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| Andrew Wetzel's simulation shows stars in the Milky Way-like galaxy on the left and the same region's dark matter on the right. Credit: Andrew Wetzel. |
Dwarf galaxies are enigmas wrapped
in riddles. Although they are the smallest galaxies, they represent some of the
biggest mysteries about our universe. While many dwarf galaxies surround our
own Milky Way, there seem to be far too few of them compared with standard
cosmological models, which raises a lot of questions about the nature of dark
matter and its role in galaxy formation.
New theoretical modeling work from
Andrew Wetzel, who holds a joint fellowship between Carnegie and Caltech,
offers the most accurate predictions to date about the dwarf galaxies in the
Milky Way's neighborhood. Wetzel achieved this by running the
highest-resolution and most-detailed simulation ever of a galaxy like our Milky
Way. His findings, published by The Astrophysical Journal Letters, help
to resolve longstanding debates about how these dwarf galaxies formed.
One of the biggest mysteries of
dwarf galaxies has to do with dark matter, which is why scientists are so
fascinated by them.
"Dwarf galaxies are at the
nexus of dark matter science," Wetzel said.
Dark matter makes up a quarter of
our universe. It exerts a gravitational pull, but doesn't seem to interact with
regular matter—like atoms, stars, and us—in any other way. We know it exists
because of the gravitational effect it has on stars and gas and dust. This
effect is why it is key to understanding galaxy
formation. Without dark matter, galaxies could not have formed in
our universe as they did. There just isn't enough gravity to hold them together
A video explaining what dark matter is anyway, and why scientists are interested in it. Credit: John Strom and Andrew Wetzel
The role of dark matter in the
formation of dwarf galaxies has remained a mystery. The standard cosmological
model has told us that, because of dark matter, there should be many more dwarf
galaxies out there, surrounding our own Milky Way, than we have found.
Astronomers have developed a number of theories for why we haven't found more,
but none of them could account for both the paucity of dwarf galaxies and their
properties, including their mass, size, and density.
As observation techniques have
improved, more dwarf galaxies have been spotted orbiting the Milky Way. But
still not enough to align with predictions based on standard cosmological
models.
So scientists have been honing their
simulation techniques in order to bring theoretical modeling predictions and
observations into better agreement. In particular, Wetzel and his collaborators
worked on carefully modeling the complex physics of stellar evolution,
including how supernovae—the fantastic explosions that punctuate the death of
massive stars—affect their host galaxy.
With these advances,
Wetzel ran the most-detailed simulation of a galaxy like our Milky Way.
Excitingly, his model resulted in a population of dwarf galaxies that is
similar to what astronomers observe around us.
As Wetzel explained: "By
improving how we modeled the physics of stars, this new simulation offered a
clear theoretical demonstration that we can, indeed, understand the dwarf
galaxies we've observed around the Milky Way. Our results thus reconcile our
understanding of dark matter's role in the universe with observations of dwarf
galaxies in the Milky Way's neighborhood."
Despite having run the
highest-resolution simulation to date, Wetzel continues to push forward, and he
is in the process of running an even higher-resolution, more-sophisticated
simulation that will allow him to model the very faintest dwarf galaxies around
the Milky Way.
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| A video of images from the simulation |
"This
mass range gets interesting, because these 'ultra-faint' dwarf
galaxies are so faint that we do not yet have a complete
observational census of how many exist around the Milky Way. With this next
simulation, we can start to predict how many there should be for observers to
find," he added.
Provided by: Carnegie Institution for Science
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