Galaxies are a fundamental part of the 13.7 billion year old universe. Understanding how a system that is as complex and distinctive as our own Milky Way after the Big Bang is one of the great themes of modern astronomy.
Our research, published this week in Nature Astronomy, has found a surprising connection between the time of a galaxy and its three-dimensional shape.
As galaxies grow older, they become rounder and fall prey to the mid-range range that catches many of us humans around the world.
We know it for a long time, in the form and age in very extreme galaxies are connected – so very flat and very round galaxies. But this is the first time we have shown that this applies to all types of galaxies ̵
Revealing the true face of a galaxy
In this study, we calculated the age and shape of galaxies with different techniques.
Allocating an age to a galaxy is difficult. They do not have a single date of birth when they suddenly appeared.
We measured the average age of stars in a galaxy as a measure of the age of the galaxy. Young galaxies have a large proportion of recently formed hot blue stars, while old galaxies usually contain colder red stars that originated shortly after the Big Bang.
Spectroscopy – which splits 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 accuracy than simply using blue or red images as it normally happens.
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 in terms of a perfect sphere. A zero ellipticity means that a galaxy is a perfect ball like a football. But as the measured ellipticity increases from zero to one, the galaxy is more and more crushed – from a roundish pumpkin shape to a thin slice like a pancake.
We see galaxies as two-dimensional images projected onto the sky, not telling us what they really look like in three dimensions. If we can measure how the stars move in a galaxy, we can conclude their true three-dimensional shape.
Spectroscopy makes this possible through the Doppler effect. We can measure shifts in the wavelength of stars, which depend on whether these stars are moving toward or away from us to measure their movements.
We did this with SAMI, the multi-object integral field spectrograph of the Sydney Australian Astronomical Observatory, on the 3.9 meter long Anglo-Australian Telescope at the Siding Spring Observatory. The SAMI instrument provides 13 optical fiber units that can "dissect" galaxies using spectroscopy and deliver unique 3D data.
In recent years, the SAMI Galaxy Survey team has performed 3D measurements for more than a thousand galaxies of all types and with a hundredfold mass range.
Changing the shape of galaxies
So what do we learn about the processes that shape galaxies out of this result?
Galaxies tend to form their stars on a pancake-like slice with high ellipticity. But these stars do not stay in this thin slice as the galaxy ages.
There are many different gentle events, called secular processes, that cause the disc to inflate and become rounder and less squeezed. A galaxy can be shot at 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 a galaxy ages, its initial thin disk of stars begins to thicken – the spread in middle age – and the galaxy gets older, rounder and less squeezed.
In some cases, a galaxy can experience more extreme events that radically change its shape. Elliptical galaxies like M87 are the oldest and most rounded galaxies in the universe.
Astronomers believe that these galaxies arise in large mergers-dramatic collisions between galaxies that cause one massive galaxy to be completely consumed by another galaxy.
Since these events are so important, they scatter all stars out of the disk of a galaxy, resulting in a much rounder shape. They also prevent the formation of new stars after the merger and the aging of the galaxy. The end result is an old, very round galaxy.
Closer to home
If we look at our own Milky Way, which is more than 10 billion years old, we can see examples of this story.
The most recent part of the Milky Way, where stars are still formed, is the thin slice, which has a heavily pinched, pancake-like shape. The Milky Way also contains rounder and older components, a thick disk and a bulge, but their origin is still largely unknown.
We know that the Milky Way will eventually merge with our galactic neighbor, the Andromeda Galaxy. Predictions are that this will lead to a very round, very old giant elliptical galaxy.
So, when we study the processes that form other nearby galaxies, we can learn a lot about the past and our own destiny.
This article was originally published in The Conversation. Read the original article.