Slow Galaxies Form Stars

They may look like they are standing still, but galaxies are all spinning.  Spiral galaxies have the lovely regular spin of a disk, while elliptical galaxies are all over the place, a buzzing hive of stars.  We don’t see this rotation in real time because it takes millions of years for it to be noticeable.  The Milky Way takes 250 Million years to spin just once around it’s axis.  Looking at this rotation rate vs. distance from the galactic center was what originally led to the discovery of dark matter. Some galaxies do in fact spin slower than others, but how does this affect their properties?

Regular spiral galaxies, such as the ‘whirlpool galaxy’ on the left, form far fewer stars than the clumpy galaxy on the right. The blue regions have the least star-forming gas and red-yellow regions have the most. Credit: Dr Danail Obreschkow, ICRAR. Image uses data from the Hubble Space Telescope.

Billions of years ago, rapid star formation was common among galaxies, which were young and full of the gas and dust needed to forms stars.  Today, most spiral galaxies, including our own Milky Way, form stars at a much slower rate.  The exception to the rule is so-called ‘clumpy’ galaxies, which form stars about 50 times as quickly as the Milky Way, even in the local universe.

Looking into the distant universe, and subsequently back in time, there were a lot more of these clumpy galaxies present, which slowly evolved into the stable spiral galaxies we see today.  But if that is the case, why do we still see the clumpy galaxies? Shouldn’t they have evolved by now?

The answer is that the clumpy galaxies are spinning three times as slow as modern spiral galaxies.  Whatever is happening to keep the rapid star formation going, it’s related to the spin of these strange galaxies.

Understanding galactic evolution is a fundamental part of understanding the evolution of the universe itself, and helps us to understand the eventual formation of the Sun and Earth.  Each step forward in our understanding reveals a little bit more of the entire picture of the cosmos.



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