It is no surprise that the Milky Way is the most studied galaxy in the universe since we live there.
However, studying just one galaxy can only tell us so much about the complex processes by which galaxies form and evolve.
A crucial question that cannot be resolved without looking further into the distance is whether the Milky Way is an ordinary galaxy, or whether it is unusual or even unique.
Our research, published today in The Astrophysical Journal Letters, suggests the former is true. Important details of the structure of our galaxy are shared with other nearby galaxies, suggesting that our home is nothing special.
At first glance, there is no reason to believe that our galaxy is remarkable anyway. Of the billions of galaxies in the observable universe, ours is not the largest, oldest, or most massive. It looks pretty similar to all of the other spiral galaxies, which are the most common type of galaxies.
However, if we take a closer look at the structure and chemistry of the Milky Way, it becomes apparent.
From the side (where it’s impossible to see the spiral arms) it looks like a pancake with a peach in the middle. Astronomers have known this for at least a century.
That simple picture changed, however, in 1983 when researchers using Australian telescopes discovered an ancient “thick disk” in the Milky Way. This faint structure is invisible to the naked eye, in contrast to the dominant thin disk (the pancake-shaped part), which is clearly visible as streaks of stars across the sky on a clear night.
The thin disk on which our sun is located is about 1,000 light years thick and about 100,000 light years in diameter. It runs in the same plane through the middle of the thick disk. The thick disk is appropriately much thicker, a few thousand light years thick, but much less densely populated with stars.
Read more: Under the Milky Way: What a new map reveals about our galaxy
An interesting new discovery is that the thick and thin disks contain very different types of stars. Stars in the thin disk tend to have a high proportion of heavy elements such as iron (“metals” in astronomical parlance) and relatively low amounts of “alpha elements” (carbon, oxygen, magnesium, silicon and a few others). Thick disk stars now have about 100 times fewer metals, but significantly more alpha elements.
This double disk structure with its very different populations of stars is very difficult to replicate in computer simulations. For a long time, computer models with the same structure could only be created in one specific scenario, in which a medium-sized galaxy collided with our own about nine billion years ago. Simulations indicated that this process was incredibly rare: only one in 20 galaxies that were superficially similar to the Milky Way suffered a collision that resulted in disks of different thickness and thickness.
If this scenario were correct, galaxies like the Milky Way should be as rare as chicken teeth.
Our research aims to test this clear prediction. We examined a handful of galaxies that are broadly similar to the Milky Way using the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope at the European Southern Observatory in Chile.
Spectroscopy – the division of light from a galaxy into many different colors – enables us to determine the chemical composition of its stars. What makes MUSE an extremely powerful tool is that we get 90,000 spectra in a single observation that convert every location in the galaxy into a spectrum.
One particular galaxy – UGC10738, which is roughly 320 million light years away – was notable for its lateral orientation, which allowed us to separate the thin and thick disk stars and then compare them.
We found that the chemical compositions of the stars in UGC 10738 are very similar to those in the Milky Way. We found metal-rich, magnesium-rich stars concentrated in a thin disk along the center of the galaxy, with a certain group of metal-poor, magnesium-rich stars above and below in the area of the thick disks.
This distant galaxy is remarkably similar to ours. Which in turn means that the Milky Way is probably nothing special after all.
A. Russell / ESO
Our discovery has several implications. First, it suggests that the disk features in the Milky Way could be the result of a standard formation path that all galaxies follow. This is aided by the identification of similar structures in non-Milky Way galaxies.
Second, the fact that our galaxy is relatively normal is extremely exciting. This implies that the Milky Way can serve as a blueprint or template for galaxy formation.
This means that our home galaxy (which is obviously the easiest for us to study) could hold the key to unlocking the cosmic history of the entire universe.
After all, the Milky Way is the only galaxy we know to contain life as it is a little speculative here. New research has shown that galactic-scale events may have played a crucial role in the formation of our solar system. The recent explosion of exoplanet discoveries has shown systems like this to be common across the galaxy, suggesting that life could find many possible homes within.
Read more: From pancakes to soccer balls, a new study shows how galaxies change shape as they age
Now we know that the Milky Way’s history was likely similar to many billions of other galaxies. It seems more and more likely that they too will create a good home for life.
Whatever future research teaches us, ultimately the Milky Way will remain our home. And that is what makes it special – although our research suggests that in another sense it is not special at all.