High Velocity Clouds Comprise Less of the Milky Way’s Mass Than We Thought

Sometimes in astronomy, a simple question has a difficult answer. One such question is this: what is the mass of our galaxy?

On Earth, we usually determine the mass of an object by placing it on a scale or balance. The weight of an object in Earth’s gravitational field lets us determine the mass. But we can’t put the Milky Way on a scale. Another difficulty with massing our galaxy is that there are two types of mass. There is the mass of dark matter that makes up most of the Milky Way’s mass, and there is all the regular matter like stars, planets, and us, which is known as baryonic matter.

We have several approaches to determining the total galactic mass, which usually involves measuring the speed of things such as stars, globular clusters, or nearby galaxies. Each of these approaches have strengths and weaknesses, though they all give a total value of a trillion solar masses, give or take a few hundred billion. All of these methods, however, only tell us the total mass. They say nothing about how much of the galaxy is baryonic mass. While baryonic mass is only a fraction of the total, it is what gives us all sorts of cool things like star formation, planetary systems, and digital watches.

Calculating the baryonic mass of our galaxy is even more difficult because you have to count up all the mass of regular stuff without counting dark matter. That’s relatively easy to do for things like stars and dense molecular clouds, but it’s much more challenging for things such as diffuse interstellar clouds. This is particularly true for the halo of stars and gas surrounding the Milky Way. No matter how much stuff we see at the fringes of our galaxy, there may be even more lurking about we haven’t seen. Which is why a new study looks at high-velocity clouds (HVCs) in the halo.

Most of the baryonic matter we have accounted for moves around the galaxy at the same rate. It’s easier to track things if you have an idea about how they move. But high-velocity clouds are different. They are interstellar clouds of hydrogen that can speed through the galactic halo at up to 500 km/s, and they often travel in directions very different from the galactic plane. Some astronomers have argued that HVCs might comprise a good portion of baryonic matter in the halo. So the team looked at data from the Galactic All Sky Survey (GASS) to determine whether this is true.

The GASS survey was made by the Parkes radio telescope in Australia and captured radio emissions from neutral hydrogen gas seen in the Southern Hemisphere. Since HVCs are mostly made of neutral hydrogen, they are contained in the GASS data. But GASS only tells us the direction and relative motion of these clouds, so the team had to estimate their distance. They did this by comparing the motion of the HVCs relative to the motion of the Magellanic clouds. Also, since GASS only observed portions of the southern sky, the authors used Bayesian statistics to calculate the distribution of HVCs within the entire galaxy.

Previous observations of high-velocity clouds within the galactic disk of the Milky Way show that HVCs comprise a fraction of a percent of baryonic matter there. A simple extrapolation to the halo would suggest that up to 10% of halo baryonic mass could be due to HVCs. But this new work estimates the true value is closer to 0.1%, meaning that they comprise an insignificant fraction of baryonic mass in our galaxy’s halo. But the authors stress that their calculations are based on their assumptions of cloud distances, which could be wrong. Further radio surveys would be needed to pin down the HVC distances to obtain a better value.

Reference: Tahir, Noraiz, Martín López-Corredoira, and Francesco De Paolis. “The baryonic mass estimates of the Milky Way halo in the form of high-velocity clouds.” New Astronomy 115 (2025): 102328.

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