Astronomers have long thought that the Andromeda Galaxy was 2-3 times the mass and size of the Milky Way, and that our own Galaxy would ultimately be engulfed by the bigger neighbor. New research now evens the score between the two galaxies.
The Andromeda Galaxy. Image credit: Ivan Bok / CC BY 4.0.
“We used a new technique to measure the speed required to escape a galaxy,” said lead author Dr. Prajwal Kafle, a researcher at the University of Western Australia.
“When a rocket is launched into space, it is thrown out with a speed of 11 km/s to overcome the Earth’s gravitational pull.”
“The Milky Way is over a trillion times heavier than our tiny planet Earth so to escape its gravitational pull we have to launch with a speed of 550 km/s.”
“We used this technique to tie down the mass of the Andromeda Galaxy.”
According to Dr. Kafle and colleagues, the Andromeda Galaxy has a mass of 800 billion solar masses, on par with the Milky Way.
“Our research suggests scientists previously overestimated the amount of dark matter in the Andromeda Galaxy,” he said.
“By examining the orbits of high speed stars, we discovered that this galaxy has far less dark matter than previously thought, and only a third of that uncovered in previous observations.”
The Milky Way and Andromeda are two giant spiral galaxies in our local Universe, and light takes a cosmologically tiny two million years to get between them.
With Andromeda no longer considered the Milky Way’s big brother, new simulations are needed to find out what will happen when the two galaxies eventually collide.
The study authors used a similar technique to revise down the weight of the Milky Way in 2014.
“The latest finding had big implications for our understanding of our nearest galactic neighbors,” they said.
“It completely transforms our understanding of the local group.”
“We had thought there was one biggest galaxy and our own Milky Way was slightly smaller but that scenario has now completely changed.”
“It’s really exciting that we’ve been able to come up with a new method and suddenly 50 years of collective understanding of the local group has been turned on its head.”
The team’s results appear in the Monthly Notices of the Royal Astronomical Society (arXiv.org preprint).
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Prajwal R. Kafle et al. 2018. The need for speed: escape velocity and dynamical mass measurements of the Andromeda galaxy. MNRAS 475 (3): 4043-4054; doi: 10.1093/mnras/sty082
