12 ASTRONOMY • OCTOBER 2019
The Milky Way likely collided with recently
discovered dwarf galaxy Antlia 2 less than
a billion years ago, according to research
presented June 12 at the 234th meeting of
the American Astronomical Society.
The work, led by Sukanya Chakrabarti
of the Rochester Institute of Technology,
supports a prediction she made a decade
ago about how the Milky Way obtained a
unique ripple pattern in its outer disk of
gas. The study has been submitted for pub-
lication in The Astrophysical Journal Letters.COSMIC WHODUNIT
In 2009, Chakrabarti published work that
concluded a collision between a dark
matter-dominated dwarf galaxy and the
Milky Way could explain the curious ripples,
which have no obvious cause. To find the
culprit, her team first rounded up the usual
suspects: known satellites of the Milky
Way, such as the Magellanic Clouds and
the Sagittarius Dwarf Galaxy. However, the
Magellanic Clouds are too distant and the
Sagittarius Dwarf has too little heft to explain
our galaxy’s scars. This led Chakrabarti to
predict that another dwarf galaxy — one that
hadn’t yet been found at the time — was
responsible for the galactic drive-by.
Fast-forward to last year, when research-
ers used data from the Gaia satellite to
uncover a previously unknown, faint dwarf
galaxy orbiting the outskirts of the Milky
Way: Antlia 2. The galaxy, which is about
400,000 light-years away and is roughly
as wide as the Large Magellanic Cloud, isalmost invisible. According to Chakrabarti,
it’s the galaxy with the lowest-known sur-
face brightness, which measures a galaxy’s
light per area of the sky that it covers.
Chakrabarti set out to determine whether
this newly discovered galaxy could be the
elusive dwarf she had predicted. First, she
calculated the past trajectory of Antlia 2
based on its movement and current location.
Lo and behold, Antlia 2 does appear to have
smashed into the Milky Way in the past. Next,
she compared the Milky Way’s observed rip-
ples with those produced by a simulated col-
lision between our galaxy and Antlia 2. “It’s
almost dead on,” Chakrabarti tells Astronomy
of the match between the two.
Now she and her team are waiting for the
next batch of data from the Gaia mission,
which is working to map over a billion stars
in the Milky Way. More data will allow them
to test their prediction, Chakrabarti says, with
regard to how the stars within Antlia 2 should
currently be moving. If the motions line up
with predictions, it should clinch the case for
Antlia 2 as the cause of our galaxy’s ripples.TEST SUBJECT
The dwarf galaxy could be used to investigate
dark matter, Chakrabarti says. Based on its
run-in with our galaxy, scientists could deter-
mine how dark matter is spread throughout
Antlia 2. With that information, she says, “you
can now start to tell the difference between
different dark matter models.” That would
certainly bring astronomers closer to under-
standing the elusive nature of dark matter. — J.P.Milky Way still bears scars
The dwarf Antlia 2 may have caused
strange ripples in our galaxy’s outer disk.
SMALL
GALAXY,
BIG BLACK
HOLE
At just 3,000 light-years
wide, the dwarf galaxy
ESO 495–21 is only about
3 percent the diameter
of the Milky Way. But
it now seems the pint-
sized galaxy packs a
supermassive black hole
more than a million times
the mass of the Sun.
That puts ESO 495–21’s
black hole in the same
weight class as our own
galaxy’s central black
hole, Sagittarius A*, which
weighs in at 4.3 million
solar masses. If confirmed,
researchers think such a
big black hole at the heart
of such a petite galaxy
strongly suggests that
black holes formed first
in the early universe, with
galaxies later developing
around them. — J.P.QUANTUM GRAVITY
LEAVE YOUR MARK.
Our Milky Way,
shown here in an
artist’s concept, has
strange ripples in
its outlying regions.
New research
indicates those
ripples were caused
by a collision with
a dim dwarf galaxy
called Antlia 2.
ESA/C. CARREAUESA/
HUBBLE,^ NASA