Detailed scan of the Milky Way reveals possible “ancient” spiral arms

(ORDO NEWS) — As we learn more about space, it becomes interesting when some of the greatest discoveries take place near our home. This is expected to continue in the future, when observations of distant galaxies will be carried out along with studies of the outer solar system and our galaxy.

In this latter respect, ESA’s Gaia Observatory will continue to play a vital role. As an astrometric mission, Gaia had to determine the correct position and radial velocity of over a billion stars in order to create a three-dimensional map of the Milky Way.

Using data from Gaia’s early third data release (eDR3) and previous data from the Sloan Digital Sky Survey (SDSS), an international team of astronomers have created a new map of the Milky Way’s outer disk.

In the process, they uncovered evidence of structures in the region that include the remains of fossilized spiral arms. This discovery will shed new light on the formation and history of the Milky Way and could lead to a breakthrough in our understanding of galactic evolution.

The study was led by Chervin Laporte of the Institute of Space Sciences at the University of Barcelona (ICCUB-IEEC) and the Kavli Institute for Physics and Mathematics of the Universe at the University of Tokyo.

He was joined by Sergey E. Koposov and Vasily Belokurov, astronomers at the Royal Observatory of the University of Edinburgh (ROE) and the University of Cambridge (respectively). Their findings were described in a study published in the Monthly Reports of the Royal Astronomical Society.

Using data from the Gaia mission released in December 2020, Laporte and his team have identified coherent structures in the Milky Way’s outer disk to create a clearer map of the region.

While this map provided a clearer picture of previously known structures, it also revealed the existence of previously unknown filamentous structures. At first glance, this discovery was not so surprising, since numerical simulations have already predicted the existence of such threadlike structures.

This is due to past interactions with satellite galaxies, of which our galaxy has had many. Currently, our galaxy is surrounded by 50 satellites, such as the dwarf galaxy Sagittarius, which has periodically disturbed the structure of the Milky Way over the past 5-6 billion years. However, the sheer number of substructures observed in these filamentous structures was unexpected and left Laporte and his colleagues puzzled.

One possible reason is that they are remnants of the tidal arms of the Milky Way’s disk, which were excited by different satellite galaxies at different times in the past. For example, before the Sagittarius dwarf galaxy, the Milky Way interacted with

Gaia Sausage, the remnants of a dwarf galaxy that merged with the Milky Way about 8-11 billion years ago.

In a previous study, Laporte and colleagues showed that one of the filamentous structures in the outer disk (Anticenter Stream) contained stars that are mostly 8 billion years old or more.

This means they were too old to be from Sagittarius and must have experienced an interaction with Gaia Sausage as well. Another option is that some of these structures are not fossil spiral arms at all, but are the crest of large-scale vertical distortions in the disk of the Milky Way.

“We believe the disks are responding to satellites, which create vertical waves that travel like ripples on a pond,” Laporte said in a recent press release from the RAS.

To understand this in more detail, the team developed a special scientific program with the William Herschel telescope on the island of Las Palmas. Using optical and spectroscopic instruments, Laporte and his colleagues will try to study the properties of stellar populations in each substructure.

Their efforts and other research in the near future will help shed light on the nature and origins of these fine structures, which will improve our understanding of how galaxies evolve through interactions and mergers.

“Typically, this region of the Milky Way remains poorly understood due to dust, which greatly obscures much of the midplane of the Galaxy. While dust affects the luminosity of a star, its motion remains unchanged.

We were, of course, very pleased to see that Gaia’s motion data helped us uncover these filamentous structures! Now the main task remains – to find out what it is, how they appeared, why in such a large number and what they can tell us about the Milky Way, its formation and evolution. ”


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