- 4357 - MILKY WAY - dark matter and galaxies? Astronomers measure the mass of the Milky Way by calculating how hard it is to escape. If you want to determine your own mass, it’s pretty easy. Just step on a scale and look at the number it gives you. That number tells you the gravitational pull of Earth upon you.
------------------- 4357 - MILKY WAY - dark matter and galaxies?
- The same scale could also be used to
measure the mass of Earth. If you place a kilogram mass on the scale, the
weight it gives is also the weight of Earth in the gravitational field of the
kilogram. With a bit of mass measured, you have the mass of Earth.
-
- Things aren’t quite that simple. The Earth
is not a perfectly spherical, perfectly uniform mass, so its gravitational pull
varies slightly across the globe. But this method gives a reasonable ballpark
value, and we can use it to estimate the masses of other objects in the solar
system.
-
- How can we determine the mass of the Milky
Way? One method is to estimate the number of stars in the galaxy and their
masses, then estimate the mass of all the interstellar gas and dust, and then
rough out the amount of dark matter… It all gets very complicated.
-
- A better way is to look at how the orbital
speed of stars varies with distance from the galactic center. This is known as
the “rotation curve” and gives an upper mass limit on the Milky Way, which
seems to be around 600 billion to a trillion solar masses.
-
- The wide uncertainty gives you an idea of
just how difficult it is to measure our galaxy’s mass. But a new study
introduces a new method, and it could help astronomers pin things down.
-
- The method looks at the escape velocity of
stars in our galaxy. If a star is moving fast enough, it can overcome the
gravitational pull of the Milky Way and escape into interstellar space. The
minimum speed necessary to escape depends upon our galaxy’s mass, so measuring
one gives you the other.
-
- Unfortunately, only a handful of stars are
known to be escaping, which is not enough to get a good handle on galactic
mass. So astronomers looked at the statistical distribution of stellar speeds
as measured by the “Gaia spacecraft”.
-
- The method is similar to weighing the Moon
with a handful of dust. If you were standing on the Moon and tossed dust
upward, the slower-moving dust particles would reach a lower height than faster
particles. If you measured the speeds and positions of the dust particles, the
statistical relation between speed and height would tell you how strongly the
Moon pulls on the dust, and thus the mass of the Moon. It would be easier just
to bring our kilogram and scale to measure lunar mass, but the dust method
could work.
-
- In the Milky Way, the stars are like dust,
swirling around in the gravitational field of the galaxy. The astronomers used
the speeds and positions of a billion stars to estimate the escape velocity at
different distances from the galactic center. From that, they could determine
the overall mass of the Milky Way. They calculated a mass of 640 billion Suns.
-
- This is on the lower end of earlier
estimates, and if accurate it means that the Milky Way has a bit less dark
matter than we thought.
-
- Dark matter has been detected in a cluster
of thousands of galaxies not just our own.
Astronomers have detected dark matter hanging from massive filaments
that stretch across the universe and form a "cosmic web" that trap
galaxies like a spiderweb.
-
- An 8.2-meter optical-infrared telescope
near the summit of Maunakea in Hawaii was used to measure an effect that
gravity has on light to indirectly observe dark matter sitting on cosmic web
filaments in the Coma Cluster.
-
- This marks the first-ever detection of dark
matter on the cosmic web, and could help confirm how this structure, with
strands that run for tens of millions of light-years, has influenced the
evolution of the universe.
-
- Also known as “Abell 1656”, the “Coma
Cluster” is a collection of over a thousand galaxies and is located some 321
million light-years away from us in the direction of the constellation Coma
Berenices. Because of this tremendous size and relative proximity, the Coma
Cluster is an ideal place for scientists to hunt dark matter on cosmic web
strands.
-
- The cosmic web is a network of filaments,
made up of matter, that feed gas into galaxies, helping them grow. This web
also helps channel galaxies together, leading them to cluster.
-
- The main filaments of the cosmic web are
themselves the walls of galaxy superclusters, with the wall corresponding with
the Coma Cluster known as the
"great wall." The
great wall was actually the first superlarge structure in the universe to be
discovered.
-
- Clusters of galaxies are believed to gather
at points where filaments intersect, but these filaments are believed to
terminate between galaxies and form
"intracluster filaments." Dark matter is expected to run along
these cosmic web filaments dangling from those intracluster filaments.
-
- Dark matter is acting as a cosmic
scaffold. Though the cosmic web, the
largest structure in the universe, has been known about for decades,
astronomers have only seen the faint glow of its gas filaments when they have
been illuminated by bright regions at the hearts of galaxies powered by feeding
supermassive black holes. Those active black holes are called “quasars”.
