Galaxies are a fundamental part of the 13,700 million year old universe. Understanding how a system as complex and surprising as our own Milky Way was formed after the Big Bang is one of the great themes of modern astronomy.
Our research, published this week in Nature Astronomy, has identified a surprising connection between the age of a galaxy and its three-dimensional shape.
As galaxies age, they become rounder and fall victim to the mid-life spread that traps many of us, humans here on Earth.
We have known for a long time that form and age are linked in very extreme galaxies, that is, very flat and very round. But this is the first time we have shown that this is true for all types of galaxies: all forms, all ages, all masses.
Revealing the true face of a galaxy
In this study we calculate the age and shape of galaxies using different techniques.
Assigning an age to a galaxy is complicated. They do not have a single date of birth for when they suddenly appeared.
We evaluate the average age of stars in a galaxy as a measure of the age of the galaxy. Young galaxies have a large fraction of newly formed hot blue stars, while ancient galaxies contain mostly cooler red stars formed shortly after the Big Bang.
Spectroscopy by dividing the light of a galaxy into many different colors allows us to measure the average age of stars in a galaxy. This technique provides a much higher precision than the simple use of blue or red images as is usually done.
To measure the true three-dimensional shape and ellipticity of a galaxy, you must measure how its stars move.
Ellipticity is simply a measure of how crushed a galaxy is with respect to a perfect sphere. An ellipticity of zero means that a galaxy is a perfect sphere like a soccer ball. But as the measured ellipticity increases from zero to one, the galaxy flattens itself more and more, from a rounded shape of pumpkin to a thin disk like a pancake.
We see galaxies as two-dimensional images projected in the sky, but that does not tell us how they really are in three dimensions. If we can also measure how stars move in a galaxy, we can infer their true three-dimensional shape.
Spectroscopy allows us to do this through the Doppler effect. We can measure the changes in the wavelength of the light emitted by the stars, which depends on whether those stars are moving towards us or away from us, and thus measure their movements.
We did this using SAMI, the Australian-Australian Astronomical Observatory multi-object integral field spectrograph, on the 3.9 meter Anglo-Australian telescope at the Siding Spring observatory. The SAMI instrument provides 13 fiber optic units that can "dissect" galaxies using spectroscopy, providing unique 3D data.
In recent years, the SAMI Galaxy Survey team has gathered 3D measurements for more than a thousand galaxies of all types, and with a range of one hundred times its mass.
Changing the shape of galaxies
So, what do we learn about the processes that shape galaxies from this result?
Galaxies tend to form their stars in a pancake-like disk with high ellipticity. But these stars do not remain in that thin disk as the galaxy ages.
There are many different soft events, known as secular processes, that cause the disk to swell, become rounder and less squashed. A galaxy can be bombarded by other smaller galaxies. Even if a galaxy is isolated, internal dynamic processes can cause the disk to thicken.
The net result is that, as the galaxy ages, its thin initial disk of stars begins to thicken (the spread of middle age) and the galaxy becomes older, rounder and less squashed.
In some cases, a galaxy may experience more extreme events that radically change its shape. Elliptical galaxies, such as M87, are the oldest and most rounded galaxies in the universe.
Astronomers believe that these galaxies are formed in large mergers: dramatic collisions between galaxies that cause one massive galaxy to be completely consumed by another.
Because these events are so significant, they disperse all the stars in a galaxy's disk, resulting in a much more round shape. They also prevent new stars from forming after fusion, which causes the galaxy to age rapidly. The final result is an ancient, very round galaxy.
Closer to home
If we look at our own galaxy of the Milky Way, which is more than 10 billion years old, we can see examples of this history.
The youngest part of the Milky Way, where the stars are still forming, is the thin disk, which has a shape similar to a crushed pancake. The Milky Way also contains older, more round components, a thick disk and a protuberance, but its origin remains unknown.
We know that, finally, the Milky Way will merge with our galactic neighbor, the Andromeda galaxy. The predictions are that this will result in a very round and very old giant elliptical galaxy.
So, by studying the processes that shape other nearby galaxies, we can learn a lot about the past and the destiny of ourselves.
This article was originally published in The Conversation. Read the original article