Free Astronomy Magazine January-February 2025

26 JANUARY-FEBRUARY 2025 ASTRO PUBLISHING sure has stripped off most of the original mass of the LMC’s halo. There’s only a lit- tle bit left, and it’s this small, compact leftover that we’re seeing now.” As the ram pressure pushes away much of the LMC’s halo, the gas slows down and eventually will rain into the Milky Way. But because the LMC has just passed its closest approach to the Milky Way and is moving outward into deep space again, scien- tists do not expect the whole halo will be lost. To conduct this study, the re- search team analysed ultravi- olet observations from the Mikulski Archive for Space Telescopes at STScI. Most ul- traviolet light is blocked by Earth’s atmosphere, so it cannot be observed with ground-based telescopes. Hubble is currently the only space telescope that is tuned to detect these wavelengths of light, so this study was only possible with Hubble. The team surveyed the halo by using the background light of 28 bright quasars. The brightest type of active galactic nucleus, quasars are believed to be powered by supermassive black holes. Shining like lighthouse bea- cons, they allow scientists to ‘see’ the intervening halo gas indirectly through the absorption of the back- ground light. Quasars reside throughout the Universe at extreme distances from our galaxy. The scientists used data from Hubble’s Cosmic Origins Spectrograph (COS) to detect the presence of the halo gas by the way it absorbs certain colours of light from back- ground quasars. A spectro- graph breaks light into its component wavelengths to reveal clues to the object’s state, temperature, speed, quantity, distance, and com- position. With COS, they measured the velocity of the gas around the LMC, which allowed them to determine the size of the halo. Because of its mass and prox- imity to the Milky Way, the LMC is a unique astrophysics laboratory. Seeing the LMC’s interplay with our galaxy helps scientists understand what happened in the early Universe, when galaxies were closer together. It also shows just how messy and complicated the process of galaxy interaction is. “This is a fantastic example of the cutting-edge science still being enabled by Hub- ble’s unique capabilities,” said Professor Carole Mundell, Director of Science at the Eu- ropean Space Agency. “This result gives us precious new insights into the complex his- tory of the Milky Way and its nearby satellite galaxies.” The team will next study the front side of the LMC’s halo, an area that has not yet been explored. “In this new programme, we are going to probe five sightlines in the region where the LMC’s halo and the Milky Way’s halo are col- liding,” said co-author Scott Lucchini of the Center for Astrophysics | Harvard & Smithsonian. “This is the lo- cation where the halos are compressed, like two bal- loons pushing against each other.” T his artist’s concept illustrates the Large Magellanic Cloud’s (LMC’s) encounter with the Milky Way galaxy’s gaseous halo. In the top panel, at the middle of the right side, the LMC begins crashing through our galaxy’s much more massive halo. The bright purple bow shock represents the leading edge of the LMC’s halo, which is being compressed as the Milky Way’s halo pushes back against the incoming LMC. In the mid- dle panel, part of the halo is being stripped and blown back into a streaming tail of gas that eventually will rain into the Milky Way. The bottom panel shows the progression of this interaction, as the LMC’s comet-like tail becomes more defined. A compact LMC halo re- mains. Because the LMC is just past its closest approach to the Milky Way and is moving outward into deep space again, scientists do not expect the residual halo will be lost. [NASA, ESA, R. Crawford (STScI)] !