(DIR) Home
Scientists finally found 2 of the Milky Way's missing satellite
galaxies. What could this mean for astronomy?
(HTM) Source
----------------------------------------------------------------------
Astronomers have discovered two new satellite galaxies of the Milky
Way, and these findings could help us better understand dark matter —
the mysterious stuff that accounts for around 85% of the matter in the
universe yet remains effectively invisible to us.
The discoveries also move scientists a step closer to solving a
lingering problem with the standard model of cosmology, or the "lambda
cold matter model," also known as " _Λ_ CDM," in which the word "cold"
assumes dark matter is composed of particles moving slower than the
speed of light.
The newly found small gatherings of stars have been designated Sextans
II and Virgo III. They join the around 60 known dwarf galaxies that
swarm around our much larger home spiral galaxy at maximum distances
of 1.4 million light-years. The most famous and largest of these dwarf
galaxy satellites of the Milky Way are the Large Magellanic Cloud
(LMC) and the Small Magellanic Cloud (SMC).
"How many satellite galaxies does the Milky Way have? This has been an
important question for astronomers for decades," team leader Masahi
Chiba, a professor at Tohoku University, said in a statement.
**Related:** **Amateur astronomer finds 5 fascinating new galaxies —
and they're now named after him**
Many tiny dwarf galaxy satellites of the Milky Way remain undiscovered
due to their distant and faint nature, but Chiba and colleagues were
determined to start finding these elusive objects. So, they turned to
the Subaru telescope. This powerful ground telescope, located near the
summit of Maunakea, Hawaii, is well-suited to hunting dwarf galaxies,
and the same team had previously used it to detect three new
satellites of the Milky Way.
The position of the newly found dwarf galaxy Virgo III and its member
stars. (Image credit: NAOJ/Tohoku University)
## Help! Our dwarf galaxies are missing
Dark matter is a lingering cosmological problem because it neither
interacts with light nor the ordinary matter that composes stars,
planets, moons and us. And, well, if it does interact with those
things, those interactions are far too weak for us to notice.
Breaking space news, the latest updates on rocket launches,
skywatching events and more!
That means dark matter could be composed of particles that are
currently undiscovered, though there are potential explanations that
don't require extensions to particle physics. For instance, scientists
have explored the idea that dark matter could be made of tiny
primordial black holes left over from just after the Big Bang.
However, dark matter indeed interacts with gravity, which can
influence the motion and dynamics of light and everyday matter. This
has allowed scientists to infer the presence of dark matter, and
eventually determine that large galaxies are surrounded by vast haloes
of this mysterious substance. Those halos, it's believed, extend far
beyond galactic disks and halos of visible matter.
The _Λ_ CDM predicts that these dark matter halos have played a
significant role in the evolution of galaxies. In the early universe,
they formed gravitational wells into which the gas and dust that
formed stars within galaxies were drawn. Eventually, these halos also
drew together, forming large galaxies like the Milky Way.
A diagram showing the known satellite galaxies of the Milky Way,
including the newly discovered Sextans II and Virgo III. The central
grey dot is the disk of the Milky Way (Image credit: NAOJ/Tohoku
University)
This model also suggests there should really be hundreds of satellite
galaxies around the Milky Way and other major galaxies. For instance,
simulations using _Λ_ CDM predict that our neighboring galaxy,
Andromeda, should be surrounded by around 500 satellite galaxies. Yet,
astronomers have seen just 39 dwarf galaxies swirling around
Andromeda.
For the Milky Way, some simulations based upon the standard model of
cosmology indicate that our galaxy should be orbited by about 220
dwarf galaxies, yet scientists can't figure out where they all are.
The discovery of Sextans II and Virgo III helps to redress that
balance. Still, results spinning out of these findings could present
cosmologists with the opposite problem they were facing before this.
The area of the sky over Earth observed by the HSC-SSP (area
surrounded by red lines). Previously known satellite galaxies are
indicated by black squares, and newly discovered satellite galaxies
are indicated by white triangles and stars (Image credit: NAOJ/Tohoku
University)
## Not enough dwarf galaxies or too many?
Though the number of Milky Way galaxies identified is still well below
the predicted 220 dwarf galaxies, the team behind this research took
into account the fact that the Subaru can't see the entirety of the
night sky over Earth.
They combined the distribution of dwarf galaxies Subaru has been able
to see with its night-sky "footprint" to calculate an estimate for how
many satellites _should,_ in reality, surround our galaxy. This led to
the calculation that 500 galaxies surround the Milky Way — over double
the amount predicted by simulations based on the _Λ_ CDM.
So, have scientists gone from a "not enough dwarf galaxies problem" to
a "too many dwarf galaxies" problem?
The dwarf galaxies Donatiello I to Donatiello IX as seen by the Dark
Energy Spectroscopic Instrument. (Image credit:
DECam/CTIO/NOIRLab/NSF/AURA, G. Donatiello)
Maybe not. Recently, amateur astronomer Giuseppe Donatiello discovered
five new satellite galaxies around the Sculptor galaxy, officially
known as NGC 253.
When he and a team of astronomers looked at the distribution of
satellite galaxies around the sculptor galaxy, including three
previously found by Donatiello himself, they found that the
distribution of these galaxies, which sit around 11.5 million light-
years away from Earth, was uneven. In other words, the tiny galaxies
seemed to have a "favored direction," with more galaxies lying to one
side of the Sculptor galaxy than the other.
If there is also a favored direction to dwarf galaxies around the
Milky Way and the Suburu telescope happened to be looking this way,
then the estimates based on Subaru's observations would be inflated.
The team behind these Milky Way dwarf galaxy findings now intends to
further investigate the actual number of satellite galaxies
surrounding us with yet another ground telescope.
"The next step is to use a more powerful telescope that captures a
wider view of the sky," Chiba concluded. "Next year, the Vera C. Rubin
Observatory in Chile will be used to fulfill that purpose. I hope that
many new satellite galaxies will be discovered."
The team's results were published on June 8 in the Publications of the
Astronomical Society of Japan.
______________________________________________________________________
Served by Flask-Gopher/2.2.1