- 4527 - MILKY WAY GALAXY - is changing its shape? - 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.
------------------------------------- 4527 - MILKY WAY GALAXY - is changing its shape?
- Astronomers were 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 neighbourhood 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.
-
- 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.
-
- This result 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.
-
- The warp is possibly the result of a
collision with another galaxy billions of years ago. The warped disk of the Milky Way is
surrounded by a slightly flattened dark matter halo. The warp in the spiral disk is precessing
backward under the influence of the enormous mass of dark matter that forms an
invisible halo around our galaxy.
-
- 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.
-
- 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."
-
- 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.
-
- 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.
-
- 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.
-
- 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, The team
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.
-
- 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.
-
- 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 at a rate of 1.24 miles per second for every
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.
-
- What's more, 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.
-
- 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).
-
- 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.
-
-
July 16, 2024 MILKY WAY
GALAXY - is changing its shape? 4527
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--------------------- --- Wednesday, July 17,
2024
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