Reading magnetism from light and motion

Magnetic fields in space cannot be seen directly, but they do leave fingerprints in light and in the motion of gas.

To reconstruct the plane-of-the-sky magnetic field in and around G111, the team combined several complementary tracers:

• Polarised sub-millimetre emission from interstellar dust recorded by the Planck satellite, which reveals large-scale magnetic orientations along the line of sight.

• Polarised starlight in visible and near-infrared bands from the Kanata 1.5-m telescope in Japan, combined with Gaia distances, to separate foreground dust from the magnetic field within G111 itself.

• High-sensitivity molecular gas observations of carbon monoxide isotopologues (¹²CO, ¹³CO, C¹⁸O) from the IRAM 30-m telescope in Spain, used with a relatively new approach called the Velocity Gradients Technique (VGT).

Dust grains tend to align their short axes with the local magnetic field, so their emission or the light they polarise encodes the field’s orientation. The VGT, on the other hand, exploits the fact that turbulent eddies in magnetised gas are elongated along field lines. By analysing how the gas velocity changes across the cloud, the technique can infer magnetic directions in dense regions where polarisation is weak or confused.

A curved magnetic “backbone” that follows the ring

The resulting map showed something striking:

Across much of G111, the magnetic field is coherent yet curved, closely following the ring’s filamentary structure rather than cutting across it.

• In the southern and eastern parts of the ring, the field lines derived from IRAM 30-m CO data bend gracefully along the dense JCMT-traced dust ridges.

• Near-infrared starlight polarisation and WISE 12 µm emission from warmer dust show the same curved pattern on slightly larger scales.

• Even in the more diffuse gas traced by ¹²CO, the Velocity Gradients Technique reveals a consistent large-scale magnetic geometry that wraps around the ring.

This level of agreement across independent tracers and density regimes indicates that magnetic forces have remained ordered throughout the cloud’s evolution, resisting the tendency of turbulence and gravity to twist and tangle the field.

“The magnetic field behaves like a flexible but resilient backbone,” notes Dr Alina. “It seems to have guided the gas flows during the formation of the ring and helped preserve its symmetry over time.”

A rare laboratory for magnetic fields and star formation

This makes G111 a rare laboratory for testing theories of molecular cloud formation, feedback and magnetic regulation of star formation.

The work also showcases the power of combining:

• space-based all-sky surveys (Planck, WISE, Gaia),

• targeted ground-based polarimetry and spectroscopy (Kanata, IRAM 30-m, TRAO), and

• advanced analysis methods such as the Velocity Gradients Technique.

“Our study demonstrates that to really understand how stars form, we have to consider not only gravity and turbulence, but also magnetic fields and feedback, all at once,” says Dr Alina. “G111 gives us a unique, three-dimensional case study of how these ingredients interact.”

What shaped the ring? The mystery continues

Despite the newly revealed magnetic blueprint, the ultimate origin of the G111 ring remains open.

Was it inflated by a powerful stellar wind, carved by the expanding shell of a supernova, or assembled gradually by large-scale turbulent flows and shear? Each possibility leaves subtler signatures in the gas chemistry, shock tracers and fine-scale kinematics that current data only begin to probe.

The team’s next steps include:

• using additional molecular tracers (such as SiO and CS) that are sensitive to shocks,

• running numerical magnetohydrodynamic simulations tailored to G111, and

• pursuing higher-resolution polarimetric and spectroscopic observations to zoom in on individual clumps and filaments.

By comparing these future datasets with the magnetic map presented in the new study, the researchers hope to finally reconstruct the full formation history of this enigmatic ring.

Until then, G111 stands as a striking demonstration that even in the turbulent interstellar medium, magnetic fields can impose order, quietly sculpting the clouds that will eventually collapse into new generations of stars.

Figure Caption

Magnetic field map of G111.
Segments show magnetic orientations inferred from multiple techniques overlaid on WISE 8–12 μm emission. Black contours trace cold, dense dust. Pink segments (IRAM CO + VGT) follow the cold ridge on the eastern side. Red segments (near-IR starlight polarisation) trace the magnetic field associated with warmer dust highlighted by the green contour. Together they reveal a coherent ring-like magnetic structure enveloping G111.