Astronomers using NASA's Hubble Space Telescope have mapped the immense halo enveloping the Andromeda Galaxy (M31), revealing it already interacts with our Milky Way's own halo.
Galaxies like Andromeda and the Milky Way are encased in vast, spherical envelopes of diffuse gas and plasma, extending thousands to millions of light-years. These halos are nearly invisible, as their sparse molecules emit little light.
To probe them, scientists use distant quasars as backlights. Their intense emissions pass through foreground halos, imprinting absorption signatures detectable from Earth.
In a recent study, researchers analyzed 43 quasars with Hubble's Cosmic Origins Spectrograph (COS), examining how Andromeda's halo—our nearest major galactic neighbor—absorbs their light. This approach unveiled the halo's detailed structure.
The halo comprises two distinct layers. The inner shell, extending roughly 500,000 light-years, is complex and dynamic, enriched with heavy elements from supernova explosions in Andromeda's disk.
The outer layer is cooler and more diffuse, stretching up to 1.3 million light-years toward the Milky Way and two million light-years in other directions—meaning Andromeda's halo is already brushing against ours.
Andromeda lies about 2.5 million light-years away, yet the galaxies are on a collision course. Data from ESA's Gaia mission, which is 3D-mapping the Milky Way, confirms the merger in roughly 4.5 billion years. Though distant, their halos are already making first contact.
Whether the Milky Way survives intact remains debated. Recent mass measurements suggest Andromeda is comparable in size—not twice or thrice as massive—potentially allowing our galaxy to endure the encounter.
