Planck takes magnetic fingerprint of our galaxy

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Our Galaxy’s magnetic field is revealed in a new image from ESA’s Planck satellite. This image was compiled from the first all-sky observations of ‘polarised’ light emitted by interstellar dust in the Milky Way. Many researchers and engineers belonging to the CNRS, the CEA, the CNES and French Universities are involved in the Planck mission which continues his harvest results. These analyzes are to be submitted in four articles to the journal Astronomy & Astrophysics.

Light is a very familiar form of energy and yet some of its properties are all but hidden to everyday human experience. One of these – polarisation – carries a wealth of information about what happened along a light ray’s path, and can be exploited by astronomers. In space, the light emitted by stars, gas and dust can be polarised in various ways. By measuring the amount of polarisation in this light, astronomers can study the physical processes that caused the polarisation. In particular, polarisation may reveal the existence and properties of magnetic fields in the medium light has travelled through (1).

The magnetic field of the Milky Way as seen by the Planck satellite. The darker regions correspond to a stronger polarized emission and the striations indicate the direction of the magnetic field projected onto the sky plane. ESA/Planck collaboration

The map presented here was obtained using detectors on Planck that acted as the astronomical equivalent of polarised sunglasses. Swirls, loops and arches in this new image trace the structure of the magnetic field in our home galaxy, the Milky Way. This image reveals the large-scale organization of part of the galactic magnetic field. The dark band running horizontally across the centre corresponds to the Galactic Plane. Here, the polarisation reveals a regular pattern on large angular scales, which is due to the magnetic field lines being predominantly parallel to the plane of the Milky Way.

The data also reveal variations of the polarisation direction within nearby clouds of gas and dust. This can be seen in the tangled features above and below the plane, where the local magnetic field is particularly disorganised.

Areas of high galactic latitude have been hidden. The signal is lower and further work is required to measure and separate the polarization of our galaxy than the microwave background radiation.

Beyond our Galaxy

The brightness of the CMB has already been mapped by Planck in unprecedented detail and scientists are now scrutinising the data to measure the polarisation of this light. This is one of the main goals of the Planck mission, because it could provide evidence for gravitational waves generated in the Universe immediately after its birth.

In March 2014, scientists from the BICEP2 collaboration claimed the first detection of such a signal in data collected using a ground-based telescope observing a patch of the sky at a single microwave frequency. Critically, the claim relies on the assumption that foreground polarised emissions are almost negligible in this region.

Later this year, scientists from the Planck collaboration will release data based on Planck’s observations of polarised light covering the entire sky at seven different frequencies. The multiple frequency data should allow astronomers to separate with great confidence any possible foreground contamination from the tenuous primordial polarised signal. This will enable a much more detailed investigation of the early history of the cosmos, from the accelerated expansion when the Universe was much less than one second old to the period when the first stars were born, several hundred million years later.

About Planck

Launched in 2009, Planck was designed to map the sky in nine frequencies using two state-of-the-art instruments: the Low Frequency Instrument, which includes three frequency bands in the range 30–70 GHz, and the High Frequency Instrument, which includes six frequency bands in the range 100–857 GHz. HFI completed its survey in January 2012, while LFI continued to make science observations until 3 October 2013, before being switched off on 19 October 2013. The HFI instrument was designed and built under the direction of the Institut d’astrophysique spatiale (CNRS / Université Paris-Sud) with funding from CNES and CNRS.

The contribution of French research in the Planck mission

France is the leader in the high-frequency Planck-HFI instrument whose data are essential for cosmological results but also for many galactic and extragalactic results. Its building costed € 140 million and mobilized 80 researchers from ten laboratories of the CNRS, the CEA and Universities. France has provided more than 50% of funding for this construction as well as 100% of the processing of its data: this funding comes, half from the CNES, half from the CNRS, the CEA and Universities. It also helps funding the mission itself through its financial contribution to the scientific program of the ESA, i.e., 15% of the cost of the mission.

The exploitation of scientific results is mainly provided by the CNRS, including Jean-Loup Puget (IAS) as the “Principal Investigator” and François Bouchet (IAP) as the “Co-Principal Investigator.” The following French laboratories were involved in the construction then the analysis of the data coming from the HFI instrument (from raw measurements to cards by frequency) and in the astrophysical and cosmological interpretation of all Planck mission data :

  • APC, AstroParticule et cosmologie (Université Paris DiderotlCNRSlCEAlObservatoire de Paris), à Paris.
  • IAP, Institut d’astrophysique de Paris (CNRSlUPMC), à Paris.
  • IAS, Institut d’astrophysique spatiale (Université Paris-SudlCNRS), à Orsay.
  • Institut Néel (CNRS), à Grenoble.
  • IPAG, Institut de planétologie et d’astrophysique de l’Observatoire des sciences de l’Univers de Grenoble (CNRSlUniversité Joseph Fourier), à Grenoble.
  • IRAP, Institut de recherche en astrophysique et planétologie de l’Observatoire Midi-Pyrénées (Université Paul SabatierlCNRS), à Toulouse.
  • CEA-IRFU, Institut de recherche sur les lois fondamentales de l’Univers du CEA, à Saclay.
  • LAL, Laboratoire de l’accélérateur linéaire (CNRSlUniversité Paris-Sud,), à Orsay.
  • LERMA, Laboratoire d’étude du rayonnement et de la matière en astrophysique (Observatoire de ParislCNRSlENSlUniversité Cergy-PontoiselUPMC), à Paris.
  • LPSC, Laboratoire de physique subatomique et de cosmologie (Université Joseph-FourierlCNRSlGrenoble-INP), à Grenoble.
  • CC-IN2P3 du CNRS, Centre de calcul de l’Institut national de physique nucléaire et de physique des particules (IN2P3) du CNRS.

Further details

  • Website dedicated to the Planck mission :
  • In 2015, a giant stratospheric balloon of the CNES will carry at nearly 40 km altitude an experience weighing a ton or so developed by the CNRS, the CEA and the CNES, with contributions from the Universities of Rome and Cardiff. This experiment, called Pilot, will map the polarized emission of the disk of our galaxy with even more fine detail (about one-twentieth of a degree) and at a wavelength complementary to those of Planck. Further details :

Notes :

(1) The knowledge of the magnetic field of our galaxy is fundamental because it is believed to govern or influence many phenomena, such as the trajectory of electrically charged particles (cosmic rays) and star formation.

References :

The four articles whose authors are members of the Planck collaboration were submitted to the journal Astronomy & Astrophysics:

  • Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust : Consulter le site web
  • Planck intermediate results. XX. Comparison of polarized thermal emission from Galactic dust with simulations of MHD turbulence : Consulter le site web
  • Planck intermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353 GHz : Consulter le site web
  • Planck intermediate results. XXII. Frequency dependence of thermal emission from Galactic dust in intensity and polarization : Consulter le site web

IRAP Contact

Jean-Philippe Bernard l Tel : 33 5 61 55 75 38 l

Other IRAP people involved

D. Alina, T. Banday, K. Ferrière, M. Giard, T. Jaffe, L. Montier, I. Ristorcelli, A. Sauvé



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