The Large Magellanic Cloud, or LMC, is one of the Milky Way galaxy’s nearest neighbors. Located nearly early 200,000 light-years from Earth, it is a dwarf galaxy and satellite galaxy of the Milky Way.
Researchers propose that the Large Magellanic Cloud (LMC) is not orbiting the Milky Way but merely passing by it. They suggest that the LMC has just completed its closest approach, which has stripped away most of its gas halo.
For the first time, astronomers have measured the size of this halo using the Hubble Space Telescope. Their findings reveal that the LMC’s halo is unusually small—about 50,000 light-years across, roughly 10 times smaller than halos of other galaxies of similar mass.
This compactness supports the theory that the LMC’s interaction with the Milky Way has significantly impacted its halo.
Andrew Fox of AURA/STScI for the European Space Agency in Baltimore, who was the principal investigator on the observations, said, “The LMC is a survivor. Even though it’s lost a lot of its gas, it’s got enough left to keep forming new stars. So, new star-forming regions can still be created. A smaller galaxy wouldn’t have lasted—there would be no gas left, just a collection of aging red stars.”
Hubble Explores the Formation and Evolution of Star Clusters in the Large Magellanic Cloud
Though damaged, the Large Magellanic Cloud (LMC) still holds onto a slight halo of gas, which it couldn’t retain if it were less massive. At 10 percent the mass of the Milky Way, the LMC is more massive than most dwarf galaxies. Despite the Milky Way’s massive halo truncating the LMC’s gas due to gravitational interactions, the LMC’s high mass allows it to retain about 10 percent of its gas halo.
Most of the LMC’s halo was stripped away by ram-pressure stripping, a process where the dense environment of the Milky Way pushes against the incoming LMC. This interaction creates a wake of gas trailing the LMC, similar to the tail of a comet.
Fox said, “I like to think of the Milky Way as this giant hairdryer, and it’s blowing gas off the LMC as it comes into us. The Milky Way is pushing back so forcefully that the ram pressure has stripped off most of the original mass of the LMC’s halo. There’s only a little bit left, and it’s this small, compact leftover that we’re seeing now.”
As the ram pressure strips away much of the LMC’s halo, the gas slows down and eventually falls into the Milky Way. However, scientists don’t expect the entire halo to be lost since the LMC has just passed its closest approach to the Milky Way and is now moving away into deep space.
To conduct this study, the research team used ultraviolet observations from the Mikulski Archive for Space Telescopes at STScI, as most ultraviolet light is blocked by Earth’s atmosphere and cannot be detected with ground-based telescopes. Hubble, the only space telescope capable of observing these wavelengths, made this study possible.
The team surveyed the LMC’s halo by analyzing the background light from 28 bright quasars powered by supermassive black holes. These quasars, located at extreme distances, act like beacons, allowing scientists to observe the halo gas through their light absorption indirectly.
The scientists used Hubble’s Cosmic Origins Spectrograph (COS) to detect the gas in the LMC’s halo by analyzing how it absorbs specific wavelengths of light from background quasars. A spectrograph breaks light into its component wavelengths, providing insights into the object’s state, temperature, speed, quantity, distance, and composition. With COS, the team measured the velocity of the halo gas, which helped them determine its size.
Due to its mass and proximity to the Milky Way, the LMC serves as a unique astrophysical laboratory. Studying the interaction between the LMC and our galaxy helps scientists understand the early universe when galaxies were closer together and illustrates galaxy interactions’ complex, chaotic nature.
The team plans to study the front side of the LMC’s halo, an area that has yet to be explored. They will probe five sightlines in the region where the LMC’s halo and the Milky Way’s halo are colliding.
Journal Reference:
- Sapna Mishra, Andrew J. Fox, Dhanesh Krishnarao et al. The Truncated Circumgalactic Medium of the Large Magellanic Cloud. DOI: 10.48550/arXiv.2410.11960
