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Virtual "Universe Machine" illuminates galaxy evolution



  Virtual
A UA-led team of scientists created millions of different universes on a supercomputer, each of which followed different physical theories on how galaxies should form. Picture credits: NASA, ESA and J. Lotz and the HFF team / STScI

How do galaxies evolve like our Milky Way? How do they grow and change over time? The science behind the formation of galaxies has been a mystery for decades, but a team of scientists led by the University of Arizona has come a step closer to finding answers thanks to supercomputer simulations.

The observation of real galaxies in space can only provide snapshots in time. Therefore, researchers who want to study how galaxies evolve over billions of years need to resort to computer simulations. Traditionally, astronomers have used this approach to sequentially invent and test new theories of galaxy formation. Peter Behroozi, an Assistant Professor at the UA Steward Observatory, and his team overcame this hurdle by creating millions of different universes on a supercomputer, each of which followed a variety of physical theories on how galaxies should form.

Monthly communications from the Royal Astronomical Society question fundamental ideas about the role of dark matter in galaxy formation, how galaxies evolve over time, and how they produce stars.

"At the computer, we can create many different universes and compare them to the actual ones, and so we can derive what rules lead to the ones we see," said Behroozi, the lead author of the study are so exact replicas of the Reality: Computer simulations, each representing a sizeable portion of the actual cosmos, containing 1

2 million galaxies and spanning the time from 400 million years after the Big Bang to the present.

Every "ex-machina" universe has undergone a series of tests to assess how similar galaxies occurred in the created universe compared to the true universe. The universes most similar to our own are based on similar physical rules that suggest a new approach to the study of galaxy formation.

The results of "UniverseMachine," as the authors call their approach, have helped solve the long-standing paradox of why galaxies do not make new stars, even though they contain a lot of hydrogen gas, the raw material from which stars are forged.

Common ideas about how galaxies form stars include a complex interaction of cold gas that collapses under the influence of gravity in dense pockets forming stars, while other processes counteract star formation.

For example, it is thought that most galaxies harbor supermassive black holes in their centers. Matter falling into these black holes radiates tremendous energies acting as cosmic blowlamps that prevent the gas from cooling enough to collapse into starry kindergartens. Likewise, stars that give their lives in supernova explosions contribute to this process. Dark matter also plays a major role as it applies most of the gravitational force to the visible matter in a galaxy, sucking in cold gas from around the galaxy and heating it up.

"Like us When we go back in the universe earlier and earlier, we would expect the dark matter to be denser and the gas therefore getting hotter, which is bad for star formation, so we thought many galaxies would be in the early The universe should become denser long ago stopped forming stars, "said Behroozi. "However, we found the opposite: galaxies of a certain size, contrary to expectations, tend to form stars at a higher speed."

In order to match the observations of actual galaxies, his team had to create virtual galaxies in which the opposite was true – universes where galaxies made stars much longer.

On the other hand, when the researchers created universes based on current theories of galaxy formation – universes in which the galaxies ceased to form stars early – these galaxies appeared much redder than the galaxies we see in the sky.

Galaxies appear red for two reasons. The first is obvious in nature and has to do with the age of a galaxy – if it formed earlier in the history of the universe, it will move faster and shift the light into the red spectrum. Astronomers call this effect redshift. The other reason is intrinsic: – If a galaxy has stopped forming stars, it contains fewer blue stars, which usually die out earlier, and leave older, redder stars.

"But we do not see that." Said Behroozi. "If galaxies behave as we thought and no longer form stars, our actual universe would be completely wrong colored, in other words, we are forced to conclude that galaxies in the early days make stars more efficient than we thought does that tell us? " is that the energy generated by supermassive black holes and exploding stars less effectively suppresses star formation than we predicted. "

According to Behroozi, the creation of pseudo-universes of unprecedented complexity required a completely new approach that was not limited by computational power and memory and enough resolution to scale from" small "- single objects like supernovae – to a considerable extent

"The simulation of a single galaxy requires 10 to 48 arithmetic operations." Explained, "All computers on Earth together could not do this in a hundred years. So to simulate only one galaxy, let alone 12 million, we had to do it differently. "

In addition to using computer resources at the NASA Ames Research Center and the Leibniz Computing Center in Garching, the team deployed the supercomputer" Ocelote " Cluster UA High Performance Computing in. Two thousand processors shredded the data simultaneously over three weeks, and Behroozi and his colleagues generated more than 8 million universes during the research project.

"We have used the last 20 years of astronomical observations "Compared with the millions of pseudo-universes that we have created," explained Behroozi, "We've assembled thousands of pieces of information to determine which ones are the same. Did the universe we created look right? If not, we would go back and make changes and re-examine. "

To better understand the formation of galaxies Behroozi and his colleagues plan to expand the UniverseMachine with the morphology of individual galaxies and the evolution of their shapes over time.


Hubble watches a tiny galaxy with a big heart


Further information:
Peter Behroozi et al., UniverseMachine: The Correlation Between Galaxy Growth and Halo-Assembly of Dark Matter of z = 0-10, Monthly Notices of the Royal Astronomical Society (2019). DOI: 10.1093 / mnras / stz1182

Provided by
University of Arizona




Quote :
Virtual "Universe Machine" provides information on the evolution of the galaxy (2019, 10 August)
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