Sky & Telescope - June 2021 - 22

Galactic Shakeup

after all, at just over 160,000 light-years away, the LMC is
already inside our galactic borders. The revision did not bode
well for our galaxy.
Although scientists were initially reluctant to completely
revise their understanding of the LMC's history, research over
the past decade is proving Kallivayalil and Besla's interpretation correct.
" So many parts are coming together, " says Besla. " It's a
global effort. "

Some of his colleagues actually
laughed in surprise at the results.
Peñarrubia. " We're now realizing that was completely and
absolutely wrong. "

Weighing Galaxies

One of the first important steps was to confirm the LMC's
predicted higher mass. But there is no scale large enough to
weigh a galaxy. Counting stars is insufficient, because most of
The finding that the LMC was a new neighbor to the Milky
a galaxy's mass is hidden in its dark matter halo.
Way made waves in the community. But initial studies
Instead, astronomers must measure how galaxies interact
were insufficient to cause most researchers to abandon the
with each other and other objects to determine their mass:
multiple-passage argument. At times, Besla felt at odds with
The orbital speed of two objects depends on their relative
the community, giving talks at conferences where people told
masses. So astronomers turned to the skies to
her later that her conclusions were wrong.
" weigh " the LMC.
" I've been yelling about this for a really long
In 2016, Peñarrubia worked with Besla
time, " she says.
and others to model 35 nearby galaxies,
Understanding the situation has been styFraction of Milky
along with the LMC and the Milky Way,
mied by the fact that such a giant companion
Way-mass galaxies
to help nail down the dwarf galaxy's mass.
is hard to find among galaxies. Observations
The results suggested that the LMC was 250
indicate that only 12% of Milky Way-like
with a companion
billion times the mass of the Sun, roughly a
galaxies have an LMC-like companion at a
of similar mass and
fourth the size of the Milky Way. This value
similar distance. The new models suggest
proximity as the Large
lay within the error bars suggested in earlier
that the LMC made its closest approach to
Magellanic Cloud
theoretical work and was 10 times higher
the Milky Way roughly 50 million years ago,
than estimated in previous decades.
an eyeblink in astronomical time. Such close
Some of his colleagues actually laughed in surprise at the
LMC-like neighbors are hard to spot. Those with an addiresults, Peñarrubia says. " They thought it was crazy that the
tional companion like the Small Magellanic Cloud are even
LMC was so massive. "
rarer, and less than 1% of Milky Way-LMC/SMC analogs lie
But Peñarrubia himself wasn't shocked - before the 2016
as close together as our pair.
results, he says he was " agnostic " when he heard Besla's
Besla and other scientists who study the Local Group -
initial results. What convinced him was the realization
the collection of the Milky Way and its neighbors - realized
that a larger LMC solved another problem that Local Group
that a new arrival would also be more massive and thus have
researchers had long struggled with. Historical measurements
a completely different effect on the region than a smaller,
using similar methods had suggested that the Andromeda
ancient companion.
Galaxy was as much as three times larger than the Milky
Astronomers had assumed that frequent passages around
Way, but Peñarrubia says he was always wary of so much
the Milky Way would have stripped the LMC of its dark mathidden mass having to be in Andromeda. Given the interacter, leaving a galaxy dominated by visible stars and gas. " The
tions among the three objects, a larger LMC suggests that
classic picture was that the LMC was not massive enough
Andromeda is closer in size to the Milky Way, a finding later
to affect the stellar disk, " says Jorge Peñarrubia (University
supported with independent observations.
of Edinburgh, UK). But if the LMC is coming in for the first
" It was another sign that the LMC was having a big
time, then it should still have the bulk of its initial dark mateffect, " says coauthor Denis Erkal (University of Surrey, UK)
ter, with a correspondingly stronger gravitational effect on
about the 2016 mass measurement. " I think the commuour galaxy.
nity took a while to warm up to [a large LMC] because there
A more massive LMC would also merge faster with the
wasn't a smoking gun. "
Milky Way - too fast to still be around today if it had made
Erkal himself managed to help change that.
multiple passes already. So if astronomers could measure the
mass and confirm that it's as high as theorists inferred, then
they would know the galaxy is a newcomer.
" Smoking Gun "
All of this has contributed to a major rethink of our
The Milky Way is surrounded by thin lines of stars that are
galactic neighborhood. " I remember when I was doing my
remnants of destruction. As smaller galaxies move closer to
PhD, people thought the Milky Way was a really boring place.
the Milky Way, their stars and gas are stretched into long
Nothing had happened in the last 10 billion years, " says
" strings of pearls " by the more massive giant. These stellar

Not Boring After All

12%

22

JUNE 2 021 * SK Y & TELESCOPE



Sky & Telescope - June 2021

Table of Contents for the Digital Edition of Sky & Telescope - June 2021

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