When you are right in the middle of something, it can be difficult to tell exactly how big it is. For example, the Milky Way galaxy.
We cannot accurately photograph it from the outside, so our best estimates are based on distance measurements to objects in the suburbs.
An estimate based on last year's Gaia mapping data gave us a disk diameter of about 260,000 light years. But just as the Sun's influence travels farther than the Kuiper belt, the gravitational influence and density of the Milky Way – its invisible dark matter halo – travels farther than the disk.
How much further? Well, as new calculations have shown, quite a bit. In a new paper submitted to the Royal Astronomical Society's Monthly Notices and uploaded to arXiv, astrophysicist Alice Dison of Durham University in the UK and colleagues found a diameter of 1.9 million light years.
There's more to the Milky Way than what we can see – all the stars and gas orbiting Sagittarius A, the supermassive black hole at the center of the galaxy. We know this because the stars at the outer edges of the galactic disk are moving much faster than they should be given the gravitational influence of the detected matter.
The additional gravitational force that gives this spin a boost is interpreted as coming from dark matter – the vast spherical halo of matter that surrounds the galactic disk. But since we cannot detect dark matter directly, we must infer its presence based on how it affects the surrounding matter.
Here's what Deason and her colleagues did.
First, they performed high-resolution cosmological simulations of the dark matter halos of galaxies with the mass of the Milky Way, both individually and in analogues of the Local Group, a small group of galaxies about 9.8 million light-years across to which the Milky Way belongs.
Diagram of the dark matter halo of our galaxy. (Digital Universe / American Museum of Natural History).
They were particularly focused on the Milky Way's proximity to M31, AKA, the Andromeda galaxy, our closest large neighbor, and which the Milky Way is set to collide in about 4.5 billion years. The two galaxies are currently about 2.5 million light-years apart – close enough to already interact gravitationally.
Using several different programs, the team modeled the Milky Way's dark matter halo by looking at radial velocity – the orbital speed of objects orbiting the galaxy at various distances – and density to try to define the edge of the dark matter halo.
All these simulations have shown that, outside the dark matter halo, the radial velocity of objects such as dwarf galaxies is noticeably lower.
They then compared it to the Local Group's database of dwarf galaxies around the Milky Way. And, as their simulations predicted, there was a sharp drop in the radial velocity. The team's radial distance to this boundary was about 292 kiloparsecs – about 950,000 light years.
Double that in diameter and you get just over 1.9 million light years.
This distance can still be refined and needs to be refined as it was not the main goal of this study, but the calculation helps to impose important constraints on the Milky Way, and can be used to find such boundaries for other galaxies.
In many analyzes of the Milky Way's halo, its outer edge is a fundamental limitation. The choice is often subjective, but as we have said, it is preferable to define physically and / or observationally the outer edge. Here we tied the boundary of the distribution of dark matter to the observed stellar halo, 'the researchers wrote in their article.
“There is great hope that the new data will provide more reliable and accurate measurements of the boundaries of the Milky Way and nearby galaxies.”
The study was accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available on the arXiv website.
Sources: Photo: (ESA / Gaia / DPAC, CC BY-SA 3.0 IGO)
