Earlier this year, we released a story about the Andromeda Galaxy, which is due to collide with the Milky Way 4 billion years from now. If you missed it, be sure to watch the great video illustrating our night sky in the course of that titanic collision. Astronomers at the National Radio Astronomy Observatory (NRAO) later added to the story of Andromeda’s rampage through our Local Group, over timescales inconceivably long to puny humans. They announced that Andromeda closely encountered another Local Group spiral galaxy in the past.
We know this other galaxy as M33 aka the Triangulum Galaxy. It’s tough but not impossible to spot with the eye alone under perfect sky conditions. The press announcement of Andromeda’s likely brush past M33 was accompanied by this beautiful artist’s illustration of the two galaxies, with a tenuous gas bridge between them, shown here in red.
It’s this gas bridge that’s the smoking gun in revealing Andromeda’s encounter with M33. Presumably the Andromeda galaxy pulled gas from M33 (or M33 pulled gas from Andromeda) when the encounter took place. Jay Lockman of NRAO said:
The properties of this gas indicate that these two galaxies may have passed close together in the distant past. Studying what may be a gaseous link between the two can give us a new key to understanding the evolution of both galaxies.
Andromeda, by the way, is the closest and one of the largest galaxies in our Local Group, which has about 30 member galaxies. The Milky Way and M33 are both large spiral galaxies in the Local Group, too. The Andromeda Galaxy is about 2.6 million light-years away. M33 is more like 3 million light-years away.
Astronomers in the Netherlands announced the gas “bridge” between Andromeda and M33 in 2004, but other scientists questioned the discovery on technical grounds. Now, according to the NRAO astronomers, the gas bridge has been confirmed. The astronomers don’t say exactly when Andromeda has its close encounter with M33, only that it was in the “distant past.” That’s “distant past” in human terms to you … punk.