When Galaxies Collide Size Matters
Published:
Many moons ago, Prof. Geraint Lewis and I wrote a piece on the Conversation. This was when our work had turned up an interesting result on measuring the mass of the Andromeda galaxy, which at a distance of only two million light years is our cosmic next-door neighbour.
The Andromeda and the Milky Way galaxies are very similar, giant spirals containing hundreds of billions of individual stars. But astronomers have struggled to work out which of these two galaxies is the most massive.
The answer to this question is vitally important if we are to understand the dynamic history of all nearby galaxies, both large and small, as the gravitational field of the most massive will command the action.
Our work took a new approach to measuring the mass of the Andromeda galaxy. We figured that if we can trace out the escape velocity for stars within the massive gravitational halo of Andromeda, we can work out its gravitational pull, and the mass that is ultimately responsible.
While the calculation was complicated, the result was unequivocal: previous estimates had overstated the total mass of Andromeda. It and the Milky Way are equals in the local universe.