Scientists solve weighty matter of Milky Way mass

Milky Way's mass mostly made up of dark matter, new research into the galaxy reveals - 10-Mar-2019 - NZ International news

Scientists determine more accurate weight of Milky Way

"Global clusters extend out to a great distance, so they are considered the best tracers astronomers use to measure the mass of our galaxy" said Tony Sohn (Space Telescope Science Institute, USA), who led the Hubble measurements.

The result was that our home galaxy weighs around 1.5 trillion solar masses within a radius of 129,000 light years around the galactic center.

Previous estimates for our galaxy's mass ranged widely from 500 billion to three trillion solar masses.

'We just can't detect dark matter directly, ' explains Laura Watkins of the ESO, who led the team performing the analysis.

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When the Hubble and Gaia measurements are combined as anchor points, like pins on a map, astronomers can estimate the distribution of the Milky Way's mass out to almost 1 million light years from Earth.

Astronomers used the data from these two telescopes to measure the motion of 46 "globular clusters" - the distinct groups of stars orbiting the centre of Milky Way.

"The more massive a galaxy, the faster its clusters move under the pull of its gravity" co-author N. Wyn Evans of the University of Cambridge says in another press release. Previous investigations used the speed of clusters relative to Earth to figure out the mass.

Instead, the researchers were able to use data collected by the Gaia probe and NASA's Hubble telescope to measure the sideways motion of clusters.

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Astronomers used these calculations to extrapolate outward and finally estimate the mass of the Milky Way. A tiny, tiny fraction is the monstrous supermassive black hole at the center of the galaxy, coming in at four to five million solar masses, plus the stuff like the planets, and you and I. The remaining 90-odd per cent is dark matter.

Observations from Hubble allowed faint and distant globular clusters, as far as 130,000 light-years from Earth, to be added to the study. An global team of astronomers from the European Southern Observatory (ESO) in Garching near Munich, the British University of Cambridge, the Johns Hopkins University Center for Astrophysical Sciences, and the Space Telescope Science Institute in Baltimore, USA could at least approximately determine the weight of the Milky Way. Now, scientists have solved one of the riddle of the Milky Way's weight by combining fresh data from the Gaia mission and the Hubble Space Telescope. The objects (in this case globular clusters) orbit inside a larger object (in this case the Milky Way). That means there's a pretty large margin of error in the estimate, meaning the true mass of the Milky Way may be somewhere between 0.79 and 2.29 trillion solar masses-but the current estimate is a good start.

ESA's Gaia satellite is a space telescope created to measure the positions of billions of stars with unprecedented precision. Since Hubble has been observing some of these objects for ten years, it was also possible to accurately track the velocities of these clusters as well. Knowing the mass of our own galaxy would also help us better understand how it evolved, giving us more insight into how other galaxies came to be.

The mass of the Milky Way is one of the most fundamental measurements astronomers can make about our galactic home. Due to their great distances, globular star clusters allow astronomers to trace the mass of the vast envelope of dark matter surrounding our galaxy far beyond the spiral disk.

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"We were lucky to have such a great combination of data", said Roeland van der Marel, of the Space Telescope Science Institute in the United States.

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