Can we measure the mass of the Milky Way?

Our Galaxy’s smaller siblings could hold the secret to its size.

Illustration showing what our galaxy the Milky Way likely looks like.

At times our ignorance about the Universe around us is breathtaking. Often I realise that a fact that I’d taken for granted is being called into question.

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The identity of the closest star to the Sun, for example, and the length of the day on Saturn are both up for debate.

On larger scales, we don’t really have a satisfactory measure of our own Galaxy’s mass. Indeed, how do you measure the mass of the Milky Way?

Weighing the Milky Way is an exercise in more than just idle curiosity, too.

Studying our Galaxy is one way that cosmologists test their ideas about how still-mysterious dark matter shapes the formation of galaxies.

More from Chris Lintott:

Large and Small Magellanic Clouds. Credit: CEDIC Team + Christoph Kaltseis, CCDGuide.com
Data from the Milky Way’s satellite galaxies, such as the Large and Small Magellanic Clouds, is helping with research into our Galaxy’s mass. Credit: CEDIC Team + Christoph Kaltseis, CCDGuide.com

One particular problem is that our home system seems to have too few satellite galaxies in orbit around it, a problem that goes away if we have a Milky Way which is less massive than expected because lighter central galaxies have a less populated retinue.

Those dwarf galaxies we do have are a little unexpected in themselves. They turn up in odd places, preferring to sit above the Galaxy’s poles than along its disc, and more of them seem to have ceased substantial star formation than in similar systems we see elsewhere.

Turning to ESA’s Gaia satellite for help

The opportunity to see whether these mysteries can be resolved by sorting out the Milky Way’s mass comes from the latest data release from ESA’s Gaia satellite, which is busy mapping and recording the movements of nearly two billion stars.

Though the bulk of the data describes stars within our Galaxy’s main disc, enough have been identified in satellite galaxies that the team behind a new paper have been able to work out how these miniature systems are moving in relation to the Milky Way.

Gaia's all-sky view of the Milky Way based on the measurements of almost 1.7 billion stars. Credit: ESA
Gaia’s all-sky view of the Milky Way based on the measurements of almost 1.7 billion stars. Credit: ESA
ESA/Gaia/DPAC, CC BY-SA 3.0 IGO

Because their motion is controlled by the pull of the Milky Way’s gravity, watching the satellites move, and comparing those movements to the esoterically named Phat ELVIS suite of simulated galaxies, gives us a measure of mass.

In contrast to methods that involve counting stars, because we’re measuring the effect of gravity itself we get a value for the total mass, including both normal baryonic matter and that pesky dark matter.

In practice, the team are comparing properties of the galaxy population, such as the speed in which satellites are moving towards or away from the Milky Way.

In an ideal world they’d be able to measure the time since each satellite first entered the Milky Way halo.

The Milky Way over Maine Prashant Naik, Maine, USA, 10 June 2021 Equipment: Nikon D810 DSLR, Nikkor 14–24mm lens, Induro CLT204 tripod
The Milky Way in the night sky over Maine, USA, by Prashant Naik, 10 June 2021.

Unfortunately, that’s best determined by travelling back in time, and as the team note that they lack a Back to the Future-style DeLorean, they can’t compare this value with simulations.

Nonetheless, the results look convincing, with the Milky Way weighing between 1 and 1.2 trillion solar masses. That’s high enough that the missing satellite problem still exists.

Future surveys, including that carried out by the Vera Rubin Observatory (now under construction in the Atacama desert), should expect to detect more small companions of the Milky Way, each of which will ease the worries of cosmologists and, as a bonus, add to our ability to measure the mass of the Galaxy we call home.

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Chris Lintott was reading Sizing from the Smallest Scales: The Mass of the Milky Wayby MK Rodriguez Wimberly et al. Read it online at arxiv.org.