A near collision between two galaxies in the dim and distant past has left a stream of hydrogen tenuously strung between M31 and M33, crossing a distance of 782,000 light years.
The M31 (Andromeda) and M33 (Triangulum) galaxies are part of the Local Group of galaxies along with the Milky Way. In 2004 a Dutch study by the WesterborkSynthesis Radio Telescope strongly indicated that the bridge existed but could not confirm it as the resolution of the array was not good enough to give a categorical answer one way or the other.
Earlier this year, the National Radio Astronomy Observatory took a fresh look at the part of space that contains the bridge, confirming the earlier Dutch study. The bridge is a strong indication that the two galaxies passed close to each other a long time ago, each pulling hydrogen out of the other due to their combined gravity.
The NRAO data was of a high enough resolution to distinguish six regions of higher density in the hydrogen bridge. Measurements of the velocity of these regions have revealed that the hydrogen bridge is moving at the same speed as the two galaxies, lending further weight to theory that the bridge is connecting the pair together.
“We think it’s very likely that the hydrogen gas we see between M31 and M33 is the remnant of a tidal tail that originated during a close encounter, probably billions of years ago,” Spencer Wolfe, of West Virginia University in Morgantown, said in a statement. “The encounter had to be long ago, because neither galaxy shows evidence of disruption today.”
Jay Lockman, one of the paper’s principal authors had this to say: “The gas we studied is very tenuous and its radio emission is extremely faint — so faint that it is beyond the reach of most radio telescopes,” Lockman said. “We plan to use the advanced capabilities of the GBT to continue this work and learn more about both the gas and, hopefully, the orbital histories of the two galaxies.”