It is no surprise that the Milky Way is the most studied galaxy in the universe since we live there.
But studying just one galaxy can only tell us so much about the complex processes that create and evolve galaxies.
A crucial question that cannot be resolved without a second look 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 by other nearby galaxies, suggesting that our home is not all that special.
A sky map of the Milky Way galaxy from ESA’s Gaia satellite.
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 other spiral galaxies, which is the most common type of galaxy.
But when we look at the structure and chemistry of the Milky Way in detail, it is noticeable.
From the side (where you can’t 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 in 1983, however, when researchers using Australian telescopes discovered an ancient component of the “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 a star streak in the sky on a clear night.
The thin disk in which our sun is located is about 1,000 light years thick and about 100,000 light years in diameter and runs through the center of the thick disk in the same plane. 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.
Artist’s impression of the Milky Way.
This double disk structure with its very different star populations is very difficult to reproduce in computer simulations. For a long time, computer models with the same structure could only be created in one particular scenario, in which a medium-sized galaxy collided with ours, about nine billion years ago. Simulations suggested that this process was incredibly rare: only one in 20 galaxies that looked superficially similar to the Milky Way experienced a collision that resulted in disks of different thickness and thickness.
If this scenario were correct, galaxies like the Milky Way would be as rare as chicken teeth.
An example of spectral imaging with the MUSE instrument.
Our research aimed to test this clear prediction. We examined a handful of galaxies that are largely similar to the Milky Way with the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope of the European Southern Observatory in Chile.
Spectroscopy – the splitting of light from a galaxy into many different colors – lets us determine the chemical composition of its stars. What makes MUSE such an extremely powerful tool is that in a single observation we get 90,000 spectra that convert any location in the galaxy into a spectrum.
One particular galaxy – UGC10738, which is roughly 320 million light years away – stood out for its lateral orientation, which allowed us to separate the thin and thick disk stars and then compare them.
We found that the chemical makeup of the stars in UGC 10738 is very similar to that in the Milky Way. We found metal-rich, magnesium-poor stars concentrated in a thin disk along the center of the galaxy, with a specific group of metal-poor, magnesium-rich stars above and below in the thick disk region.
This distant galaxy is remarkably similar to our own. Which in turn means that the Milky Way is probably nothing special.
The Milky Way over the Very Large Telescope at ESO’s Paranal Observatory.
A. Russell / ESO
Our discovery has several implications. First, it suggests that the disk features in the Milky Way are the result of a standard formation path that all galaxies follow. This is underpinned 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, and since it’s a little speculative here, the Milky Way is the only galaxy we know to contain life. New research has shown that galactic-scale events may have played a crucial role in the formation of our solar system. The recent explosion in exoplanet discoveries has shown that systems like this one are common across the galaxy, suggesting that life could find many possible homes in them.
Read more: From pancakes to soccer balls: New study shows how galaxies change their shape with age
Now we know that the Milky Way’s history was likely similar to that of many billions of other galaxies, and it seems increasingly likely that they too could make good homes for life.
Whatever future research teaches us, the Milky Way will ultimately remain our home. And that’s what makes it special – even if our research shows that it is not special at all in another sense.
Comments are closed.