- 3202 - GALAXIES - with and without Dark Matter? Dark matter is invisible and its nature is unknown, but its existence is inferred from galaxies behaving as if they were shrouded in significantly more mass than we can see. There is thought to be about five times as much dark matter in the Universe as ordinary, visible matter. This is assuming that the laws of gravity are the same everywhere.
- 14 June 2021. The spin of the Milky Way’s galactic bar, which is made up of billions of clustered stars, has slowed by about 25% since its formation. What causes that?
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- For 30 years, astrophysicists have predicted such a slowdown, but this is the first time it has been measured. Dark Matter acts like a counterweight slowing the galaxy spin.
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- The “Gaia Space Telescope” was used to observe a large group of stars, the Hercules Stream, which are in resonance with the bar, revolving around the galaxy at the same rate as the bar’s spin.
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- These Hercules Stream stars are gravitationally trapped by the spinning bar. The same phenomenon occurs with Jupiter's Trojan and Greek asteroids, which orbit Jupiter's Lagrange points ahead and behind Jupiter.
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- If the bar’s spin slows down, these stars would be expected to move further out in the galaxy, keeping their orbital period matched to that of the bar’s spin.
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- The stars in the stream carry a chemical fingerprint, richer in heavier elements, proving that they have traveled away from the galactic center, where stars and star-forming gas are about 10 times as rich in metals compared to the outer galaxy.
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- Using this data, they calculated that the bar, made up of billions of stars and trillions of solar masses, had slowed down its spin by at least 24% since it first formed.
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- The counterweight slowing this spin must be “dark matter”. Astronomers have only been able to infer dark matter by mapping the gravitational potential of galaxies and subtracting the contribution from visible matter. This new measurement of dark matter was not of its gravitational energy, but of its inertial mass, which slows the bar’s spin.
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- The Milky Way, like other galaxies, is thought to be embedded in a ‘halo’ of dark matter that extends well beyond its visible edge.
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- The Milky Way is a barred spiral galaxy, with a thick bar of stars in the middle and spiral arms extending through the disc outside the bar. The bar rotates in the same direction as the galaxy.
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- ‘Supermassive blackholes” are blackholes with masses that are several million to billion times the mass of our sun. The Milky Way hosts a blackhole that is much smaller with mass 4 million times the solar mass.
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- Billion solar mass blackholes are found when the universe was just 6% of its current age which 6% of 13.7 billion years.
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- A “Dark Matter Halo” is the halo of invisible matter surrounding a galaxy or a cluster of galaxies. Although dark matter has never been detected in laboratories, physicists remain confident this mysterious matter that makes up 85% of the universe’s matter exists. Were the visible matter of a galaxy not embedded in a dark matter halo, this matter would fly apart.
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- A “seed blackhole” is a blackhole at its initial stage. The seed blackhole is either much more massive or it grows much faster than we thought, or both. The question that then arises is what are the physical mechanisms for producing a massive enough seed blackhole or achieving a fast enough growth rate?”
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- It takes time for blackholes to grow massive by accreting surrounding matter. If dark matter has self-interactions then the “gravothermal” collapse of a halo can lead to a massive enough seed blackhole.
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- A mechanism used to explain blackholes is the collapse of pristine gas in protogalaxies in the early universe. This mechanism, however, cannot produce a massive enough seed blackhole to accommodate newly observed Supermassie Blackholes, unless the seed blackhole experienced an extremely fast growth rate.
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- An alternative explanation is a self-interacting dark matter halo that experiences gravothermal instability and its central region collapses into a seed blackhole.
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- Dark matter particles first cluster together under the influence of gravity and form a dark matter halo. During the evolution of the halo there are two competing forces, gravity and pressure.
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- While gravity pulls dark matter particles inward, pressure pushes them outward. If dark matter particles have no self-interactions, then, as gravity pulls them toward the central halo, they become hotter, that is, they move faster, the pressure increases effectively, and they bounce back.
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- In the case of self-interacting dark matter, dark matter self-interactions can transport the heat from those “hotter” particles to nearby colder ones. This makes it difficult for the dark matter particles to bounce back.
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- The central halo, which would collapse into a blackhole, has angular momentum with its rotation. The self-interactions can induce viscosity, or “friction,” that dissipates this angular momentum. During the collapse process, the central halo, which has a fixed mass, shrinks in radius and slows down in rotation due to its viscosity.
