Cambridge, Mass. – For nearly fifty years, astronomers have come up empty-handed in their
search for stars within the sprawling structure known as the Magellanic
Stream. A colossal ribbon of gas, the Magellanic Stream spans nearly 300
Moon diameters across the Southern Hemisphere’s sky, trailing behind
the Magellanic Cloud galaxies, two of our Milky Way Galaxy’s closest
cosmic neighbors.
Now the star search is finally over. Researchers at the
Center for Astrophysics | Harvard & Smithsonian
(CfA) and colleagues have identified 13 stars whose distances, motion,
and chemical makeup place the stars squarely within the enigmatic
stream.
Locating these stars has now pinned down the true distance to the
Magellanic Stream, revealing that it extends from 150,000 light-years to
more than 400,000 light-years away. The findings pave the way to map
and model the Magellanic Stream in unprecedented detail, offering new
insights into the history and characteristics of our Galaxy and its
neighbors.
"The Magellanic Stream dominates the Southern Hemisphere's sky and
our work has at last found a stellar structure that people have sought
for decades," says
Vedant Chandra, a PhD student in Astronomy & Astrophysics at the CfA and lead author of a new
study published in
The Astrophysical Journal reporting the findings.
"With these results and more like them, we hope to gain a far greater
understanding of the formation of the Magellanic Stream and the
Magellanic Clouds, as well as their past and future interactions with
our Galaxy," said co-author Charlie Conroy, a Professor of Astronomy at
the CfA and Chandra’s advisor.
The Large and Small Magellanic Clouds are dwarf satellite galaxies of
the Milky Way. Visible to the naked eye as gauzy luminances, the Clouds
have been known since antiquity. With the advent of increasingly
powerful telescopes able to perceive phenomena too faint for our eyes to
see, astronomers discovered a gigantic plume of hydrogen gas apparently
cast out of the Clouds in the early 1970s.
Studies of the gas within this Magellanic Stream further showed the
Stream to have two interwoven filaments, with one originating from each
Cloud. These features suggest the gravity of the Milky Way might have
pulled the Magellanic Stream out of the Clouds. Yet how exactly the
Stream formed has remained difficult to nail down, in no small part
because of its presumed stellar component remaining irksomely
indiscernible.
Chandra came at this problem through an ambitious project started in
2021 for his PhD at the CfA. Chandra consulted with Conroy about
interesting topic areas to study, and Conroy pointed Chandra to the
uncharted frontier of the Milky Way. The scant stars dotting the
Galaxy’s outskirts have been little studied because our Solar System is
smack dab in the starry disk of the Milky Way itself—akin to a
concertgoer near the stage attempting to see somebody all the way out at
the crowd’s periphery.
Over the last decade though, deep observational catalogs compiled by
new instruments—especially the European Space Agency's Gaia
spacecraft—have started to spy stellar objects that just might be these
elusive frontier stars. With access granted to the 6.5m Magellan Baade
Telescope at Las Campanas Observatory in Chile through the CfA and MIT,
Chandra undertook a project to perform spectroscopy on 200 far-flung
Milky Way stars, which when completed will be the largest such sample
set to date.
Spectroscopy involves collecting enough light from an object to
detect certain signatures imprinted within the light’s color bands that,
like fingerprints, uniquely identify individual chemical elements.
These signatures thus disclose the chemical makeup of an object,
speaking to its origins. In addition, the signatures shift based on the
distance to an object, enabling astronomers to tell where an object,
such as a star, is going, and correspondingly where it came from.
In the case of Chandra's study, the spectroscopic analysis revealed a
set of 13 stars with distances and velocities that fall right within
the range expected for the Magellanic Stream. What’s more, the stars’
chemical abundances matched those of the Magellanic Clouds, for instance
by being distinctively deficient in the heavier elements astronomers
call metals. “These 13 stars just fell right out of our dataset,” says
Rohan Naidu, co-author on the study and former CfA graduate student,
currently a Hubble postdoctoral fellow at MIT.
By obtaining solid distance and extent measurements of the Magellanic
Stream via these stars, the researchers buttressed its origin story as a
gravitational grab by the Milky Way. The researchers were additionally
able to calculate the Stream's overall gas distribution with higher
confidence compared to prior estimates. The distribution indicates that
the Stream is actually about twice as massive as generally reckoned.
That result, in turn, presages a future full of new star formation in
the Milky Way, because the Stream is actively falling into our Galaxy,
according to previous observations. In this way, the Stream serves as a
primary provider of the cold, neutral gas needed for making fresh Milky
Way stars.
"The Magellanic Stream is the dominant source of stellar calories for
the Milky Way—it's our breakfast, lunch, and dinner," says Ana Bonaca,
co-author on the study and former ITC postdoctoral fellow at the CfA,
now staff scientist at Carnegie Observatories. "Based on the new, higher
mass estimates for Magellanic Stream, the Milky Way may end up packing
on more pounds than initially thought."
Further studies of the Magellanic Stream should also help astronomers
learn more about the composition of our Galaxy. Because the Stream is
thought to trace the past paths of the Magellanic Clouds, modeling the
evolution of the relatively massive Large Magellanic Cloud via the
Stream will improve measurements of the Milky Way’s mass distribution.
Much of that mass is in the form of dark matter—a poorly understood,
gravity-exerting substance. Better gauging the mass of our Galaxy out in
its distant hinterlands will aid in accounting for ordinary matter
versus dark matter contents, constraining the possible properties of the
latter.
"The beauty of having a vast stellar stream like the Magellanic
Stream is that we can now perform so many astrophysical investigations
with it," says Chandra. "As our spectroscopic survey continues and we
find more stars, we're excited to see what other surprises the Galactic
outskirts have in store for us."
