In a surprising discovery, thanks to the application of a new technique to measure the mass of galaxies, a team of astrophysicists from the International Center for Radio Astronomy Research has discovered that the Andromeda galaxy – our next door neighbor – has approximately the same size as the Milky Way, and not two or three times bigger than our galaxy, which was previously thought.
Both the Milky Way and the Andromeda galaxy (M31) are giant spiral galaxies in our local universe. And in about 4 billion years, the Milky Way and Andromeda will collide in a sumogravitational combat that will finally unite them forever. In light of this new discovery it means that when the two galaxies merge, the Milky Way will not be consumed by the Andromeda galaxy as previous models suggested.
Because astronomers previously thought that Andromeda was up to three times more massive than the Milky Way, they expected our galaxy to be absorbed by our larger neighbor, but it seems that we had overestimated our opponent.
Calculating mass
The experts, led by astrophysicist Prajwal Kafle, found that the mass of the Andromeda galaxy is about 800 billion times the mass of the Sun. Thus, our closest galactic neighbor is about the same size as the Milky Way. (700,000 million times the mass of the Sun). This finding places the two galaxies, about 2.5 million light-years apart, on par in terms of size.
To determine the size of the Andromeda galaxy, the team used a technique that calculates the speed required for a fast-moving star to escape the gravitational pull of its host galaxy. This required velocity necessary for ejection is known as the escape velocity of an object.
“When a rocket is launched into space, it is launched with a speed of 11 km / s to overcome the gravitational pull of the Earth. Our galaxy, the Milky Way, is more than a trillion times heavier than our little planet Earth, so to escape its gravitational pull we must launch it with a speed of 550 km / s, “explained Kafle.
Modifying the Local Group map
Our galaxy and Andromeda are the two largest galaxies in what is known as the Local Group, a collection of more than 30 galaxies spanning approximately 10 million light years.
This finding “completely transforms our understanding of the local group. We thought that there was a larger galaxy and that our own Milky Way was a little smaller, but that scenario has now completely changed,” Kafle clarifies.
This is not the first time that the weight of a galaxy has been recalculated based on analysis of the escape velocities of the objects it contains. In 2014, Kafle used a similar technique to check the mass of the Milky Way, showing that our galaxy has far less dark matter – a mysterious form of matter that has gravity but does not interact with light – than previously thought.
The upshot of all this is that we now need new simulations to find out what would happen when the two galaxies inevitably merge. But it also means that we have a new way of collecting data about our universe.
“It is really exciting that we have been able to come up with a new method and suddenly 50 years of collective understanding of the local group has radically changed,” concludes Kafle.
Reference: The Need for Speed: Escape velocity and dynamical mass measurements of the Andromeda galaxy. Monthly Notices of the Royal Astronomical Society 2018. Prajwal R. Kafle, Sanjib Sharma, Geraint F. Lewis, Aaron SG Robotham, Simon P. Driver. DOI: 10.1093 / mnras / sty082