-
- In 2021 the “Keck Cosmic Web Imager”, also
atop Maunakea, caught the first direct light emanating from web filaments that
cross one another and stretch across the darkest corners of space. These are
filaments that sit isolated between galaxies, in the largest and most hidden
portions of the cosmic web.
-
- Seeing the location of dark matter around
these cosmic web strands is a completely different story, however. That's because, despite making up an
estimated 85% of all the matter in the universe, dark matter is invisible
because it doesn't interact with light like everyday matter that comprises
stars and dust does.
-
- Dark matter's dominance over everyday matter
also means it dominates the filaments of the cosmic web, forming an invisible
scaffold along which the universe's structure takes shape.
-
- However, even though dark matter doesn't
interact with light, it does interact with gravity, and this interaction
impacts the movement of everyday matter and light that we can see.
-
- The team behind this research took
advantage of this concept, using it to detect dark matter on cosmic web
filaments threaded throughout the Coma Cluster.
-
- Albert Einstein's 1915 theory of gravity,
called general relativity, suggests that objects with mass cause the fabric of
spacetime to curve. In turn, the theory explains, what we perceive as gravity
arises from this curvature. Furthermore, when light from a background source
passes through this curvature, its path gets diverted.
-
- This can lead to background sources
appearing to shift in the sky, to be amplified, or in some extreme cases, even
to appear at multiple points in the same image. This is called “gravitational
lensing”.
-
- Using light from galaxies and stars behind
the Coma Cluster and assisted by the high sensitivity, high resolution and wide
field of view of the Subaru telescope's “Hyper Suprime-Cam” (HSC), the team
detected a weak lensing effect of the dark matter component of intracluster
filaments for the first time.
-
- This first-ever detection of dark matter on
terminal segments of the cosmic web helps to further confirm the existence of
the large-scale structure spreading across the universe.
-
- Stars have been caught crawling around the
outskirts of the Milky Way more sluggishly than expected, a slow motion that
scientists say can only be explained if our dark matter galaxy map is wrong.
-
- Specific velocities of stars around the
edges of galaxies have historically been dead giveaways for the presence of
dark matter in those galaxies. This is because astronomers can measure a
galaxy's "rotation curve," which charts the orbital velocities of
stars against their distances from the center of a galaxy.
-
- If no dark matter were present (and hence
the gravitational influence it offers) were absent, stars would begin to slow
down the farther they orbit from the center of a galaxy. Instead, however, in
the 1960s and early 1970s, astronomers
noticed that the rotation curves of galaxies were flat.
-
- The orbital motion of stars did not drop
off with distance. They maintained pace. The explanation for this, scientists
believe, is that galaxies are ensconced within haloes of dark matter. These
haloes are thought to be densest at the center of the galaxy; it is the gravity
from this dark matter that keeps stars moving.
-
- Because we sit inside our galaxy measuring
the rotation curve of our Milky Way has proven more difficult. What is needed is accurate distance
information so that we know how far from the galactic center various outlying
stars are.
-
- In 2019, Anna-Christina Eilers of the
Massachusetts Institute of Technology (MIT) led a research team that used the
European Space Agency's star-measuring Gaia mission to chart the orbital
velocities of stars out to 80,000 light-years from the galactic center. As expected, the researchers found a flat
rotation curve with only the merest hint of a decline in velocity for only the
outermost stars in that sample.
-
- However, new results that combine Gaia
measurements with those from APOGEE (Apache Point Observatory Galactic
Evolution Experiment), performed on a ground-based telescope in New Mexico and
which measures the physical properties of stars to better judge their distance,
have indeed measured the Milky Way's rotation curve for stars out farther than
ever before, to about 100,000 light years.
-
- What we were really surprised to see was
that this curve remained flat, flat, flat out to a certain distance, and then
it started slowing. This means the outer
stars are rotating a little slower than expected, which is a very surprising
result.
-
- The decline in orbital velocity at these
distances implies that there is less dark matter in the center of our galaxy
than expected. The research team describe the galaxy's halo of dark matter as
having been "cored," somewhat like an apple.
-
- There's not enough gravity from what dark
matter there seems to exist there, to reach all the way out to 100,000 light
years and keep stars moving at the same velocity. This puts this result in tension with other
measurements.
-
- The next step is to employ high-resolution
computer simulations to model different distributions of dark matter within our
galaxy to see which best replicates the falling rotation curve. Models of
galaxy formation could then try to explain how the Milky Way galaxy arrived at
its specific, cored-out distribution of dark matter and why other galaxies
didn't that.
-
-
February 16, 2024 MILKY WAY - dark matter and galaxies? 4357
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--------------------- --- Saturday, February 17,
2024
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