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The Milky Way is just 2 million light years in diameter, says new research



When you're in the middle of something, it's very difficult to say exactly how big it is - like the
Milky Way galaxy, for example. We can't exactly take a picture from outside, so our best
estimates are based on measurements of distance to objects in the vicinity.

An estimate based on mapping data last year gave us a disk diameter of around 260,000 light
years. But just as the Sun's influence extends beyond the Kuiper Belt, the gravitational
influence and density of the Milky Way - its invisible halo of dark matter - extends farther
away from the star disk.




In a new article sent to the Monthly Notices of the Royal Astronomical Society and sent to
arXiv , astrophysicist Alis Deason, from the University of Durham in the United Kingdom, and
her colleagues revealed a diameter of 1.9 million light years.

There is more to the Milky Way than we can see. We know this because the stars at the outer
edges of the galactic disk are moving much faster than they should be just because of the
gravitational influence of detectable matter.

The additional gravitational influence that pushes this rotation is interpreted as coming from
dark matter - a vast spherical halo of the material surrounding the galactic disk. But, as we
cannot directly detect dark matter, we have to infer its presence based on how it affects things
around it.

So, this is what Deason and his international team of colleagues did. First, they performed
high-resolution cosmological simulations of the dark matter halos of the Milky Way galaxies,
alone and in analogs of the Local Group, a small group of galaxies about 9.8 million light years
in diameter, for which the Milky Way belongs.

They were particularly focused on the Milky Way's proximity to the M31, also known as the
Andromeda Galaxy, our closest big neighbor and the Milky Way is likely to collide in about 4.5
billion years. The two galaxies are currently about 2.5 million light years away, close enough
to already be interacting gravitationally.

Using several different simulation programs, the team modeled the halo of dark matter in the
Milky Way, observing the radial velocity - the orbital velocity of objects moving through the
galaxy at various distances - and the density to try to define the edge of the matter's halo dark.
All of these simulations showed that, in addition to the halo of dark matter, the radial velocity
of objects such as dwarf galaxies dropped noticeably.

They then compared this to a database of observations of dwarf galaxies around the Milky
Way in the Local Group. And just as their simulations predicted, there was a sudden drop in
radial speed. The radial distance that the team calculated up to that limit was after a distance
of about 950,000 light years.

Fold this to the diameter and you will have just over 1.9 million light years. This distance can
still be refined, and it should, since it was not the main focus of this research, but it helps to
impose important restrictions on the Milky Way and could be used to find these limits for other
galaxies.

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