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EI2GYB > ASTRO 21.12.23 18:38l 106 Lines 5783 Bytes #999 (0) @ WW
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Subj: Astronomers Find 100,000-light-year Bow Shock in the Milky
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Astronomers Find 100,000-light-year Bow Shock in the Milky Way's Outskirts
By: Monica Young December 19, 2023
New simulations showed astronomers where to look for evidence of the Large
Magellanic Cloud's bow shock as it crashes through the Milky Way's halo.
What's invisible except in a handful of wavelengths and extends across 100,000
light-years? According to new simulations and a new look at archival data, the
answer to this riddle is a humongous bow shock in the Milky Way's outskirts.
If you've ever seen a fighter jet fly (whether in real life or in Top Gun), you
know that when the plane overtakes the speed of sound, it creates a shock wave
that encompasses and trails behind the plane. The sudden change in pressure
accompanying the shock wave is known as a sonic boom - like when Top Gun's
Maverick "buzzed the tower" and spilled the officer's coffee.
Something similar, minus the coffee, is happening in the bath of hot, sparse
gas that surrounds the Milky Way Galaxy. The largest satellite, the Large
Magellanic Cloud (LMC), is falling into this gas faster than the local speed of
sound (that is, the speed at which pressure waves can propagate). In the sparse
gas, the speed of sound is 165 kilometers per second (370,000 mph). The LMC is
crashing through this medium at almost twice that speed (320 km/s, or Mach 2.)
Astronomers have long suspected that the LMC's infall should create a bow
shock. Now, David Setton (Princeton University) and colleagues have simulated a
"wind tunnel" to predict how large it should be. In this computer simulation,
the LMC is a bundle of gas and stars that sits at rest in a box.
The gas around the Milky Way rushes like a headwind toward and around this
bundle. While the stars ignore the headwind, passing straight through it, the
gas pushes into the wind to generate a bow shock that extends three times the
size of the dwarf galaxy.
With an idea of what to look for, and where, Setton and colleagues then
searched recent measurements taken by the Wisconsin H-alpha Mapper (WHAM). The
hydrogen-alpha emission that WHAM is mapping marks the presence of hot gas.
Projecting WHAM data onto their shock simulation, the researchers found good -
though not perfect - alignment. They've posted these results on the arXiv
astronomy preprint server.
Marcel Pawlowski (Leibniz-Institute for Astrophysics, Germany), who was not
involved in the work, thinks it's likely a bow shock is there, but he also
notes that more work is needed. "The morphological match isn't that clear," he
says. "However, I expect that more sensitive data and a quantitative comparison
would allow [us] to evaluate this better, so the paper can be a good motivation
for such follow-up observations."
The team acknowledges that the simplified "wind tunnel" model is a first step,
and more detailed work is already underway.
Studying the bow shock will help astronomers understand the behavior of the gas
around the Milky Way. Sparse as it is, there's still a lot of it around our
galaxy, and it serves as a reservoir for future star formation in the disk.
It's also the dumping ground of gas fountains sprayed out by supernovae, and
thus is continuously evolving.
As gas-rich dwarf galaxies such as the LMC and its smaller companion (the SMC)
approach the Milky Way, they mix up this reservoir, bringing the whole soup up
to a hotter, common temperature, team member Gurtina Besla (University of
Arizona) explains. That might prevent gas from cooling and falling in toward
the spiral disk. At the same time, she adds, "there might be other cooling
processes that are not hindered by this effect though, so we need to study this
better."
The LMC and SMC are so close, Besla notes, that they've become "the perfect
astrophysical laboratory" with which to study galactic dynamics and evolution,
not to mention the role of dark matter, star formation, and stellar feedback on
these processes. "I think it is awe-inspiring to be working to understand this
pair of galaxies that have been known to humans since we first looked to the
sky," she adds.
"It is a complex system with many moving and interacting parts," Pawlowski
agrees, "so it is good to see another, thus-far overlooked aspect being
explored now."
More info & images at:
https://skyandtelescope.org/astronomy-news/astronomers-find-100000-light-year-b-
owshock-in-the-milky-ways-outskirts/
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