Posts
Topics of this Blog include-
1.1- A brief about Milky Way Spiral Galaxy and Dark Matter
1.2- Details about Topic -Dark Matter Slowing down Milky Way
1.3 - Conclusion and Learnings from this discovery
1.4 - Summary of Topic
1.1) A brief about Milky Way galaxy and Dark Matter
Milky Way Galaxy
Milky way ( or Akashganga ) is our home galaxy. Approximately two-thirds of all spiral galaxies contain a bar, so does our galaxy - It is a barred spiral galaxy which means that it has a central bar with spiral body structure ( Fig. 1and 2)
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| Fig 1 |
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| Fig 2 |
This bar-shaped core region is surrounded by disk of gas, dust and stars
Dark matter
- To know more about dark matter you can refer to this link : Dark matter
Dark matter constitutes 27 % of content of whole universe. It is made up of 'something' that cannot be detected by electromagnetic force ( basically by touch or sight ). It's gravitational effects are responsible for flattening of velocity vs radius curves in the outer regions of galaxies. According to theories It forms a spherical halo around the galaxy.
1.2) Details about Topic - How dark matter is slowing down Milky Way
"Astrophysicists have long suspected that the spinning bar at the center of our galaxy is slowing down, but we have found the first evidence of this happening," study co-author Ralph Schoenrich, in a statement.
Galaxy models have long predicted that galactic bars slow down by losing angular momentum to their postulated dark haloes.
Hercules stream is a group of stars currently rotating away from center of Milky Way. This stream is in resonance with the central bar - i.e. it is gravitationally trapped by the central bar and thus, revolves around at the same rate as bar's spin ( like Jupiter's Trojan )
If the bar's rotation slows, the stars in the stream move outward to have orbit in sync with rotation of bar. Researchers investigated the chemical composition of stars in Hercules stream and found them to be rich in heavier elements which is possible only when they have been formed close to the center of Galaxy.
Thus they have been sweeping out of the center of galaxy which implies that galactic rotation has been slowing down and has slowed down by 24%.
The one possible and seemingly correct explanation is that dark matter has been slowing down the spin by dynamical friction. Loss of the bar's angular momentum has been attributed to dark matter.
This possible explanation, although seeming the most correct till now has run into rivals. Schoenrich said "Our finding also poses a major problem for alternative gravity theories — as they lack dark matter in the halo, they predict no, or significantly too little slowing of the bar ". There are also alternating theories of gravity which propose tweaks in Einstein's relativity and disregard dark matter possibility. But Einstein's theory has passed all of the tests till now ( Our GPS works on basis of that, the famous black hole picture that we have obtained is also consistent with his theory )
1.3) Conclusion and Learnings from this discovery
• This model supports the presence of Dark Matter, while physical detection is yet to be done.
•It again put a shade on alternating theory of gravity and dark matter. ( Like MOND )
• We have got new constraints on galactic history and the unique opportunity to differentiate between different dark matter models.
• Due to position of Sun’s position far from the center we currently see only the outer region of the resonance of spin of bar and Hercules stream . By performing the analysis at a spatial coordinate closer to the Lagrange points, we could probe deeper into the inner region of the resonance, where we may find traces of events that happened in the early epoch of bar formation and also determine the size of the initial core of the resonance which stems from the formation of the bar. However author mentions that this will be possible in the future with extended data covering the full range of resonance and a proper chemo-dynamical model predicting the age-dependent effects.
1.4) Summary of Topic
Analysing data from Gaia, a European Space Agency Mission to map position of stars in milky way, researchers at University College London (UCL) and the University of Oxford have shown that the spin of the Milky Way's galactic bar has slowed by about 24 % or a quarter since its formation. Most plausible explanation is Dark matter acts as a counterweight slowing the spin.
Also Check :
Milky way : https://en.m.wikipedia.org/wiki/Milky_Way
Dark matter: https://en.wikipedia.org/wiki/Dark_matter
