- 4161 - GALAXIES - how they get their shapes?! 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.
-------------- 4161 - GALAXIES - how they get their shapes?!
- 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
-
- Astronomers old and new observations show
how the evolution of galaxies from one shape to another takes place . The
research shows that clashes and subsequent mergers between galaxies are a form
of "natural selection" that drives the process of evolution.
-
- 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.
-
- 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.
-
- 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”.
-
- What this widely-used system has lacked
until now were the evolutionary paths that link one galaxy shape to
another. To understand 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. They
compared the mass of all the stars in each galaxy to the mass of their central
supermassive black holes.
-
- This cimparison 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 the 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.
-
- The surprising result is the conclusion
that spiral galaxies like the Milky Way actually lie between dust-rich and
dust-poor lenticular galaxies on the Hubble sequence.
-
- Things fell into place once it was
recognized that the lenticular galaxies are not the single bridging
population. This re-draws our much-loved
galaxy sequence, and, importantly, we now see the evolutionary pathways through
a galaxy wedding sequence, or
acquisitions and mergers.
-
- 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.
-
- These dust-poor lenticular galaxies 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 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.
-
-
September 19, 2023 GALAXIES - how
they get their shapes?! 4161
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--------------------- --- Wednesday, September
20, 2023
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