Free Astronomy Magazine January-February 2025

25 JANUARY-FEBRUARY 2025 ASTRO PUBLISHING This passage has blown away most of the spherical halo of gas that sur- rounds the LMC. Now, for the first time, astronomers have been able to measure the size of the LMC’s halo — something they could do only with Hubble. In a new study published in the Astrophysical Journal Letters, researchers were surprised to find that it is so ex- tremely small — about 50,000 light- years across. That’s around 10 times smaller than the halos of other galaxies that are the same mass as the LMC. Its compactness tells the story of its encounter with the Milky Way. “The LMC is a survivor,” said An- drew Fox of AURA/STScI for the Eu- ropean Space Agency in Baltimore, who was principal investigator on the observations. “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 would- n’t have lasted — there would be no gas left, just a collection of aging red stars.” Though quite a bit the worse for wear, the LMC still retains a com- pact, stubby halo of gas — some- thing that it wouldn’t have been able to hold onto gravitationally had it been less massive. The LMC is 10 percent the mass of the Milky Way. “Because of the Milky Way’s own giant halo, the LMC’s gas is getting truncated, or quenched,” explained STScI’s Sapna Mishra, the lead au- thor of the paper chronicling this discovery. “But even with this cata- strophic interaction with the Milky Way, the LMC is able to retain 10 percent of its halo because of its high mass.” Most of the LMC’s halo was blown away by a phenomenon called ram- pressure stripping. The dense envi- ronment of the Milky Way pushes back against the incoming LMC and creates a wake of gas trailing the dwarf galaxy — like the tail of a comet. “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,” said Fox. “The Milky Way is pushing back so forcefully that the ram pres- T his artist’s concept shows the Large Magellanic Cloud, or LMC, in the fore- ground as it passes through the gaseous halo of the much more massive Milky Way galaxy. The encounter has blown away most of the spherical halo of gas that surrounds the LMC, as illustrated by the trailing gas stream reminiscent of a comet’s tail. Still, a compact halo remains, and scientists do not expect this residual halo to be lost. The team surveyed the halo by using the background light of 28 quasars, an exceptionally bright type of active galactic nucleus that shines across the Universe like a lighthouse beacon. Their light allows scientists to ‘see’ the in- tervening halo gas indirectly through the absorption of the background light. The lines represent the Hubble Space Telescope’s view from its orbit around Earth to the distant quasars through the LMC’s gas. [NASA, ESA, R. Crawford (STScI)]