- 4352 - GALAXY - That shouldn’t exist? - Astronomers working with the JWST found a dwarf galaxy they weren’t looking for. It’s about 98 million years away, has no neighbors, and was in the background of an image of other galaxies. This isolated galaxy shows a lack of star-formation activity, which is very unusual for an isolated dwarf.
------------------------- 4352 - GALAXY - That shouldn’t exist?
- Most isolated dwarf galaxies form stars. The
JWST’s “PEARLSz” (Prime Extragalactic Areas for Reionization and Lensing
Science) observing program is aimed at understanding the epoch of galaxy
assembly, active galactic nucleus (AGN) growth, and “First Light”. As part of
its work, it observed a galaxy cluster called “CLG1212”. The isolated dwarf
galaxy, named “PEARLSDG”, was found serendipitously.
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- “PEARLS is a “ Potentially Isolated
Quiescent Dwarf Galaxy” with a Tip of the “Red Giant Branch” distance of 30
Megaparsec. Dwarf galaxies contain far
fewer stars than galaxies like our Milky Way. Nobody’s certain how many stars
are in the Milky Way exactly. But well-reasoned estimates point to an upper
number of about 400 billion. In contrast, dwarf galaxies like PEARLSDG contain
up to about 100 million stars.
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- Besides its lack of star formation, PEARLSDG
is unusual for another reason. The JWST is able to discern individual “red
giant branch” (RGB) stars in the dwarf galaxy because the stars are bright in
JWST’s observed wavelengths. It’s almost too far away for the JWST to see the
stars, so PEARLSDG is one of the most distant galaxies in which we can see
individual stars.
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- Being able to see individual red giant
branch (RGB) stars makes studying the dwarf galaxy much easier. RGB stars have
a specific intrinsic brightness, and that means that the astronomers behind the
discovery can measure the galaxy’s distance: about 98 million light-years away.
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- They can also measure the stars’ ages,
showing that the PEARLSDG stellar population is older. If it were still forming
stars, some of the stars would be much younger. The dwarf galaxy hasn’t formed a star in at
least one billion years.
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- Part of the evidence is in the lack of UV
energy from the galaxy. Young stars emit powerful UV, yet PEARLSDG displays
only low levels of UV radiation. Consistent with its low level of UV emission
and the lack of emission lines in its spectrum.
Suggesting that its star formation shut off over 1 Gyr ago.
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- When a galaxy ceases to form stars, it’s
called a “quiescent galaxy”. In a quiescent galaxy, the supply of gas used in
star formation has been quenched. It’s usually caused by another neighboring
galaxy that has interacted with the quiescent galaxy to halt star formation.
Somehow, the interaction has stripped gas from the quiescent galaxy or
disrupted the flow of gas.
- But PEARLSDG has no close neighbors. These types of isolated quiescent dwarf
galaxies haven’t really been seen before except for relatively few cases. They
are not expected to exist given our current understanding of galaxy evolution,
so the fact that we see this object helps us improve our theories for galaxy
formation. Generally, dwarf galaxies that are out there by themselves are
continuing to form new stars.
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- Despite the fact that PEARLSDG has no close
neighbors we cannot completely rule out past interactions with other galaxies
that may have affected its formation history.
The recessional velocity and luminosity distance of PEARLSDG are
consistent with it being in the Hubble Flow, and there are no visible
signatures of tidal interactions.
-
- The “Hubble Flow” is what makes galaxies
recede from one another as the Universe expands. Some galaxies interact and
even merge despite the expansion because other forces have acted on them. But
there’s no indication that anything has interacted with the dwarf galaxy that
could’ve quenched its star formation.
-
- When galaxies interact with one another, the
tidal forces distort their shapes and can create tails and streams of
stretched-out gas, dust, and stars. But PEARLSDG shows none of these symptoms.
It’s a fairly non-descript, normal-shaped dwarf galaxy.
-
- Discoveries like this make astronomers
pause and reconsider their models of galaxy evolution. But it’s likely that the
JWST will find more isolated and quiescent dwarf galaxies. As more are
observed, things will become clearer, and eventually, there’ll be an
explanation.
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- But for now, it’s just one more mystery in
the cosmos.
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-
February 11, 2023 GALAXY
- That shouldn’t exist? 4352
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--------------------- --- Monday, February 12,
2024
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