milkyway-galaxy

Highlights:

  • The actual Milky Way disc, made up of stars, black holes, planets moons, and other stellar objects has a diameter of 260,000 light-years.
  • Most part of the galaxy is made up of invisible dark matters
  • Scientists used the gravitational force applied by neighbouring galaxies to calculate the size of our galaxy.

A recently submitted research to the Royal Astronomical Society unveils the size of the Milky Way galaxy is across 1.9 million light-years. But most of it is made up of dark matters.

An estimate based on Gaia mapping data last year gave us a disc diameter of about 260,000 light-years, give or take. But, just as the Sun’s influence extends farther than the Kuiper Belt, the gravitational influence and density of the Milky Way – its invisible dark matter halo – extends farther than the disc.

Astronomers took the help of, the gravitational pull force applied by smaller neighbouring galaxies to calculate the true size of Milky Way galaxy.

The actual Milkyway disc, made up of stars, black holes, planets moons and other stellar objects is 260,000 light-years across – a fraction of the total galactic size.

The rest of the Milky Way is made up of the invisible ‘dark matter halo’ – that is a vast sphere of dark matter enveloping the galactic disc than can’t be directly observed.

Dark matter can only be detected by the impact its gravity has on other objects – it can’t be directly observed, according to astronomers.


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Astronomers from Durham University realised the Milky Way is surrounded by a sphere of dark matter after observing that the stars on the outer edge of the disc move faster than they should be moving based on the gravitational influence of detectable matter.

A team led by astrophysicist Alis Deason from Durham University set out to see just how far this sphere of invisible matter actually extended from the edge of the disc.

‘In many analyses of the Milky Way halo its outer boundary is a fundamental constraint,’ Deason wrote in the paper. ‘Often the choice is subjective, but as we have argued, it is preferable to define a physically and/or observationally motivated outer edge. Here we have linked the boundary of the underlying dark matter distribution to the observable stellar halo and the dwarf galaxy population.’

They used the way dark matter interacts objects around it as a starting point to see whether there was an obvious drop off in interactions towards the edge.

The team started by creating a simulation of the dark matter haloes of Milky Way sized galaxies on their own and alongside other smaller galaxies.

They created a virtual version of our local group – a collection of galaxies about 9.8 million light-years across that includes the Milky Way and Andromeda.

andromeda-galaxy
andromeda-galaxy, image: NASA

Andromeda, our nearest large galactic neighbour, was a major focus for the team as the Milky Way is due to collide with the galaxy in 4.5 billion years and already has some gravitational interactions.

It is about 2.5 million light-years away from the Milky Way at the moment but is getting closer.

The simulations allowed them to model the way the Milky Way’s dark matter halo would look and interact with other objects.


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They were able to show that beyond the dark matter halo the radial velocity – that is the orbital speed of objects moving around the galaxy – dropped off noticeably.

Deason and team compared their simulations to actual observations of nearby dwarf galaxies  – because they can’t observe the entire Milky Way from within it.

The observations of other galaxies in our Local Group matched the simulations and proved the sudden drop in radial velocity – allowing them to calculate the diameter.

In the dwarf galaxies, this boundary was 950,000 light-years.

They doubled that figure to get the diameter of the dark matter halo surrounding the Milky Way – and the actual full diameter of the Milky Way – 1.9 million light-years.

Deason says more refinement is needed of this figure as calculating the diameter wasn’t the main focus of their work – so it is likely to vary.

The team says it could be used to find the boundaries for other galaxies and determine exactly how far apart galaxies actually are.

‘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 team said.

the paper submitted to the Monthly Notices of the Royal Astronomical Society and can be read on arXiv, astrophysicist Alis Deason of Durham University in the UK.


dark matterWhat is dark matter?

Dark matter is a hypothetical invisible mass thought to be responsible for adding gravity to galaxies and other bodies.

By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that the Universe has, ~68% dark energy, ~27% dark matter, ~5% normal matter.

The dark matters are not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations.

It is also not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them.

Dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter.

In the early 1990s, one thing was fairly certain about the expansion of the universe. It might have enough energy density to stop its expansion and recollapse. Though it has enough energy to expand, but theoretically the universe had to slow its expansion due to gravity.

In 1998 the Hubble Space Telescope changed the conception as the observations of distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today.

So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. Then came the concept of dark matter and dark energy as Astronomers do not have a correct explanation of these mysteries. Because there is some invisible matter in the universe which is changing playing a major role in its expansion.

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