Recent analysis of past observations made by the orbiting Hubble Space Telescope have found a massive halo of hot, heavy gas surrounding our neighboring galaxy, Andromeda. This expansive mass of material around Andromeda could mean that it will begin merging with the Milky Way ahead of schedule.
Previously, we knew that the two galaxies were heading for a fender bender in about 4 billions years. Ultimately, this event is the culmination of a collision course of intergalactic proportions (literally), and it’s an event that no one is going to be able to put the brakes on. But the million light-year halo discovered around Andromeda means wisps of that galaxy will be entering the Milky Way ahead of schedule. Indeed, it raises the possibility that the two galaxies may already be in contact.
This last scenario may be a reality if the Milky Way also possesses a similar halo. If that’s the case, the currently estimated 2.5 million light-year gap between the galaxies could already have been covered by the respective galactic halos, which could even now be mingling at the edges.
The data on Andromeda was unearthed as scientists examined observations that were conducted of quasars over the course of the the last several years. Upon inspection, the researchers found a dip in the brightness of quasars where the light had to pass through the region that (we now understand) is covered by the galactic halo. Quasars along a line of sight outside that span exhibited no such dip in brightness.
By analyzing the range of the ultraviolet light that is absorbed by the halo, the researchers were able to determine that the gases composing the halo are heavier than hydrogen and helium. That means that the only likely source is a supernova, or several, which explode from with in the main disk of the galaxy and expel that material far beyond the central body of the galaxy.
Galactic halos have been observed previously, but Andromeda’s appears larger and more massive than any other detected so close to the Milky Way. In fact, Andromeda’s halo contains perhaps half of all mass generated by supernovae exploding within the galaxy since its formation. Unsurprisingly, it is around 1000 times more massive than previously estimated.
The scale suits Andromeda (also known as M31, for it’s Messier number): The galaxy is already the most massive in the Local Group of galaxies. Housing around one trillion stars in the last galactic census, Andromeda has more than double the number of stars that are in the Milky Way.
But with our impending merger accelerated, soon (well, within a few million years, anyway) we’ll be able to claim the glory of belonging to the largest Local Group galaxy along with any hypothetical Andromedaites out there.