Is the Milky Way galaxy typical or special? Australian astronomers have tackled this question by taking a detailed cross-section of a galaxy very similar to our own – and the results suggest we’re more average than we thought.
Like many other galaxies, the Milky Way is a spiral galaxy flattened into a disk shape. It actually has two distinct disks: a thin disk, which is the main component containing dust, gas and the bulk of the stars (including our Sun), and a thick disk, which is entirely made up of stars, usually older ones.
But not all galaxies have thick disks, and astronomers are still trying to understand how they form.
One theory suggests our thick disk formed after a rare and violent accident, when a smaller galaxy smashed into ours – but new research suggests a more peaceful evolution.
“Our observations indicate that the Milky Way’s thin and thick discs didn’t come about because of a gigantic mash-up, but a sort-of ‘default’ path of galaxy formation and evolution,” says Nicholas Scott, from the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the University of Sydney.
“From these results we think galaxies with the Milky Way’s particular structures and properties could be described as the ‘normal’ ones.”
Scott and his colleague Jesse van de Sande are co-lead authors of the new paper published in The Astrophysical Journal Letters.
The team used MUSE, a powerful spectroscopic instrument on the European Southern Observatory’s Very Large Telescope (VLT) in Chile, to study the galaxy UGC 10738, 320 million light-years away. We can’t get a big-picture view of our own galaxy because we’re inside of it, so galaxies like UGC 10728 provide a good analogue.
It was found to have both a thin and thick disk, similar to the Milky Way. Other galaxies had previously been found to have thick disks, but his research looked at the metal ratios of the stars to compare how different stellar populations are distributed compared to in our own galaxy.
“They were pretty much the same as those in the Milky Way – ancient stars in the thick disc, younger stars in the thin one,” Scott explains. “We’re looking at some other galaxies to make sure, but that’s pretty strong evidence that the two galaxies evolved in the same way.”
This suggests that our own galaxy’s shape evolved naturally, without the need for rare and violent interventions – so Milky-Way-type galaxies are likely quite common.
Michael Cowley, an astronomer at the Queensland University of Technology who was not involved in the research, says that this kind of research has only been made possible with instruments such as MUSE.
“It’s only now, with recent technological improvements (e.g., the integral field spectrograph), we’ve been able to start comparing the chemical construct of distant galaxies to our own to understand if our galaxy is unique or generic in nature,” he explains.
Another independent astronomer, Brent Groves from the International Centre for Radio Astronomy Research (ICRAR), agrees: “Getting time on MUSE…is very competitive, thus to achieve that demonstrates that the astronomical community already thinks this is an interesting question.”
Groves explains that different types of stars produce different elements – for example, type II supernovae produce elements like oxygen and magnesium, while type Ia supernovae produce heavier elements like iron – so looking at the chemical signatures allows researchers to determine the types and ages of stars in different regions.
“Measuring these elements is hard, and needs very sensitive spectra, only possible with big telescopes like the VLT and on nearby galaxies,” he explains.
But while UGC 10728 and the Milky Way do appear similar in the results of this new study, there are a few apparent differences, including the fact that UGC10738 appears to be more metal-rich.
“What the authors don’t discuss is whether these differences mean something, or are just because UGC10738 is not the Milky Way – meaning, for example, it doesn’t have neighbours like the Magellanic Clouds,” Groves says. “Maybe we need [to study] more galaxies like the Milky Way, or a better understanding of the differences between UGC10738 and the Milky Way to understand this.”
Cowley agrees that this has the potential to be a powerful result – but “the sample size needs to be increased before a conclusive result can be confirmed – i.e., they need to look at more galaxies”.
Research like this can improve the accuracy of our understanding of galaxy formation, which Cowley says has implications across many questions in astronomy, including his own work into supermassive black holes.