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- The central halo eventually collapses into a singular state or a seed blackhole. This seed can grow more massive by accreting surrounding baryonic, visible matter, such as gas and stars.
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- The mass of the seed blackhole can be high since it is produced by the collapse of a dark matter halo. It can grow into a supermassive blackhole in a relatively short timescale
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- The presence of baryons, such as gas and stars, can significantly speed up the onset of this gravothermal collapse of a halo. The self-interactions can induce viscosity that dissipates the angular momentum remnant of the central halo. General relativistic instability of the collapsed halo ensures a seed blackhole could form.
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- Self-interacting dark matter can provide a good explanation for the observed motion of stars and gas in galaxies. Stars and gas dominate their central regions. The presence of this baryonic matter affects the collapse process by speeding up the onset of the collapse. The self-interactions also lead to viscosity that can dissipate angular momentum of the central halo and further help the collapse process.
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- Once we think all galaxies have central blackholes and are surrounded by dark matter we find a “see through” galaxy with no evidence of dark matter?
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- The galaxy is a giant cosmic cotton ball so diffuse with ancient stars so spread out that distant galaxies in the background can be seen through it. Called an “ultra-diffuse galaxy“, this galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy.
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- The ghostly galaxy doesn't appear to have a noticeable central region, spiral arms, or a disk. Researchers calculated a more accurate distance to the galaxy using Hubble telescope to observe about 5,400 aging red giant stars.
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- Red giant stars all reach the same peak brightness, so they are reliable yardsticks to measure distances to galaxies. The research team estimates that this galaxy is 72 million light-years from Earth.
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- This galaxy lacks dark matter, the invisible glue that makes up the bulk of the universe's contents. The galaxy contains at most 1/400th the amount of dark matter that the astronomers had expected.
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- Dark matter is thought to be the invisible glue that makes up the bulk of the universe's matter. All galaxies appear to be dominated by it; in fact, galaxies are thought to form inside immense halos of dark matter.
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- Determining the amount of the galaxy's dark matter hinges on accurate measurements of how far away it is from Earth. If this galaxy is as far from Earth as these measurements then galaxy's dark-matter content may only be a few percent.
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- This measurement is based on the motions of the stars within the galaxy and how their velocities are influenced by the pull of gravity. The researchers found that the observed number of stars accounts for the galaxy's total mass, and there's not room left for dark matter.
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- The research team targeted aging red giant stars on the outskirts of the galaxy that all reach the same peak brightness in their evolution. Astronomers can use the stars' intrinsic brightness to calculate their vast intergalactic distances.
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- For almost every other galaxy we look at, we say that we can't see most of the mass because it's dark matter. This stealthy Galaxy is a giant cosmic cotton ball where the stars are spread out. The galactic oddball is almost as wide as the Milky Way, but it contains only 1/200th the number of stars as our galaxy.
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- The researchers are hunting for more of these oddball galaxies. In 2020, a group of researchers uncovered 19 unusual dwarf galaxies they say are deficient in dark matter. However, it will take uncovering many more dark matter-less galaxies to resolve this mystery.
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- Finding a galaxy lacking dark matter tells astronomers something about the invisible substance. If you have a galaxy without dark matter, and other similar galaxies seem to have it, that means that dark matter is actually real and it exists. "It's not a mirage."
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- 3146 - GALAXIES - millions more discovered ? In the universe, galaxies are distributed along extremely tenuous filaments of gas millions of light years long separated by voids, forming the cosmic web. Astronomers have captured an image of several filaments in the early universe, revealing the unexpected presence of billions of dwarf galaxies in the filaments.
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- 3100 - GALAXIES - Don’t Follow the Laws of Gravity? Astronomers do not have a good idea of what Dark Matter really is. It is something that has mass and is an attractive force for gravity. But, it does not interact with the electromagnetic forces. It does not absorb or emit light or any other part of the electromagnetic spectrum.
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- 3018 - GALAXIES - the most distant galaxies? Most of the galaxies in the Universe are “ over the horizon” and beyond what we can see. Astronomers estimate that 98.4% of the galaxies in the Universe lie in the zone that we can never observe. (Unless we find something we can detect traveling faster that the speed of light.)
- June 29, 2021 GALAXIES - with and without Dark Matter? 3200
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