|
| The nature of dark matter remains a mystery. Image Credit: Smithsonian Institute |
Galaxies come in many different shapes and sizes, but the basic ingredients
seem fairly consistent. There's usually a big black hole at the center, a
bunch of stars and gas, and a generous serving of dark matter that helps glue
the whole thing together.
While dark matter is, well, dark, the stars, gas, and swirling core of
heated material stand out with the radiant beauty of a city in the night.
However, one newly discovered dwarf galaxy located a mere 94 million
light-years away is defying expectations. It's named FAST J0139+4328, and
it's not emitting any optical light. In fact, it's barely emitting any light
at all.
FAST J0139+4328 appears to be what is known as a dark galaxy. Aside from a
small smattering of stars, the galaxy seems to be made up almost entirely of
dark matter. A paper describing the discovery has been accepted for
publication in The Astrophysical Journal Letters, and is available on
preprint
server arXiv.
"These findings provide observational evidence that FAST J0139+4328 is an
isolated dark dwarf galaxy,"
write a team of astronomers
led by Jin-Long Xu of the Chinese Academy of Sciences in Beijing. "This is
the first time that an isolated dark galaxy has been detected in the nearby
Universe."
Dark matter is currently the leading explanation for a weird discrepancy
between the amount of normal, or baryonic, matter observed in corners of the
Universe and the strength of the gravity required to hold it together. Put
simply, there's just not enough baryonic matter to account for all the
gravity. Galaxies are spinning so fast that they should fly apart without
something else binding it all together.
Whatever is responsible for this extra gravity remains elusive. It doesn't
seem to interact with normal matter in any way other than through gravity;
nor does it emit any form of radiation we can currently detect. We simply
can't see the source of this extra mass. Still, reserving a space for some
kind of unknown material goes a long way towards resolving the problems we
observe.
However, dark matter theory isn't perfect either. One problems is a
discrepancy between simulations of the dark matter distribution in the
Universe and the number of dwarf galaxies we see out there orbiting larger
galaxies. There are way fewer dwarf galaxies than the simulations suggest
there ought to be. This is known as the dwarf galaxy problem.
It is possible we're simply unable to detect some kinds of dwarf galaxy,
such as those with very few stars, consisting primarily of gas and dark
matter. Finding enough of them would help resolve the whole shortfall in
dwarf galaxies.
Some candidate dark galaxies have been identified, but they are very close
to other structures, which makes them hard to distinguish from blobs of
debris ripped free by stronger gravitational forces.
An ideal dark galaxy candidate would be drifting by itself, isolated in
space, where its identity could not be mistaken.
|
| The location of FAST J0139+4328 in (a) optical, (b) near-infrared, (c) near-ultraviolet, and (d) mid-infrared. The yellow circles in a and b are foreground stars. (Xu et al., arXiv, 2023) |
To search for such a galaxy, Xu and his colleagues used the Five-hundred-meter
Aperture Spherical radio Telescope (FAST) in China. They used the telescope
during gaps in its observation schedule as a "filler" project to conduct a
search for the radio emission of large clouds of neutral atomic hydrogen (HI)
gas in intergalactic space, looking for features consistent with a galaxy.
And they got a hit: the radio waves emitted by a cloud of HI 94 million
light-years away were consistent with a rotating disk galaxy, without the
optical light expected of one. Follow-up observations in infrared and
ultraviolet revealed a faint smattering of stars.
All together, the data allowed the researchers to determine the properties
of the galaxy, which they named FAST J0139+4328.
According to the team's calculations, the galaxy has an upper limit of
690,000 solar masses' worth of stars – and it contains 83 million solar
masses' worth of HI. The total baryonic mass of the galaxy clocks in at
around 110 million solar masses.
This, however, is just a drop in the bucket of the galaxy's total mass. The
team was able to calculate FAST J0139+4328's rotation speed, and from that
its total mass, which came in at 5.1 billion solar masses. That would mean
that the galaxy is made up of around 98 percent dark matter.
It's expected that other scientists will attempt to confirm the nature of
the object. In which case, it may turn out to be something different, as so
happened with a galaxy called Dragonfly 44. In 2016 the galaxy was found to
consist of 99.99 percent dark matter. Four years later, however,
astronomers determined that Dragonfly 44 wasn't so abnormal.
But if it is validated as a dark galaxy, FAST J0139+4328 will have some very
interesting things to tell us about the Universe around us.
"This is the first time that a gas-rich isolated dark galaxy has been
detected in the nearby Universe," the
researchers write.
"In addition, a galaxy is assumed to form from gas, which cools and turns
into stars at the center of a halo. FAST J0139+4328 has a rotating disk of
gas and is dominated by dark matter, but is starless, implying that this
dark galaxy may be in the earliest stage of the galaxy formation."
The research has been accepted into The Astrophysical Journal Letters, and
is available on
arXiv.
Source: ScienceAlert
Tags:
Space & Astrophysics