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MILKYWAY GALAXY -
Your Galactic home just got a remodel. Peering through the dust and gas
that intersperse our Galaxy, astronomers have found that the Milky Way’s core
is less dense than originally thought.
The stars and other components of the Milky Way seemed more compact than
those of galaxies similar to ours that scientists have been able to see and
measure directly. The measurements made our Galaxy look “cute and small” in
comparison, why it is different.
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----------------------------- 4534 -
MILKYWAY GALAXY
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- This Galactic revision was made by
measuring the locations and distances of almost a quarter of a million red
giants, massive old stars, using survey data from the Apache Point Observatory
Galactic Evolution Experiment (APOGEE). Normally, light from celestial bodies
such as red giants can be obscured from the view of telescopes on Earth by
interstellar dust, but the Apache observatory, in Sunspot, New Mexico, can
detect near-infrared wavelengths, which pass through the dust.
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- Dust spread throughout our Galaxy obscures
our view of faraway stars, especially those near its center. Our Solar System resides on one of the Milky
Way’s spiral arms, about halfway between its visible outer edge and its
core. Previous estimates of the
Galaxy’s size and shape have been made by determining the distribution of stars
in our Sun’s neighborhood and extrapolating this measurement on the basis of
models of simple galaxies.
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- The latest census of red giants is the best
one ever collected. It enabled the
researchers to get a better idea of the distribution of the Galaxy’s stars,
particularly in its central ‘bulge’.
Instead of seeing the population of red giants increase exponentially
from the edge of the Galaxy towards the bulge, the researchers observed that it
levelled out near the midway point, implying that the central portion of the
Milky Way’s disk is not as densely packed as models had assumed. And if the
matter contained in the Galaxy isn’t stuffed into the center, it must be more
spread out than previously thought.
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- One way to measure a galaxy’s diffuseness
is to determine its half-light radius: the distance from its center at which
you can draw a circle that will encompass half the total light emitted by the
galaxy. The researchers, for the first time, used direct measurements to
calculate the Milky Way’s half-light radius, and it was roughly twice as large
as that estimated from the current best models of the Galaxy.
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- This intriguing result shows that our
Galaxy is complex. There are a lot of
things that need to be revisited. It
might lower our estimate of the total mass of the Milky Way, and in turn, that could imply that our Galaxy
holds more dark matter than originally thought.
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- The warp in the Milky Way's spiral disk is
precessing backward under the influence of the enormous mass of dark matter
that forms an invisible halo around our galaxy.
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- About one-third of all spiral galaxies have
a distinct warp to their disk-shaped structure, like a vinyl record that has
been bent. It's usually the result of a variety of factors; a collision with
another galaxy in the past is believed to be the primary culprit in causing the
Milky Way's warp in the first place, but further interactions with satellite
galaxies and the intergalactic magnetic field, as well as the infall of vast
clouds of gas, can also play their part.
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- However, in the case of the Milky Way at
least, the major player in maintaining the warp is the dark matter halo that
surrounds the disk and exerts a torque on it.
This warp isn't fixed. Its alignment with the rest of the galaxy moves,
specifically, it "precesses."
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- Precession describes how the alignment of
the warp changes with respect to the rotational axis of the galaxy, meaning
that the peak, or node, of the warp precesses around the galaxy. It's a
variation of the same phenomenon that causes spinning tops to wobble.
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- Measuring the warp's rate of precession has
proven challenging in the past. Previous estimates have attempted to use the
vertical motion of bright, but old, giant stars as tracers to calculate the
rate of precession. However, such tracers are notoriously imprecise, and
results based on them had suggested that the disk is precessing prograde (in
the same direction as the rotation of the rest of the galaxy) and not
retrograde (backward with respect to the galaxy), as had been expected.
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- Using a more accurate tracer in the form of
Cepheid variable stars to make the most accurate measure of the warp's
precession yet, finding it to be moving retrograde after all.
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- Cepheid variables are pulsating massive
stars. Their period of pulsation is linked to how intrinsically bright they
are, and based on their luminosity, we can calculate exactly how far away they
must be. This makes them great tracers for mapping the warp.
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- Using data from the European Space Agency's
Gaia astrometric spacecraft, which is measuring the positions, motions and
properties, including the age, of more than a billion stars, astronomers
identified a sample of 2,613 Cepheids with a variety of ages.
-
- Age is key to measuring the precession rate
of the disk warp. Astronomers obtained a motion picture of the disk warp by
mapping the three-dimensional distributions for Cepheid samples of different
ages.
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- Each Cepheid retains information on its
position in the warp when it was born, so by grouping the Cepheids into
different age ranges and mapping them, astronomers were able to show the shape
and position of the warp at different points in time over the past 200 million
years.
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- By then running the individual maps
together, like a motion picture, they were able to see the warp precessing.
They found that it is precessing in retrograde fashion after all, at a rate of
2 kilometers per second for every kiloparsec (3,261 light-years) of space. Or,
in more intuitive units, it is precessing backward around the galaxy by a rate
of 0.12 degrees every million years.
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- The motion picture also shows that the
precession rate decreases with distance from the galactic center, which in the
long term will lead to greater warping of the disk. Models indicate that this
decrease is the result of the dark matter halo that is exerting the torque
being oblate, or flattened, in shape.
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- The shape of the dark matter halo is
important because it acts as a data point that theorists can plug into models
that attempt to predict what dark matter is made of (such as WIMPs or axions).
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- It also gives clues about the formation
history of the Milky Way galaxy and how it has been assembled through mergers
with other, smaller galaxies and gas clouds, collisions and interactions that
have helped shape the invisible dark matter halo.
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- If we study the nearby universe, we find
that almost one-third of all disk galaxies seem to have a warp shape to their
disk. Instead of resembling a perfect disk, they somewhat resemble a potato
chip depending on your geographical location. Not surprisingly it is known as a
disk warp and even our own Galaxy has one . Galaxies rotate just like a
spinning top and the galactic disk experiences precession or a wobble due to
the torque forces from the surrounding dark matter halo.
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- Dark matter is a mysterious and invisible
type of matter that is thought to make up about 27% of the universe’s
mass-energy content. Unlike ordinary matter, dark matter does not emit, absorb,
or reflect light, making it detectable only through its gravitational effects.
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- Its presence is inferred from the rotation
speeds of galaxies, gravitational lensing, and the cosmic microwave background.
Despite lots of research, the exact nature of dark matter is still unknown, and
it is one of the most significant unsolved problems in physics and cosmology.
It is thought that this invisible matter surrounds most galaxies.
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- A “cepheid variable” are a type of
pulsating star whose brightness varies in a regular cycle due to periodic
expansions and contractions in their outer layers. They are useful because
their pulsation period is directly related to their actual or intrinsic
brightness so can be used for measuring distances in space. By comparing the
apparent brightness of a Cepheid with its known luminosity, we can determine
its distance from Earth.
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- Analyzing a sample of 2,600 Cepheid
variable stars of different ages was used to observe the precession direction
and rate of the Milky Way’s warp. These measurements showed that the current
dark matter halo of the Milky Way is slightly oblate, like a sphere that has
been squashed at the poles and is rotating backward at 0.12 degrees every
million years! I can sort of feel it
after doing this review.
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August 7, 2024 4534
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------ “Jim Detrick” -----------
--------------------- --- Thursday, August 8,
2024
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