This is the first time this unique region of the Milky Way has been observed using the polarization of dust emissions at far-infrared wavelengths. This part of the spectrum was chosen because it is where dust emissions are most intense.
PILOT (Polarized Instrument for Long-wavelength Observations of the Tenuous interstellar matter) is designed to observe in the far-infrared portion of the spectrum at 240 microns, a domain where radiation from the sky is completely blocked by Earth’s obscuring atmosphere. To overcome this obstacle, a 1-tonne gondola carrying the scientific payload is lofted to an altitude of 40 km by the largest stratospheric balloons operated by CNES.
The colour image shows the map of the submillimetre far-infrared intensity (240 microns) towards the central molecular zone at the centre of our galaxy, as observed with the PILOT balloon-borne experiment over a wide area of 4°x2°. The striations overlaid on the image show the orientation of the magnetic field as measured from the dust-polarized emission by PILOT.
At 240 microns, the sky emission is dominated by thermal radiation from tiny dust particles. The PILOT experiment is specifically designed to measure the polarization of this radiation, which is produced because interstellar dust particles are partially aligned with the magnetic field.
The map shown was obtained from the second PILOT flight, which was conducted in Australia in April 2017. This mission, which followed a first flight from Canada in 2015, concentrated on regions of the sky only visible from the southern hemisphere, among them the molecular zone at the centre of the Milky Way. This region hosts a very dense twisted torus of gas and dust and is therefore very bright in the infrared.
However, the polarization of the signal in this region is particularly low, probably because the magnetic field is extremely tangled along the line of sight.
Anna Mangilli, an astrophysicist at IRAP, explains that the polarization direction observed here with PILOT is consistent with that seen at a lower frequency by the Planck satellite, indicating that the magnetic field is essentially aligned with the plane of our galaxy. The first image of this region shows that the instrument is working correctly. It also shows that the response of the detectors can be controlled with the high degree of accuracy required to measure polarization.
The PILOT science team is now poring over the data that are expected to provide a clearer picture of the magnetic field’s structure in the central molecular zone. With these data, they are going to study the geometry of the magnetic field, the size and composition of dust grains, the role of the magnetic field in regulating gas fluxes in galaxies, and the physical processes governing the alignment of dust grains in the magnetic field.
The science team is now hoping for a third PILOT flight at the earliest possible opportunity to conduct fresh high-resolution observations of astrophysical zones and targets visible from the northern hemisphere. Analysis of all these data will aid the search for B-mode polarization of the cosmic microwave background (CMB), today still held back by our understanding of foreground polarized emissions, which are due notably to interstellar dust. This has been a holy grail for some time for scientists seeking to gain new insights into the Universe and how it formed and evolved.
- Anna Mangilli, astrophysicist at IRAP, anna.mangilli at irap.omp.eu
- Olivier La Marle, CNES program manager for astronomy/astrophysics, olivier.lamarle at cnes.fr