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| A computer model showing the Local Bubble's enormous magnetic field stretching into the Milky Way (Image credit: Theo O'Neill / World Wide Telescope) |
Astronomers at the Center for Astrophysics | Harvard & Smithsonian (CfA)
have unveiled a first-of-its-kind map that could help answer decades-old
questions about the origins of stars and the influences of magnetic fields
in the cosmos.
The map reveals the likely magnetic field structure of the Local Bubble — a
giant, 1,000-light-year-wide hollow in space surrounding our Sun. Like a
hunk of Swiss cheese, our galaxy is full of these so-called superbubbles.
The explosive supernova deaths of massive stars blow up these bubbles, and
in the process, concentrate gas and dust — the fuel for making new stars —
on the bubbles' outer surfaces. These thick surfaces accordingly serve as
rich sites for subsequent star and planet formation.
Scientists' overall understanding of superbubbles, however, remains
incomplete. With the new 3D magnetic field map, researchers now have novel
information that could better explain the evolution of superbubbles, their
effects on star formation and on galaxies writ large.
"Putting together this 3D map of the Local Bubble will help us examine
superbubbles in new ways," says Theo O'Neill, who led the mapmaking effort
during a 10-week, NSF-sponsored summer research experience at the CfA while
still an undergraduate at the University of Virginia (UVA).
"Space is full of these superbubbles that trigger the formation of new stars
and planets and influence the overall shapes of galaxies," continues
O'Neill, who graduated from UVA in December 2022 with a degree in
astronomy-physics and statistics. "By learning more about the exact
mechanics that drive the Local Bubble, in which the Sun lives today, we can
learn more about the evolution and dynamics of superbubbles in
general."
Along with colleagues, O'Neill presented the findings at the American
Astronomical Society's 241st annual meeting on Wednesday, Jan. 11, in
Seattle, Washington. 3D interactive figures and a pre-print of the research
are currently available on Authorea . The research was conducted at CfA
under the mentorship of Harvard professor and CfA astronomer Alyssa Goodman,
in collaboration with Catherine Zucker, a Harvard PhD astronomy alumna,
Jesse Han, a Harvard PhD student and Juan Soler, a magnetic field expert in
Rome.
"From a basic physics standpoint, we've long known that magnetic fields must
play important roles in many astrophysical phenomena," says Goodman, who
wrote her PhD thesis on the importance of cosmic magnetic fields thirty
years ago. "But studying these magnetic fields has been notoriously
difficult. The difficulty perpetually drives me away from magnetic field
work, but then new observational tools, computational methods and
enthusiastic colleagues tempt me back in. Today’s computer simulations and
all-sky surveys may just finally be good enough to start really
incorporating magnetic fields into our broader picture of how the universe
works, from the motions of tiny dust grains on up to the dynamics of galaxy
clusters."
The Local Bubble has emerged as a hot topic in astrophysics by virtue of
being the superbubble in which the Sun and our Solar System now find
themselves. In 2020, the Local Bubble's 3D geometry was initially worked out
by researchers based in Greece and France. Then in 2021, Zucker, now of
Space Telescope Science Institute, Goodman, João Alves of the University of
Vienna, and their team showed that the Local Bubble's surface is the source
of all nearby, young stars.
Those studies, along with the new 3D magnetic field map, have relied on data
in part from Gaia, a space-based observatory launched by the European Space
Agency (ESA). While measuring the positions and motions of stars, Gaia was
used to infer the location of cosmic dust as well, charting its local
concentrations and showing the approximate boundaries of the Local Bubble.
These data were combined by O'Neill and colleagues with data from Planck,
another ESA-led space telescope. Planck, which carried out an all-sky survey
from 2009 to 2013, was primarily designed to observe the Big Bang's relic
light. In the process, the spacecraft compiled measurements of microwave
wavelength light from all over the sky. The researchers used a portion of
Planck observations that trace emission from dust within the Milky Way
relevant to helping map the Local Bubble's magnetic field.
Specifically, the observations of interest consisted of polarized light,
meaning light that vibrates in a preferred direction. This polarization is
produced by magnetically aligned dust particles in space. The alignment of
the dust in turn speaks to the orientation of the magnetic field acting upon
the dust particles.
Mapping the magnetic field lines in this way enabled researchers working on
the Planck data to compile a 2D map of the magnetic field projected on to
the sky as seen from Earth. In order to morph or “de-project” this map into
three spatial dimensions, the researchers made two key assumptions: First,
that most of the interstellar dust producing the polarization observed lies
in the Local Bubble’s surface. And, second, that theories predicting that
the magnetic field would be “swept up” into the bubble’s surface as it
expands are correct.
O’Neill subsequently carried out the complicated geometrical analysis needed
to create the 3D magnetic field map during the summer CfA
internship.
Goodman likens the research team to pioneering mapmakers who created some of
the first maps of Earth.
“We’ve made some big assumptions to create this first 3D map of a magnetic
field; it’s by no means a perfect picture,” she says. “As technology and our
physical understanding improve, we will be able to improve the accuracy of
our map and hopefully confirm what we are seeing.”
The 3D view of magnetic whorls that emerged represent the magnetic field
structure of our neighborhood superbubble, if the field was indeed swept-up
into the bubble’s surface, and if most of the polarization is produced
there.
The research team further compared the resulting map to features along the
Local Bubble's surface. Examples included the Per-Tau Shell, a giant
spherical region of star formation, and the Orion molecular cloud complex,
another prominent stellar nursery. Future studies will examine the
associations between magnetic fields and these and other surface
features.
"With this map, we can really start to probe the influences of magnetic
fields on star formation in superbubbles," says Goodman. "And for that
matter, get a better grasp on how these fields influence numerous other
cosmic phenomena."
Because magnetic fields only affect the movement and orientation of charged
particles in astrophysical environments, Goodman says there has been a
tendency to set aside the fields' influence when building simulations and
theories where gravity — which acts on all matter — is the primary force at
play. Further discouraging its inclusion, magnetism can be a fiendishly
complex force to model.
This omission of magnetic fields’ influence, while understandable, often
leaves out a key factor controlling motions of gas in the universe. These
motions include gas flowing onto stars as they form, and flowing away from
stars in powerful jets emanating from them as they gather matter into a
planet-forming disk. Even if the effect of magnetic fields is miniscule from
moment-to-moment in the low-density environments where stars form, given the
millions-of-year timescales it takes to gather gas and turn it into stars,
magnetic effects can plausibly add up to something substantial over
time.
Goodman, O'Neill, and their colleagues look forward to finding out.
"I've had a great experience doing this research at CfA and assembling
something new and exciting with this 3D magnetic map," says O'Neill. "I hope
this map is a starting point for expanding our understanding of the
superbubbles throughout our galaxy."
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