Small Magellanic Cloud being torn apart: Nagoya University conducts detailed analysis of astrometric satellite observation data

A research group led by Graduate Student Satoya Nakano and Associate Professor Kengo Tachihara from the Graduate School of Science at Nagoya University has announced that they have revealed the distribution of massive stars in the Small Magellanic Cloud (SMC) and discovered that the galaxy is being torn apart by the neighboring Large Magellanic Cloud (LMC). The findings also indicated that the SMC itself is not rotating. This research is expected to bring new developments to the study of interactions among the SMC, LMC, and the Milky Way. The results were published in the April 10 issue of The Astrophysical Journal Supplement Series, an international academic journal.

Massive stars are those with masses more than eight times that of the Sun, with short lifespans that explode within hundreds to thousands of years after birth. Therefore, massive stars are necessarily young. Areas where massive stars exist are thought to be regions within galaxies where star formation readily occurs, such as where hydrogen gas, the raw material for stars, is present. These observations are important for understanding the evolution of stars and galaxies, but from within the Milky Way, we cannot see the entire galaxy. It has not been understood where massive stars are predominantly distributed in galaxies.

In contrast, the LMC and SMC are among the closest galaxies to the Milky Way, with masses estimated at about one-tenth that of the Milky Way. They exert gravitational influence on each other (gravitational interaction) and orbit together around the Milky Way. Being very close galaxies, they allow for easy observation of detailed star distributions and have been the subject of much research. However, research on the distribution of massive stars has been limited.

In this study, the research group used publicly available data from the astrometric satellite "Gaia," which observed the SMC in visible light, to examine the distribution of massive stars in detail. The observations precisely measured the positions, brightness, and colors of stars, and because the observations continued for five years across the entire sky, the same stars were observed multiple times, recording data related to their movement directions. Information on approximately 2 million celestial objects in the SMC was obtained. The researchers extracted massive stars by identifying those that were sufficiently bright and blue based on their colors and brightness. 7,426 stars qualified, and their distribution matched that of light from plasma created by high-temperature stars, confirming the validity of the selection. It was also found that massive stars were distributed in several clusters.

Next, the researchers visualized the movement of massive stars within the SMC by subtracting the motion of the SMC itself from the movements of these massive stars.

As a result, consistent with previous research, they observed that massive stars distributed farther from the main body of the SMC tended to move more rapidly toward the LMC. Furthermore, massive stars in the main body of the SMC moved in different directions depending on their cluster.

To clearly grasp the movement of each cluster, they divided the stars in densely distributed areas into nine groups in the main body and outer regions. They then examined the average movement of each group.

They found that groups positioned outside the main body and those within the main body included both groups approaching and moving away from the LMC. This suggests that due to gravitational interaction, the SMC is being stretched toward the LMC, and mass from the SMC is being lost in the process. Further examination of velocities in the line-of-sight direction confirmed that groups showed opposite movements in this direction as well. Moreover, from these movements, it was determined that the SMC itself does not rotate like a spiral galaxy.

Previously, the mass of the SMC was calculated assuming rotation, and based on this assumption, past interactions and movements of the SMC, LMC, and Milky Way were estimated. In light of these new findings, these estimates may need to be reconsidered. The distribution of the SMC's massive stars revealed in this study can be downloaded by anyone from the published paper.

Tachihara commented: "The situation where the SMC is disturbed internally by the tidal forces of the LMC suggests that many massive stars would be born. Previous research has also pointed this out. With the currently operating ALMA telescope, we can now understand the structure of molecular clouds with high precision. In the future, we would like to investigate under what conditions massive stars are born from molecular clouds by combining such results."

Nakano commented: "From an overview perspective, previous studies have shown that the Small Magellanic Cloud has an elongated structure along the Milky Way. It is thought to be about 210,000 light-years away from the center of the galaxy. However, the exact distances between stars inside are not known. We are currently exploring methods to determine this distance accurately."

Journal Information
Publication: The Astrophysical Journal Supplement Series
Title: Evidence of Galactic Interaction in the Small Magellanic Cloud Probed by Gaia-selected Massive Star Candidates
DOI: 10.3847/1538-4365/adb8de