image: Velocities of massive star candidates within the SMC shown as vectors. The colors of the arrows represent the direction of motion. Relative to the LMC, located at the bottom left of the image, most red arrows show movement towards the LMC, whereas most light blue arrows show movement away from the LMC, suggesting they are being pulled apart.
Credit: Credit: Satoya Nakano
A team led by Satoya Nakano and Kengo Tachihara at Nagoya University in Japan has revealed new insights into the motion of massive stars in the Small Magellanic Cloud (SMC), a small galaxy neighboring the Milky Way. Their findings suggest that the gravitational pull of the Large Magellanic Cloud (LMC), the SMC’s larger companion, may be tearing the smaller one apart. This discovery reveals a new pattern in the motion of these stars that could transform our understanding of galaxy evolution and interactions. The results were published in The Astrophysical Journal Supplement Series.
“When we first got this result, we suspected that there might be an error in our method of analysis,” Tachihara said. “However, upon closer examination, the results are indisputable, and we were surprised.”
The SMC remains one of the closest galaxies to the Milky Way. This proximity allowed the research team to identify and track approximately 7,000 massive stars within the galaxy. These stars, which are over eight times the mass of our Sun, typically survive for only a few million years before exploding as supernovae. Their presence indicates regions rich in hydrogen gas, a crucial component of star formation.
“The stars in the SMC were moving in opposite directions on either side of the galaxy, as though they are being pulled apart,” Tachihara said. “Some of these stars are approaching the LMC, while others are moving away from it, suggesting the gravitational influence of the larger galaxy. This unexpected movement supports the hypothesis that the SMC is being disrupted by the LMC, leading to its gradual destruction.”
Another surprising finding was the absence of rotational movement among the massive stars. Unlike in our Milky Way where interstellar gas rotates along with the stars, the study revealed a distinct pattern. Typically, young massive stars move together with the interstellar gas from which they were born, as they have not yet had time to decouple from its motion. However, the massive stars in the SMC do not follow a rotational pattern, indicating that the interstellar gas itself is also not rotating.
“If the SMC is indeed not rotating, previous estimates of its mass and its interaction history with the Milky Way and LMC might need to be revised,” Nakano, a collaborator on the study who also made a video explaining the findings, explained. “This could potentially change our understanding of the history of the three-body interaction between the two Magellanic Clouds and the Milky Way.”
The study has broader implications for understanding the dynamics of interactions between neighboring galaxies, particularly in the early universe. Astronomers consider the SMC to be an ideal model for studying the universe's infancy because it shares many conditions with primordial galaxies, such as low metallicity and weak gravitational potential. Therefore, the researchers' discoveries about the SMC and LMC interacting may resemble processes that shaped galaxies billions of years ago, providing valuable insights into their evolution over cosmic time. The group’s findings could create a new understanding of these processes.
“We are unable to get a ‘bird's-eye view’ of the galaxy in which we live,” Tachihara noted. “As a result, the SMC and the LMC are the only galaxies in which we can observe the details of stellar motion. This research is important because it allows us to study the process of star formation in connection with the motion of stars throughout the galaxy.”
Journal
The Astrophysical Journal Supplement Series