Key takeaways

  • The Milky Way’s diameter is now estimated to be 1.9 million light-years, far larger than previously thought.
  • This expanded size includes the galaxy’s dark matter halo, which extends well beyond the visible stars.
  • Researchers used high-resolution simulations to study the galaxy’s dark matter halo and its influence.
  • The study focused on how dark matter affects the movement of dwarf galaxies and other objects.
  • Further data and research are needed to refine this estimate and apply similar methods to other galaxies.

When you’re right in the heart of something, it’s difficult to gauge its size accurately. Consider the Milky Way galaxy, for example. We can’t really snap a picture of it from the outside, so our best estimates are based on distance measurements to objects on the outside.

Based on Gaia mapping data from last year, we estimated a disc diameter of around 260,000 light-years, give or take. However, just as the Sun’s influence extends beyond the Kuiper Belt, the Milky Way’s gravitational influence and density, including its unseen dark matter halo, extend beyond the disc.

How much farther? Well, recent computations have revealed quite a bit. In a new publication published to the Monthly Notices of the Royal Astronomical Society and uploaded to arXiv, astrophysicist Alis Deason of Durham University in the United Kingdom and colleagues discovered a diameter of 1.9 million light-years.

The Milky Way is made up of more than only the visible stars and plasma, which all orbit Sagittarius A*, the supermassive black hole at the galactic center. We know this because the stars at the galaxy disc’s farthest edges are moving much faster than expected given the gravitational impact of observable stuff.

Schematic diagram of our galaxy’s dark matter halo. (Digital Universe/American Museum of Natural History)

The additional gravitational effect giving that rotation a push is understood as coming from dark matter, a massive, spherical halo of the substance that surrounds the galactic disc. However, because we cannot directly detect dark matter, we must infer its presence by observing how it affects the surrounding matter.

So that is what Deason and her international colleagues accomplished.

First, they ran high-resolution cosmological simulations of the dark matter haloes of Milky Way-mass galaxies, both in isolation and as analogues of the Local Group, a tiny group of galaxies about 9.8 million light-years across that includes the Milky Way.

They were particularly interested in the Milky Way’s proximity to M31, also known as the Andromeda galaxy, our closest major neighbour, with which the Milky Way is set to collide in around 4.5 billion years. The two galaxies are currently around 2.5 million light-years away, close enough to interact gravitationally.

The scientists used a variety of computer programs to mimic the Milky Way’s dark matter halo, focusing on radial velocity (the orbital speed of objects moving around the galaxy at various distances) and density to try to determine the boundary of the halo.

These simulations all revealed that, beyond the dark matter halo, the radial velocity of objects like dwarf galaxies decreased significantly.

They then compared this to a database of dwarf galaxies orbiting the Milky Way in the Local Group. And, as expected by their calculations, the radial velocity dropped abruptly. The scientists concluded that the radial distance to this barrier was around 292 kiloparsecs, or 950,000 light-years.

When the diameter is doubled, the distance is somewhat more than 1.9 million light years.

This distance can still be adjusted, and should, because it wasn’t the main goal of this research, but it helps establish critical restrictions on the Milky Way and might be used to find such bounds for others galaxies.

“In many analyses of the Milky Way halo, the outer limit is a critical constraint. The decision is frequently subjective, but as we have stated, it is preferable to identify an outer edge that is physically and/or observationally driven. The researchers noted in their report, “We have linked the boundary of the underlying dark matter distribution to the observable stellar halo and dwarf galaxy population.”

“There is great hope that future data will provide a more robust and accurate measurement of the edge of the Milky Way and nearby Milky Way-mass galaxies than the one we have presented here.”

The research has been submitted to the Monthly Notices of the Royal Astronomical Society and is available on arXiv.

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