- 4139 - SPIRAL GALAXIES - how to explain them? 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.
-------------- 4139 - SPIRAL GALAXIES - how to explain them?
- 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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- How does the evolution of galaxies from one
shape to another takes place, 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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- 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 ordained 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.
-
- But what this widely-used system has lacked
until now were the evolutionary paths that link one galaxy shape to
another. To learn the 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.
The M87 galaxy,
in the middle of which is the black hole that was imaged for the first time.
The image shows the shockwaves caused by jets of plasma spewed out from the
black hole.
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- 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.
-
- 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.
-
- Spiral galaxies like the Milky Way actually
lie between dust-rich and dust-poor lenticular galaxies on the Hubble
sequence. The lenticular galaxies are
not the single bridging population they were long portrayed as. 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. 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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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September 2, 2023 SPIRAL
GALAXIES - how to explain them? 4139
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