Sky & Telescope - May 2024 - 10

NEWS NOTES
EXOPLANETS
Unexplained Aurorae on a
Lonely Brown Dwarf
OBSERVATIONS of a giant world with
the James Webb Space Telescope have
revealed the signature of aurorae - even
though the world has no obvious way
to generate the lights. Jackie Faherty
(American Museum of Natural History)
announced the find at the 243rd
meeting of the American Astronomical
Society in New Orleans.
Faherty led a team using Webb data
to examine the atmospheres of 12 of
the coldest brown dwarfs, worlds more
massive than Jupiter but not massive
enough to sustain nuclear fusion
in their cores. One of them, dubbed
W1935, jumped out from the dozen
observed. Discovered by citizen scientist
Dan Caselden (see page 11), the world
revealed an unexpected feature in its
intricate infrared spectrum.
Methane is a common molecule on
brown dwarfs and typically absorbs
most light at wavelengths shorter than
GALAXIES
Astronomers Discover " Invisible " Galaxy
ASTRONOMERS HAVE FOUND something
bizarre: a galaxy that appears
to be made only of gas. The galaxy,
J0613+52, turned up in a survey of
neutral hydrogen gas in some 350 dim,
diffuse galaxies called low surface brightness
galaxies. These are systems at least
one magnitude fainter than the ambient
glow of the night sky. They have
very few stars, making them challenging
to spot with visible-light telescopes
- which is why astronomers look for
the faint glow of their gas at radio wavelengths
instead.
But thanks to a fortuitous coordinate
mistake, Karen O'Neil (Green Bank
Observatory) and her colleagues found
something even weirder: a disk of gas
without any stars.
The dark galaxy lies 270 million
light-years away, near the head of the
constellation Auriga, and appears as
a blank patch to the eye. But seen in
radio, it contains more than 1 billion
Suns' worth of hydrogen rotating
quickly in an organized way - hallmarks
of what should be a fairly
Red and blue indicate motion away from and
toward the observer, respectively, in this illustration
of neutral hydrogen gas in the primordial
galaxy J0613+52.
10 MAY 2024 * SKY & TELESCOPE
pThis artist's concept portrays auroral emissions
(red) on the brown dwarf W1935, which is
located 47 light-years from Earth.
about 4 microns. But for W1935, a
bump among all the methane absorption
indicated methane molecules had
heated up and were emitting light. In
other words, Webb had spotted aurorae.
Astronomers have seen aurorae in
radio observations of brown dwarfs.
And charged particles from the Sun, as
well as from Jupiter's volcanic moon Io,
create aurorae on many solar system
planets (S&T: Oct. 2023, p. 74). So it
might come as no surprise that this
world would exhibit the same phenomenon.
Only, in the case of W1935, the
world is isolated, far from any obvious
source of the energetic charged particles
necessary to create aurorae.
However, there could be sources we
can't immediately see, Faherty suggests.
One possibility is that a geologically
active moon spews out charged particles
from its volcanoes, like Io does, which
then gather within the larger world's
magnetic field.
Other possibilities, such as a " burp "
of plasma in the interstellar medium,
are no less intriguing.
W1935 is too faint for ground-based
follow-up, but future Webb observations
could show the aurorae on W1935
disappear as the particle source dissipates
- or a moon might even reveal
itself by its gravitational tug.
¢MONICA YOUNG
See the spectrum at https://is.gd/
browndwarfaurora.
normal, massive spiral galaxy. Except
astronomers don't detect any stars.
" What we might have here - might
- is the discovery of a primordial galaxy,
a galaxy that is so diffuse, it hasn't
been able to form stars readily, " O'Neil
said during a press conference at the
243rd meeting of the American Astronomical
Society.
Notably, the dark galaxy is a loner:
No other galaxies huddle nearby. With
no neighbors to harass it gravitationally,
there would have been nothing to
stir and compress the galaxy's gas and
trigger star formation. That in itself is
unusual, she said, because galaxies tend
to come in groups - rarely do you find
one completely by itself.
Galaxies like J0613+52 stress-test our
theories about star formation and galaxy
evolution, O'Neil says. " This might
be our first chance to really take a look
at what happens to star formation in a
diffuse environment where nothing has
come along to tweak it. "
¢CAMILLE M. CARLISLE
BROWN DWARF AURORA: NASA / ESA / CSA / LEAH HUSTAK (STSCI);
INVISIBLE GALAXY: P. VOSTEEN / GREEN BANK OBSERVATORY / NSF / STSCI
https://www.is.gd/browndwarfaurora https://www.is.gd/browndwarfaurora
Sky & Telescope - May 2024

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