- 4122 - MILKYWAY GALAXY - what shape it's in? The Milky Way wasn't always a spiral, and astronomers may finally know why it 'shape-shifted'. A century-old mystery of how galaxies change shapes has been solved by considering 'survival of the fittest' collisions between galaxies.
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4122 - - MILKYWAY
GALAXY - what shape it's in?
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- A 100-year-old mystery surrounding the
"shape-shifting" nature of some galaxies has been solved, revealing
in the process that our Milky Way galaxy did not always possess its familiar
spiral appearance.
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- The evolution of galaxies from one shape to
another takes place is a process known as galactic speciation . The research
shows that clashes and subsequent mergers between galaxies are a form of "natural
selection" that drives the process of cosmic evolution.
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- The Milky Way's history of cosmic violence
is not unique to our home galaxy. Nor is it over. Astronomy now has a new anatomy sequence and
finally an evolutionary sequence in which galaxy speciation is seen to occur
through the inevitable marriage of galaxies by gravity.
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- Galaxies come in an array of shapes. Some,
like the Milky Way, are composed of arms of well-ordered stars revolving in a
spiral shape around a central concentration or "bulge" of stellar
bodies. Other galaxies like Messier 87 (M87) are composed of an ellipse of
billions of stars chaotically buzzing around a disordered central
concentration.
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- Since the 1920s, astronomers have
classified galaxies based on a sequence of varying galaxy anatomy called the
"Hubble sequence." Spiral galaxies like ours sit at one end of this
sequence, while elliptical galaxies like M87 sit at the other. Bridging the gap
between the two are elongated sphere-shaped galaxies, lacking spiral arms,
called “lenticular galaxies”.
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- But what this widely-used system has lacked
were the evolutionary paths that link one galaxy shape to another. To learn evolutionary paths on the Hubble
sequence, astronomers looked at 100
galaxies near to the Milky Way in optical light images collected by the Hubble
Space Telescope and compared them to infrared images from the Spitzer Space
Telescope. This allowed him to compare the mass of all the stars in each galaxy
to the mass of their central supermassive black holes.
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- This revealed the existence of two different
types of bridging lenticular galaxies: One version that is old and lacks dust,
and the other that is young and rich in dust.
When
dust-poor galaxies accrete gas, dust, and other matter, the disk that surrounds
their central region is disrupted, with said disruption creating a spiral
pattern radiating out from their hearts. This creates spiral arms, which are
over-dense rotating regions that create gas clumps as they turn, triggering
collapse and star formation.
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- The dust-rich lenticular galaxies are
created when spiral galaxies collide and merge. This is indicated by the fact
that spiral galaxies have a small central spheroid with extending spiral arms
of stars, gas and dust. Young and dusty lenticular galaxies have notably more
prominent spheroids and black holes than spiral galaxies and dust-poor
lenticular galaxies.
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- Spiral galaxies like the Milky Way actually
lie between dust-rich and dust-poor lenticular galaxies on the Hubble
sequence. The history of the Milky Way
is believed to be a series of "cannibalistic" events in which it
devoured smaller surrounding satellite galaxies to grow.
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- This research indicates that in addition to
this, our galaxy's cosmic "acquisitions" also included it accreting
other material and gradually transforming from a dust-poor lenticular galaxy to
the spiral galaxy we know today.
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- Our galaxy is set for a dramatic merger
with its closest large galactic neighbor, the Andromeda galaxy, in between 4
billion and 6 billion years. This collision and merger will see the spiral arm
pattern of both galaxies erased and the new research indicates that the
daughter galaxy created by this union is likely to be a dust-rich lenticular
galaxy still possessing a disk, albeit without a spiral structure carved
through it.
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- Should the Milky Way-Andromeda daughter
galaxy encounter a third, dust-rich lenticular galaxy and merge with it, then
the disk-like aspects of both galaxies will also be wiped clean. This would
create an elliptical-shaped galaxy without the ability to harbor cold gas and
dust clouds.
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- This could help explain the discovery by
the James Webb Space Telescope (JWST) of a massive spheroid-dominated galaxy
just 700 million years after the Big Bang. The new research could indicate,
too, that the merging of elliptical galaxies is a process that could explain
the existence of some of the universe's most massive galaxies, which sit at the
heart of clusters of over 1,000 galaxies.
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- There are regions we can’t see or map using
conventional methods. There’s no way to get outside the Galaxy to take pictures
of the whole shebang. We’re trapped
inside the Milky Way and can’t get a bird’s-eye view.
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- To get around that limitation, astronomers
used a technique called “chemical mapping” or chemical cartography. It charts
the places we can’t see using our powerful telescopes or regions for which we
have only good models and simulations.
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- When you look at charts and images of the
sky, the stars stand out. Most maps of the Milky Way take advantage of that
property. Concentrations of young stars (in particular) stand out the most
because they’re bright. They form in regions where the rotating spiral arms
compress clouds of gas and dust. The result is batches of newborn stars. So,
one of the easiest ways to map those arms is to look for new stars.
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- Wherever you have star birth, you also have
clouds of gas and dust. And, those clouds often hide the new stars, and the galaxy
arms, from our view. This is where chemical cartography comes. It relies on
knowing the metal content of stars.
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- The first stars in the Universe (and the
oldest stars in the Galaxy) are fairly metal-poor. That means they are mostly
hydrogen and helium. As they died they scattered heavier elements such as
carbon, oxygen, and so on (astronomers call them “metals”) to interstellar
space. Those elements became part of the next generations of stars. Each
subsequent batch of stars contains more complex metals than its “parent”
generations.
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- Chemical cartography tracks the distribution
of metal-rich stars in the Galaxy’s spiral arms, where young stars have higher
metallicity content. Astronomers have
seen a “belt of metallicity” in the Galaxy that might be obvious to distant
observers. If you can track the
metallicity of the stars in the arms that you can see, you can “fill in” the
view of the spiral arms we can’t see.
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- Astronomers analyzed data from the “Large Sky
Area Multi-Object Fibre Spectroscopic Telescope”(LAMOST) and “Gaia space
telescope”. Gaia has made incredibly precise and comprehensive surveys of the
Milky Way, measuring more than two billion objects. Fortunately, Gaia’s data
includes chemical composition measurements for about one percent of the Galaxy,
which was enough data to use in his chemical cartography.
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- They charted the distribution of metallicity
in our Galaxy, beginning with the region around the Sun. There’s a lot of data
for our local neighborhood—which ranged out around 32,600 light-years.
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- They found that the spiral arms in those
charts lined up with the ones in his metallicity charts. Since his map showed
the existence of spiral arms based solely on stellar metallicity (rather than
starlight), new regions showed up. Those were places not mapped by other charts
because they aren’t easily visible.
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- The spiral arms are indeed richer in metals.
This illustrates the value of chemical cartography in identifying the Milky
Way’s structure and formation. It has the potential to fully transform our view
of the Galaxy.
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- While astronomers have known for decades
that the Milky Way is a spiral galaxy, they’ve had a tough time filling in the
exact shape of the arms and the core. These days, we know that it’s a barred
spiral. The bar itself could be funneling gas in from the spiral arms to the
core. There, it eventually gets caught up in waves of star birth activity.
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- We can’t always see the activity in the
core, so chemical cartography could prove to be a very useful tool for mapping
that region of the galaxy. Gaia’s ongoing mission of charting the Galaxy and
providing even more chemical information about the stars of our galaxy will
continue to help astronomers fill in our view of the Milky Way and its
structures.
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August 19, 2023 MILKYWAY
GALAXY - what shape it's in? 4123
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