spaceSpace and Physics

Supercomputer Simulation Of Milky Way Solves Mystery Of Missing Dwarf Galaxies


Robin Andrews

Science & Policy Writer

The Milky Way isn't the only galaxy with nearby dwarf galaxies. Pictured here is dwarf galaxy NGC 5477, which resides near the Pinwheel Galaxy. NASA/ESA

It may be our stelliferous home, but there’s a lot about the Milky Way, and its surroundings, that we don’t know. A new supercomputer simulation by Caltech has provided researchers with a few new clues – and, remarkably, it appears that it’s solved a long-standing mystery.

One of the most mysterious facets of our galaxy is the nearby, low-mass dwarf galaxies, those that contain mere hundreds of millions of stars. For comparison, the Milky Way contains up to 400 billion stellar furnaces.


Simulations replicating the distribution of mass and the mass-attracting, mysterious dark matter in our universe can account for the number of galaxies we see, but models consistently show that there should be far, far more dwarf galaxies around. This so-called “dwarf galaxy problem” has baffled astronomers for ages, but this new simulation has finally managed to reconcile it by creating a Milky Way with the correct number of nearby dwarf galaxies.

After running a network of computers in parallel for 700,000 total hours, the team realized that the explosive deaths of the most massive stars – supernovae – was the missing piece of the puzzle. Writing in Astrophysical Journal Letters, the researchers describe how the stellar wind generated from these cataclysmic blasts, reaching speeds of thousands of kilometers per second, have been stripping small galaxies of gas and even individual stars.

The simulated Milky Way and its nearby dwarf galaxies. z denotes the time from the present terms of redshift, with 0 representing the present day. caltech via YouTube

These deathly winds are so powerful that they can in some instances annihilate dwarf galaxies, fragmenting them and scattering their debris into the deep, dark cosmos. These self-destructing stars, not content at taking themselves out, have also been destroying dwarf galaxies for billions of years.


“We had thought before that perhaps our understanding of dark matter was incorrect in these simulations, but these new results show we don't have to tinker with dark matter,” lead author Andrew Wetzel, a postdoctoral fellow at Caltech, said in a statement. “When we more precisely model supernovae, we get the right answer.”

Earlier research suggested that there should be thousands of these dwarf galaxies lingering around, but astronomers can only spot around 30 of them. Thanks to the most detailed simulation of our galaxy to date, now we know why. It seems that brilliant supernovae have been cleaning up the distant reaches of space for billions of years with their spectacular demises.

This revelation comes as a huge relief to astronomers. The notoriously elusive dark matter has a quantifiable effect on the evolution of galaxies and the universe as a whole, but it has yet to be directly detected. It’s poorly understood to say the least, so “tinkering” with it in a supercomputer model would be something of a highly speculative endeavor.

Although it seems strange that supernovae weren’t taken into account before in this way, it’s worth pointing out that galaxies are incredibly complex arenas. They are enormous, iridescent realms that are still regarded by researchers today as highly mysterious.


“In a galaxy, you have 100 billion stars, all pulling on each other, not to mention other components we don't see like dark matter,” coordinating research Phil Hopkins, associate professor of theoretical astrophysics at Caltech, added.

“To simulate this, we give a supercomputer equations describing those interactions and then let it crank through those equations repeatedly and see what comes out at the end.”

There’s a lot about the universe that we are still yet to unravel, but this groundbreaking study represents another significant segment of it being decrypted by human ingenuity – another bright light illuminating the darkness around us.

Solving the mystery with supercomputing power. caltech via YouTube


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