by George Halal

Magnetic fields that permeate the interstellar medium (ISM) play an important role in various astrophysical processes, such as star formation. Unfortunately, magnetic fields are difficult to detect directly, as they have no effect on neutral photons—the light by which we view the Universe. 

Magnetic fields do affect the vast clouds of dust in our galaxy by aligning the dust grains, resulting in the light they emit also being aligned, or polarized. By measuring the polarization of the dust’s emission, we can infer the structure of the interstellar magnetic field over the sky. However, we do not have a complete picture of the full 3D structure of the polarized dust emission and, and with it,  interstellar magnetic fields, as we can only observe a 2D projection of this light onto our sky. 

We do have more information about the dust itself, as we can reconstruct the full 3D structure of the dust distribution. Along with KIPAC professor Susan Clark and postdoctoral fellow Mehrnoosh Tahani, I investigated whether the 3D structure of the dust distribution affects our 2D view of the dust polarization.

3D dust mapping

Dust in the interstellar medium scatters and absorbs light from stars differently depending on the light's frequency, on the whole making the light appear redder. Light that passes through more dust is more reddened. The relative scattering of light waves along the line of sight from an observer to a star is therefore correlated with the total dust density along that line of sight. See Fig. 1, below.

The Gaia survey provided distances to more than a billion stars with unprecedented accuracy. Combining this distance information with the relative level of starlight scattered by dust has been transformative for the construction of 3D maps of the dust density distribution. In our recent paper, we used several 3D dust maps such as the ones shown below to explore the relationship between the spatial 3D dust distribution and the measured 2D dust polarization.

The Local Bubble

Our Sun’s current location is near the center of a large bubble that is relatively empty of dust and cold gas. This bubble, most commonly known as the Local Bubble, was most likely created by stars exploding and sweeping away dust and cold gas. (See Fig. 2, above.) Through simulations and observational evidence from other bubbles, we expect that magnetic field lines are also swept up by this process and squeezed into a thin layer that follows the surface of the Bubble. Since the magnetic field orientation determines the dust polarization, we looked for an imprint of the Local Bubble geometry on the dust polarization statistics.

We did not find evidence for this imprint. Our results indicated that a simple projection of the 2D dust polarization data onto the 3D Local Bubble geometry, as some previous studies have done, is not a well-motivated model for its 3D magnetic field structure. In light of this, we tested whether the 3D structure of dust beyond the wall of the Local Bubble significantly influences the 2D dust polarization structure.

Dust structure beyond the Local Bubble

If we imagine an observer looking outwards from the Sun, the dust in the observer’s line of sight can either be concentrated into a clump (low complexity) or distributed more evenly into several clumps (high complexity). If the dust is polarized differently in the different clumps along the line of sight, the net measured polarization will cancel out.

Therefore, we tested our hypothesis that the complexity of the 3D dust distribution contributes to the observed variation in the dust polarization statistics. We grouped the lines of sight into low- and high-complexity groups by pairing them based on their total dust densities (Fig. 3, above). For each pair, we subtracted the level of polarization (polarization fraction) of the line of sight with low complexity from that with high complexity. We find that, on average, the polarization fraction of the lines of sight with higher complexity is lower than those with lower complexity, as expected. We quantified this effect and found it to be statistically significant, with the broader 3D dust distribution, not just the dust linked with the Local Bubble, influencing the observed dust polarization.

Conclusion

These findings shed light on the connection between the 3D geometry of the interstellar medium and the 2D fingerprints of the interstellar magnetic field. The analysis also emphasizes the significance of mapping the 3D dust distribution over extensive distances, as it influences the measured 2D polarization fraction of the dust, which in turn has implications for the distribution of the 3D magnetic field structure. Given that the lines of sight with higher complexity typically exhibit a lower polarization fraction, a line of sight that presents a highly complex dust distribution alongside a substantial polarization fraction would therefore indicate a more uniform magnetic field.

Read more

Imprints of the Local Bubble and Dust Complexity on Polarized Dust Emission. George Halal, Susan Clark, Mehrnoosh Tahani (2024)