You may not realize it in your day-to-day life, but we are all enveloped by a giant “superbubble” that was blown into space by the explosive deaths of a dozen-odd stars. Known as the Local Bubble, this structure extends for about 1,000 light years around the solar system, and is one of countless similar bubbles in our galaxy that are produced by the fallout of supernovas.
Cosmic superbubbles have remained fairly mysterious for decades, but recent astronomical advances have finally exposed key details about their evolution and structure. Just within the past few years, researchers have mapped the geometry of the Local Bubble in three dimensions and demonstrated that its surface is an active site of star birth, because it captures gas and dust as it expands into space.
Now, a team of scientists has added another layer to our evolving picture of the Local Bubble by charting the magnetic field of the structure, which is thought to play a major role in star formation.
Astronomers led by Theo O’Neill, who conducted the new research during a summer research program at the Center for Astrophysics at Harvard & Smithsonian (CfA), presented “the first-ever 3D map of a magnetic field over a superbubble” on Wednesday at the American Astronomical Society's 241st annual meeting in Seattle, Washington. The team also unveiled detailed visualizations of their new map, bringing the Local Bubble into sharper focus.
“We think that the entire interstellar medium is really full of all these bubbles that are driven by various forms of feedback from, especially, really massive stars, where they're outputting energy in some form or another into the space between the stars,” said O’Neill, who just received an undergraduate degree in astronomy-physics and statistics from the University of Virginia, in a joint call with their mentor Alyssa Goodman, an astronomer at CfA who co-authored the new research.
Goodman added that magnetic fields outside the solar system are infamously difficult to pin down in models, because they cannot be easily mapped in 3D like those inside our solar system. As a result, she said the new map should be viewed as an initial sketch that will be refined by future observations and research methods.
“What Theo has done is to make a really good guess at a three-dimensional map of the magnetic field on the surface of a Local Bubble,” Goodman said. “That's why it's so exciting—because it's the first time there's actually an estimate outside the solar system of what the 3D field looks like.”
O’Neill and their colleagues stitched the unprecedented map together with the help of observations from two European Space Agency space missions: Gaia, which is currently building the most detailed map of the Milky Way, and Planck, which peered at the oldest light in the universe before it retired in 2013.
Both missions captured detailed observations of the distribution of dust across our galaxy, a dataset that the team used to probe the elusive structure and mechanics of the Local Bubble. Because dust particles get swept up in magnetic fields, the researchers searched for specific patterns in the light hitting dust that might reveal the dimensions and orientation of the bubble’s underlying magnetic forces.
In order to graft two-dimensional magnetic field observations into a three-dimensional model, the researchers assumed that most of the observed dust, as well as the magnetic activity, is located in the expanding surface of the bubble. While these assumptions are in line with theories, Goodman noted that future observations are likely to add much more complexity to the team’s basic map.
“It's the first attempt, and it's probably mostly right,” Goodman said.
To that point, the team hopes that their new map will provide a foundation that other scientists can build on to better understand the superbubbles that are strewn across the Milky Way. Indeed, the Sun only entered the Local Bubble a few million years ago, and it will exit in another few million years, floating out into the galaxy where it will waltz through countless other bubbles.
“Now that we have this map, there’s a lot of cool science that can be done both by us, but hopefully by other people as well,” O’Neill said. “Since stars are clustered, it's not as if the Sun is super special, and is in the Local Bubble because we're just lucky. We know that the interstellar medium is full of bubbles like this, and there's actually a lot of them nearby our own Local Bubble.”
“One cool next step will be looking at places where the Local Bubble is nearby other feedback bubbles,” they concluded. “What happens when these bubbles interact, and how does that drive start formation in general, and the overall long-term evolution of galactic structures?”