The Gaia mission recently conducted a celestial survey of 1 billion stars in the Milky Way galaxy. They found some magnificent celestial features, such as mountains, arch bridges and rivers made of stars. (Image: ESA/Gaia/DPAC, CC by sa 3.0 IGO)
To us, the night sky may look like a random scattering of stars, but astronomers have learned that in certain regions of the Milky Way galaxy, stars gather in ways that closely resemble features on Earth – such as streams, waves, arches, and ridges.
Many of the features on Earth are caused by geological tectonic activity, but scientists are unsure what exactly causes these stellar features in the Milky Way. Now, researchers are conducting tests to find the reason behind this, and they have also taken into account forces from outside the Milky Way. However, the real cause may come from the Milky Way itself.
Scientists describe the Milky Way as a barred spiral galaxy – essentially shaped like a one-sided egg, with pinwheel-shaped stars, but on a smaller scale, the galaxy’s regional "topography" conceals more detail.
A European Space Agency mission called Gaia has been conducting a census of the Milky Way since 2013, with the aim of cataloging more than 1 billion stars. Using precise measurements of the positions and motions of an additional 550 million stars released in April 2018, astronomers can now explore the Milky Way in new dimensions.
Although these galaxy exploration missions have discovered new regions of galaxies, such as those ridges and arch bridges, scientists have been unable to fully explain the origin of these stellar structures. A team led by astronomers at the University of Sydney in Australia decided to try to recreate some of the features they saw in stars in computer models.
The researchers focused on a series of eight "ridges" in the Milky Way galaxy that fold on top of each other like mountains. Gaia’s data showed that these ridges are sandwiched in the middle of the galaxy’s disk, and that each ridge has some unique stars at the top. Using data from another mission analyzing the composition of stars, they noticed that all of these stars have elements similar to those of the sun. Since the composition of the elements can suggest the age of the star, this tells the researchers that these young stars are not as scattered as older stars, which helps the researchers understand the formation of these "spines".
Theories about the formation of these "spines" and other features can be divided into two categories: internal influences and external influences. Some theories suggest that the internal mechanisms of galaxies are key to the formation of these galactic forms. For example, attractive force interactions may generate resonant waves that turn smaller clumps of matter into larger clumps. In addition, friction between stars, gas, and dust in a galaxy may also cause these morphological features to develop, just as clothes in a washing machine become entangled due to friction during washing. Other theories suggest that some external features that pass through the galaxy – such as another small dwarf galaxy – also cause stars to take on folded forms. (Imagine you shuffling your feet as you walk across a carpet that also folds.)
The team used computer simulations of these internal and external effects to see if they could reproduce the shape distribution of stars under different conditions. They found that these "ridges" of stars were more similar to simulations of independent regions affected by the interior of the galaxy. The researchers called this internal influence process phase mixing, in which multiple stellar groups gradually mix over time due to changes in the spiral arms, just as rum and cola are mixed when making cocktails. In addition, the young stars in these "ridges" also proved (the young stars were scattered for a shorter time than older stars) that these stellar forms were influenced by a nearby force. Simulations of the region affected by the attractive forces of passing galaxies show "ridges" much higher than the actual "ridges" of the Milky Way.
Shourya Khanna, an astronomer at the University of Sydney and lead author of the new paper, said the height of the ridge "could potentially be a way of distinguishing between internal and external processes".
However, there are still some limitations to these simulations. The researchers have not yet included gas in the simulations, which could affect the results. The study found evidence that a nearby galaxy once passed through the Milky Way. The study suggests that such external interactions may be responsible for the "rivers" of stars, while internal galactic influences – such as phase mixing – are more likely to be responsible for the "ridges." With so many stars still to be classified, Gaia may provide astronomers with more clues about the forces shaping the galaxy’s stunning shape.
Alice Quillen, an astronomer at the University of Rochester who was not involved in the study, said: "The region of the Milky Way that we know about so far is very close to the Sun, but upcoming Gaia data should allow us to expand the size of this region."
The scientists have published their findings on the preprint platform arXiv and submitted them to the Monthly Notices of the Royal Astronomical Society for publication.