Sky & Telescope - June 2021 - 27

ies attracted by its gravity. Chief among these is the Small
Magellanic Cloud (SMC), also visible to the naked eye in the
Southern Hemisphere.
For decades, scientists have known that the LMC and SMC
were bound together, slowly spiraling into the Milky Way. The
pair come with an enormous stream of gas, and many astronomers thought that the Milky Way had stripped this Magellanic Stream from one or both dwarf galaxies. Some researchers still point to the stream as an argument against a ﬁrst-pass
scenario. " If the clouds are just coming in for the ﬁrst time,
there's not enough time for the Milky Way to really strip the
gas from the clouds and create [the Magellanic Stream], " says
Elena D'Onghia (University of Wisconsin, Madison).
But instead of the Milky Way being the culprit, Besla
counters, a high-mass LMC tore the stream from the SMC
before their ﬁrst infall. In 2018, researchers conﬁrmed that
the composition of the stream bears a stronger resemblance
to the SMC than to the LMC, supporting that scenario.
" If you want a system where the Magellanic Stream
formed because of interactions between the LMC and SMC,
then this has to be a ﬁrst infall, " Besla says. " There is mounting evidence that they have some shared history. "
Conversely, if the SMC had followed the LMC around the
Milky Way multiple times, it would have lost its gas to our
greedy galaxy ages ago, and neither the stream nor the satellite would look the way they do - for example, today's SMC
has more of its mass in gas than in stars.
Although the most dominant, the SMC isn't the only
satellite trailing after the LMC; a number of newfound
ultrafaint dwarf galaxies are caught up in the mix. With the
second release of Gaia data, Ekta Patel (University of California, Berkeley), Besla, and others set out to hunt for the
LMC's satellite galaxies. Of the roughly 60 dwarfs surrounding the Milky Way, the team selected 18 that are in the right
place to be connected to the LMC. By tracing their orbits,
the researchers found that six of those are likely tied to the
incoming cloud, consistent with previous studies. That's
also consistent with a higher-mass LMC taking its ﬁrst turn
around the Milky Way, as a smaller galaxy (or one having
made multiple passes) would have a hard time holding onto
so many companions.
As they come closer together, the LMC and the Milky Way
will play tug of war with the small satellites. " Those satellites
are coming along for a ride [with the LMC], but that does not
necessarily guarantee they will stay on the same orbital path, "
Patel says. The Milky Way may tear away some of the satellites, sending them off on their own paths before they merge
with our galaxy. Others might continue to ride with the LMC
around the Milky Way, ultimately merging with the Milky
Way at the same time as their leader.

Magellanic Stream
The leading and trailing parts of the Magellanic
Stream together stretch more than 200° across the
sky. You'd need 20 ﬁsts held at arm's length to cover
that span.

about 2.5 billion years, the number of stars in the Milky Way's
halo will grow signiﬁcantly. The fresh inﬂux of gas will also
fuel starbirth and feed the Milky Way's central supermassive
black hole, which may grow to be nearly 10 times its current
mass. There's even a very small chance that the merger could
slightly shift the Sun's orbit around the galactic center. " It's
going to change the night sky if that happens, " says astronomer Marius Cautun (Leiden University, The Netherlands).
At somewhere between a tenth and a fourth the mass
of the Milky Way, the LMC will be an important merger in
the life of our galaxy. But it pales in comparison to another
expected impact. When the Andromeda Galaxy collides with
the Milky Way some 2 billion years later, it will do a more
thorough job of shaking things up. Andromeda will destroy
the disk of stars orbiting the heart of the Milky Way, a far
more dramatic blow than the LMC's effect on the halo of
stars on its fringes. Ultimately, the LMC will fold into the
Milky Way with some violence, but it will not completely disrupt our galaxy as thoroughly as the larger Andromeda will.
In fact, the LMC may stave off the destruction to be
wrought by Andromeda down the road. Previous predictions
of the crash's outcome relied on the masses of the LMC and
the Milky Way to determine the tug on the incoming galaxy. But a more massive LMC will pull more signiﬁcantly on
Andromeda. Instead of heading directly for the Milky Way,
Andromeda is swerving slightly toward the LMC, making the
collision with our galaxy less head-on. That deviation also
changes the deadline for the impending collision, setting it
about 5½ billion years from now, or 1 billion years later than
predicted by small-mass LMC models (S&T: June 2019, p. 12).
New instruments will help nail down just how the LMC
is affecting the Milky Way. The European Southern Observatory's 4-metre Multi-Object Spectrograph Telescope (4MOST)
is anticipated to begin observations in Chile in 2023, and the
recently built Dark Energy Spectroscopic Instrument (DESI)
at Kitt Peak National Observatory in Arizona should soon
begin its survey of the sky. The pair will precisely measure
the movement of stars in our galaxy, revealing how they are
responding to the LMC.
" Probably in a few years, we'll have [measured] thousands
and thousands of stars out there, " Erkal says, " which we can
use to really see what the LMC has done to the Milky Way. "

Slow Changes
The Large Magellanic Cloud has already invaded the Milky
Way's halo, and its intrusion will continue to be felt as it sinks
toward the center of our galaxy. When the pair collides in

NOLA TAYLOR REDD is a freelance science journalist who
can be a bit spacey - literally. She writes about space and
astronomy from Atlanta, where she lives with her four children.
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Sky & Telescope - June 2021

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