Milky Way galaxy much lighter than previously thought, astronomers find

For the first time astronomers have been able to precisely measure the weight of our Milky Way galaxy, which has turned out much lighter than previously thought, according to new research.

In the study, published in current issue of the Monthly Notices of the Royal Astronomical Society, astronomers demonstrate a new method of estimating the accurate mass of galaxies.

The Milky Way is approximately half the weight of the neighboring Andromeda galaxy, which has a similar spiral structure to our own, the team of researchers led by the University of Edinburgh has found.

Their calculations are based on the measurements of the galaxies’ mass of dark matter – the invisible matter which makes up most of the outer regions of both galaxies. Previous studies were only able to measure the mass enclosed within the inner regions.

The team also found that 90 percent of the mass of the Andromeda and Milky Way galaxies is made up of dark matter. The scientists also estimated that Andromeda contains twice as much dark matter as the Milky Way.

“We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging,” said lead author Jorge Penarrubia, from the University of Edinburgh.

“Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible,” said Penarrubia.

The two galaxies belong to the so-called Local Group – a term used to describe the group of more than 54 galaxies with the gravitational center located somewhere between Milky Way and Andromeda, which are the two largest in the group.

While most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity.

The new method allowed researchers to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda.

"By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy,” said author Matthew Walker of Carnegie Mellon University's McWilliams Center for Cosmology. “This is the first time we've been able to measure these two things simultaneously."

Previously, researchers were only able to estimate the mass of the Milky Way and Andromeda based on observations made using dwarf galaxies – much smaller galaxies than our own with only up to several billion stars.

The study was carried out in collaboration with the University of Edinburgh, the University of British Columbia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics, and supported by the UK's Science and Technology Facilities Council.