found soon with the help of simulations carried using supercomputers by a group of scientists from the University of Arizona.

Observation of galaxies can only provide their snapshots over time however understanding their evolution requires computer simulations. Astronomers have used this technique for testing different theories of the formation of galaxies. Peter Behroozi, an assistant professor at the UA Steward Observatory generated millions of universes on a supercomputer, each having different physical theories on the formation of galaxies. The paper has been published in Monthly Notices of the Royal Astronomical Society. It challenges the conventional ideas on the role of dark matter in galaxy formation and the evolution of galaxies.

Universes are created on the supercomputer and then compared to real ones which help in identifying the rules. This research managed to create self-consistent universes for the first time which are replicas of the real one and simulations which contain 12 million galaxies spanning over 400 million years.

The universes were put through several tests to understand how galaxies appeared in the simulated universe compared to the real one. The universe resembling ours had similar physical rules.

The results from “UniverseMachine” have helped to resolve as to why galaxies stop making new stars even when plenty of hydrogen gas and other raw materials are present.

The classical theories suggest the presence of supermassive black holes in the galactic centres prevent gases to cool down to form stars. Similarly, dark matter heats up the surrounding gas and prevents forming stars. However, it was found that many galaxies in the universe were more likely to form stars at higher rates which is a contradiction. The team then created virtual galaxies in which the opposite happened. The universe based on current theories which stopped star formation early on appear much redder they actually are. The galaxy appears red due to its age and moving away faster, which shifts the light into the red spectrum called “redshift”. Also if a galaxy stops forming stars, there will be lesser blue stars and old red stars will be left.

If galaxies stopped creating stars, the colour of the universe would have been entirely different, hence it can be concluded that galaxies formed stars more efficiently in the earlier than we expected and the energy from the black holes and exploding stars is less efficient in decreasing the formation of stars.

A mock universe requires huge complexity which requires an entirely new approach not limited by computing power or memory and provided enough resolution to observe both supernovae as well as a major portion of the universe. Simulating a galaxy needs 10 to the 48th computing operations. The team used the “Ocelote” supercomputer at the UA High-Performance Computing cluster. 2000 processors churned the data for three weeks and over the course of the project, the team generated 8 million universes. The team took past 20 years of observations and compared them to the millions of mock universes generated and checked for matches. They plan to expand the UniverseMachine to include the morphology of galaxies and how their shapes evolve over time.

Journal Reference: Monthly Notices of the Royal Astronomical Society