Sky & Telescope - June 2021 - 25

streams not only probe our galaxy's past but can also reveal
ongoing interactions (S&T: Mar. 2020, p. 34).
Enter the Orphan Stream, a collection of stars whose
origin remains uncertain. Erkal and his colleagues used
data from the European Space Agency's Gaia spacecraft to
trace the motion of the stream's RR Lyrae stars. As standard
candles, these variable stars do double duty, revealing not just
motions but also distances to different parts of the stream.
While those far from the LMC move along the path of the
stream, the researchers found that stars closer to the LMC
had changed direction, veering toward the incoming galaxy.
After using simulations to rule out inﬂuences within the
Milky Way, Erkal and his colleagues determined that a more
massive LMC could be responsible for the sudden change in
the stars' direction.
" The LMC today is behaving as if it has the dark matter that
it started with, " Erkal says. " It's behaving like it's still huge. "
Despite its name, the Orphan Stream is not alone. Erkal's
team has identiﬁed several other streams with similar shifts
- in fact, he says that almost all the streams visible in the
Southern Hemisphere are showing effects from the LMC.
The masses captured from those stellar collections all lend
credence to a far more massive LMC.
" We're getting the same mass over and over again with
different streams, " he says.
The Orphan Stream provided concrete proof that the
LMC is indeed massive. Erkal's collaborator Vasily Belokurov

Almost all the streams in the
Southern Hemisphere are showing
effects from the LMC.
(University of Cambridge, UK) called the study a " drastic and
clear " signal of the higher mass of the LMC.

Moving the Milky Way
When the LMC was considered a chewed-up satellite of the
Milky Way, its correspondingly small mass was insufﬁcient
to cause much damage to our galaxy. However, as it became
clear that a giant dwarf was taking its ﬁrst spin around the
Milky Way, researchers began to hunt for ways that our galaxy had begun to change.
In 2015, Facundo Gómez (University of La Serena, Chile),
Besla, and their colleagues predicted that the Milky Way
should react differently if pulled by a more massive LMC. If
the LMC were small, the center of gravity of the two galaxies would fall near the Milky Way's center. A more massive
LMC would push that center of mass to our galaxy's outskirts.
But because the Milky Way is so large, different parts would
respond independently to the LMC, with stars in the extended
halo reacting at a different rate than the central regions. The
Milky Way's inner regions should have shifted signiﬁcantly in
the last 300 to 500 million years as the LMC drew closer.

ESA / G AIA / DPAC / CC BY-SA 3.0 IGO

T GAIA'S MAGELLANIC VIEW These false-color images color-code the Magellanic Clouds' stars by age: Blue is younger (but does include some
evolved stars) and red is older (generally older than 2 billion years). The older stars are everywhere in the LMC's disk, central bar, and spiral arms, as
well as in the SMC. The intermediate-age stars (green) are more common in the spiral arms, while the younger stars appear mostly in the inner spiral
structure and the Magellanic Bridge between the two galaxies.

sk yandtelescope.org * JUNE 2 021

http://www.skyandtelescope.org

Sky & Telescope - June 2021

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