The cosmic mystery of the Milky Way galaxies has been solved

One of the new high-resolution simulations of the dark matter enveloping the Milky Way and its neighbor, the Andromeda galaxy. The new study shows that previous failed attempts to find satellite-level analogues that surround the Milky Way in dark matter simulations were due to a lack of accuracy. Credit: Till Sawala / Sibelius collaboration

Astronomers say they have solved a landmark problem that has challenged our understanding of how the universe evolved – the spatial distribution of faint satellite galaxies orbiting the Milky Way.

These satellite galaxies display a strange alignment – they appear to be lying on a very thin rotating plane – called the “satellite plane”.

This seemingly improbable arrangement has baffled astronomers for more than 50 years, leading many to question the validity of the Standard Cosmological Model, which seeks to explain how the universe came to be as it is today.

Now, new research led by the universities of Durham, UK, and Helsinki, Finland, has found that the plane of satellites is a cosmic anomaly that will decay over time in much the same way that constellations change, too.

Their research removes the challenge that the satellite plane poses to the Standard Model of cosmology.

This model explains the formation of the universe and how the galaxies we see now gradually formed inside clumps of cold dark matter – a mysterious substance that makes up about 27% of the universe.

The results have been published in the journal natural astronomy.

The moons of the Milky Way seem to be arranged in an implausibly thin plane that cuts through the galaxy, and, oddly enough, they also rotate in a long-lived coherent disk.

There is no known physical mechanism for creating satellite aircraft. Instead, it was thought that satellite galaxies would have to be arranged in a roughly circular shape to track dark matter.

Since the discovery of the satellite plane in the 1970s, astronomers have tried unsuccessfully to find similar structures in realistic supercomputer simulations tracing the evolution of the universe from the Big Bang to the present day.

The fact that the arrangement of the satellites cannot be explained has led the researchers to believe that the cold dark matter theory of galaxy formation may be wrong.

However, this latest research saw astronomers use new data from the European Space Agency’s Gaia space observatory. Gaia maps the Milky Way in six dimensions, providing precise positions and motion measurements for about a billion stars in our galaxy (about 1% of the total) and their companion systems.

The cosmic mystery of the Milky Way galaxies has been solved

The positions and orbits of 11 classic satellite galaxies of the Milky Way, projected “facing” (top) and “edge-on” (bottom), integrated for a billion years in the past and into the future. The right panels are an enlargement of the left panels. The black dot indicates the center of the Milky Way, and the arrows indicate the observed locations and transmission directions of the satellites. While they currently line up in a plane (indicated by the gray horizontal line), that plane quickly decays as the satellites move along their orbits. Credit: Till Sawala / Sibelius collaboration

This data allowed scientists to project the orbits of past and future satellite galaxies and see the plane form and melt in a few hundred million years – just a blink of an eye in cosmic time.

The researchers also looked for new, tailor-made cosmological simulations for evidence of satellite aircraft.

They realized that previous studies based on simulations had been misled by not taking into account the satellites’ distances from the galactic center, which made the virtual satellite systems appear more round than the real ones.

With that in mind, they found several hypothetical Milky Ways that feature a plane of satellite galaxies very similar to those seen through telescopes.

The researchers say this removes one of the main objections to the validity of the Standard Model of cosmology and means that the concept of dark matter remains a cornerstone of our understanding of the universe.

Study co-author Professor Carlos Frink, Professor of Fundamental Physics at the Ogden Institute for Computational Physics at Durham University, UK, said, “The strange alignment of Milky Way galaxies in the sky has puzzled astronomers for decades, so much so that it was considered a profound challenge to cosmological orthodoxy.” .

“But thanks to amazing data from the Gaia satellite and the laws of physics, we now know that planes are just a chance alignment, and it’s a matter of being in the right place at the right time, just like the clusters of stars in a sky.

“Go back in a billion years, and the plane will disintegrate, and so will the constellations today.

“We have removed one of the major challenges to cold dark matter theory. It still provides a remarkably honest account of the evolution of our universe.”

Study lead author Dr Teil Swala, from the University of Helsinki, said, “The satellite plane was really mind-boggling.

“Perhaps unsurprisingly, a puzzle that has stood for nearly fifty years requires a mixture of approaches to solve – and an international team to work together.”

more information:
Sola, the satellite plane of the Milky Way corresponds to ΛCDM, natural astronomy (2022). DOI: 10.1038/s41550-022-01856-z.

Provided by Durham University

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