- 4193 -
GALAXIES - how do they get their shape? 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 cosmic titans.
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--------------------- 4193 - GALAXIES - how do they get their shape?
- 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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- New observations
show how the evolution of galaxies from one shape to another takes place, a
process known as “galactic speciation ”. The clashes and subsequent mergers
between galaxies are a form of "natural selection" that drives the
process of cosmic evolution.
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- This means that
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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- Some galaxies, 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 this system has
lacked until now the evolutionary paths that link one galaxy shape to
another. Astronomers compared the mass
of all the stars in each galaxy to the mass of their central supermassive black
holes. 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.
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- When dust-poor
galaxies accret gas, dust, and other matter, the disk that surrounds their
central region is disrupted, with the disruption creating a spiral pattern
radiating out. 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, on the other hand, 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. This
re-draws our much-loved galaxy sequence, and, importantly, we now see the
evolutionary pathways through a galaxy wedding sequence, or what business might
refer to as acquisitions and mergers.
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- The history of the
Milky Way is believed to be punctuated with a series of
"cannibalistic" events in which it devoured smaller surrounding
satellite galaxies to grow. 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, 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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- Just as this new
galaxy will carry the story of its evolution for astronomers in the far-future,
the dust-poor lenticular galaxies could serve as fossil records of the
processes that transformed old and common disk-dominated galaxies in the early
universe.
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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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October 21, 2023 4193
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Saturday, October 21, 2023 ---------------------------------
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