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October 25, 2018
October 25, 2018, University of Michigan
This image shows an overview of the entire small magellanic cloud and was composed from two images taken from the Digitized Sky Survey 2, which digitized photographic surveys of the night sky. Credit: Davide De Martin (ESA / Hubble)
If you stand in the southern hemisphere on a clear night, you can see two bright clouds shifted from the milky way.
These star clouds are satellite galaxies of the Milky Way, called the Little Magellanic Cloud and the Great Magellanic Cloud, or SMC and LMC.
University of Michigan astronomers discovered that the southeast region, or "wing," of the small magellanic cloud was moving away from the main body of this dwarf galaxy and was providing the first information unambiguously. evidence that small and large clouds of Magellan have recently collided.
"This is really one of our exciting results," said Sally Oey, a professor of astronomy at U-M University, lead author of the study. "You can actually see that the wing is its own distinct region that stretches away from the rest of the SMC."
Their results are published in The Astrophysical Journal Letters.
In collaboration with an international team, Oey and undergraduate researcher Johnny Dorigo Jones were looking at the MSC looking for "runaway" stars or ejected clusters within the MSC. To observe this galaxy, they used a recent publication of data from Gaia, a new orbital telescope launched by the European Space Agency.
Gaia is designed to reproduce the image of the stars again and again for several years in order to trace their motion in real time. In this way, scientists can measure the movement of stars in the sky.
"We've been looking at very massive, hot young stars – the hottest and brightest stars, which are pretty rare," said Oey. "The beauty of small magellanic clouds and great magellanic clouds lies in the fact that they are their own galaxies, so we are looking at all the massive stars of a single galaxy."
The arrows indicate the relative speed and directions of movement in the plane of the sky for the 315 stars targeted in the small cloud of Magellan. The red and blue colors indicate the relative speed in the line of sight, the red and the blue corresponding to the movement away from the Earth respectively. To the left of the dotted line is the "Wing" region, showing a massive movement away from the rest of the galaxy. In this picture, the north is at the top and the left is at the left. Credit: Johnny Dorigo Jones
The examination of stars in a single galaxy helps astronomers in two ways: first, it provides a statistically complete sample of stars in a parent galaxy. Second, it gives astronomers a uniform distance from all stars, which helps them to measure their individual speeds.
"It's really interesting that Gaia got the proper motions of these stars. These motions contain everything we look at, "said Dorigo Jones. "For example, if we observe someone walking in the cabin of a plane in flight, the movement we see contains that of the aircraft, as well as the much slower motion of the person walking.
"So we removed the mass movement of the entire MSC to learn more about the speed of each star. The speed of individual stars is of interest to us because we try to understand the physical processes taking place in the cloud. "
Oey and Dorigo Jones study the fleeing stars to determine how they were ejected from these clusters. In a mechanism, called a binary supernova scenario, a star in a gravitational link, the binary pair explodes like a supernova, ejecting the other star like a slingshot. This mechanism produces binary stars emitting X-rays.
Another mechanism is that a group of gravitationally unstable stars eventually ejects one or two stars from the group. This is called the dynamic ejection scenario, which produces normal binary stars. The researchers found a significant number of fugitive stars among X-ray binaries and normal binaries, indicating that both mechanisms are important for ejecting stars from clusters.
In reviewing these data, the team also observed that all of the squadrons – this southeastern part of the MSC – were moving in the same direction and at the same speed. This shows that the SMC and the LMC probably had a collision a few hundred million years ago.
Gurtina Besla, research associate, astronomer at the University of Arizona, modeled the collision between MSC and LMC. She and her team predicted a few years ago that a direct collision would move the SMC wing region to the CML, whereas if the two galaxies simply crossed each other, the stars of the plane would be moving. wing would move in a perpendicular direction. Instead, the wing moves away from the MSC towards the CML, Oey said, confirming that a direct collision had occurred.
"We want as much information as possible about these stars to better limit these ejection mechanisms," said Dorigo Jones. "Everyone loves to marvel at images of galaxies and nebulas incredibly far apart. The SMC is so close to us, however, that we can see its beauty in the night sky to the naked eye. This fact, along with Gaia's data, allows us to analyze the complex motions of stars within the MSC and even to determine the factors of its evolution. "
Explore further:
The pair of magellanic clouds could have been a trio
More information:
"Solved Kinematics of OB Stars in the Little Magellan Cloud", M. S. Oey, J. Dorigo Jones et al., 2018, forthcoming in Astrophysical Journal Letters, arxiv.org/abs/1810.06596.
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