Older galaxies found to be more chaotic

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We have always been unsure as to what causes chaotic movements in stars that appear random, as when galaxies are born stars usually maintain a consistent and tidy rotational pattern except for some anomalies. There have been two long-standing explanations for these anomalous behaviours: either that they have been caused by the surrounding environment — for example, from galactic tidal forces stretching and forming the stars’ orbit, or the inherent mass of the galaxy itself — since if the star is closer to the centre of the galaxy, it will experience considerably stronger gravitational forces than further outside.

But a recent study, published in the monthly notices of the Royal Astronomical Society, may have rendered both of these thoughts moot and provided ample evidence that the tendency of the stars to have random motion is driven mostly by the age of the galaxy — as the galaxy ages, the motion of such stars become more and more unpredictable.

The first author, Prof. Scott Croom (University of Sydney), provides further analyses into this notion, suggesting that there is strong suggestive evidence that age plays the greatest factor in star movements. There is simply no environmental trend, but there is such for age. “If you find a young galaxy it will be rotating, whatever environment it is in, and if you find an old galaxy, it will have more random orbits, whether it’s in a dense environment or a void.”

As the galaxy ages, the motion of such stars become more and more unpredictable

It is, however, quite important not to negate the previous research entirely as ‘wrong’ or ‘outdated’. Second author Dr Jesse van de Sande (University of Sydney), suggests that age is still affected by environment in some capacity. Galaxies which fall into a dense environment will tend to shut down the star formation. And because of this, galaxies in denser environments are, on average, older. But the point of their analysis is that it is not living in dense environments that reduces their spin, but the very fact that they are older.

Let us provide an example of our own galaxy — the Milky Way. As it still has a thin star-forming disk, we still regard it as a high-spin rotational galaxy. But upon further observation, we notice that it too, like two thirds of all disk galaxies, has its own thick disk. We just don’t notice it so much due to the lack of light dominating the disk, but nevertheless older stars are prevalent on the thick disk, which “may well have been heated by the thin disk at earlier times, or born with more turbulent motion in the early Universe”, according to Prof. Croom.

What Prof. Croom, and this wider study, may have overlooked though is the extent to which turbulent motion impacted the specific properties of stars in the thick disk. Sure, turbulence had its effects in the early universe, but Croom has now potentially oversimplified the complex processes involved in galaxy formation. This may affect the validity of his claim that the thin disk played a role in the formation of thick disk stars. Whilst it may have played a role, there are plenty of other possibilities: mergers with satellite galaxies, radial migration, or dynamical heating from spiral arms or galactic bars, all could also play significant roles in shaping the properties of stars in the thick disk.

Galaxies which fall into a dense environment will tend to shut down the star formation

The research used data from observations made under the SAMI Galaxy Survey. The SAMI instrument was built in 2012 by the University of Sydney and the Anglo-Australian Observatory (now Astralis). SAMI uses the Anglo-Australian Telescope, at Siding Spring Observatory, near Coonabarabran, New South Wales. It has surveyed over 3,000 galaxies across a broad range of environments.

The study allows astronomers to rule out many processes when trying to understand galaxy formation and so enables them to fine-tune models of how the Universe has developed. The next steps will be to develop simulations of galaxy evolution with more granular detail. “One of the challenges of getting simulations right is the high resolution you need to predict what’s going on. Typical current simulations are based on particles which have the mass of maybe 100,000 stars and you can’t resolve small-scale structures in galaxy disks.”

Professor Croom’s research group could potentially tackle these issues through the usage of refinement techniques such as adaptive mesh refinement or perhaps particle splitting techniques, both of which can enhance resolution in regions of interest within the simulation. These techniques dynamically adjust the resolution based on the local density of matter, allowing for higher resolution in areas where it is most needed, such as galaxy disks or regions undergoing significant gravitational interactions.

Image: Stéphane Guisard via Wikimedia Commons

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