Polarization measurements of Galactic regions

Herschel observation of the Taurus filament. The striations and subfilaments tend to be parallel to the large scale magnetic field and are suggestive of accretion flows into the star forming regions.

Magnetic fields are believed to be a major player at almost every stage of star formation but remain very poorly constrained observationally (e.g. Crutcher et al. 2010). In particular, if sufficiently strong, magnetic fields can support star-forming clouds and pre-stellar cores against gravitational collapse and therefore at least partly explain the observed inefficiency of the star formation process. Magnetic fields also provide a viable mechanism to transport angular momentum outward during core contraction and collapse (by magnetic braking), and may be required to solve the “angular momentum problem” of star formation (e.g. Mouschovias & Ciolek 1999).

In this context, a key scientific objective for the 1 mm polarization channel of the NIKA2 camera would be to clarify the role played by magnetic fields in shaping the interconnected network of filaments and pre-stellar cores revealed by Herschel in Galactic molecular clouds (e.g. André et al. 2010 ; Molinari et al. 2010 ; Arzoumanian et al. 2011). Herschel observations indicate that pre-stellar cores and protostars form primarily in dense (“supercritical”) filaments above a threshold 15 M☉/pc in mass per unit length, roughly equivalent to AV 10 in visual extinction or column density given a characteristic filament width 0.1 pc. In several cases (e.g. Palmeirim et al. 2012, Hennemann et al. 2012), the dense star-forming filaments observed with Herschel appear to be gaining mass from a network of perpendicular striations or sub-filaments, which are apparently aligned with the ambient magnetic field according to available optical polarization vectors. This is suggestive of mass accretion along field lines into the main filaments, supporting the view that the early stages of star formation are at least partly magnetically-controlled.

Mapping observations of linearly-polarized continuum emission from magnetically-aligned dust grains at mm and submm wavelengths are a powerful tool to measure the morphology and structure of magnetic field lines in star-forming clouds and dense cores (cf. Matthews et al. 2001, Crutcher et al. 2004). Indirect estimates of the magnetic field strength can be obtained using the Chandrasekhar-Fermi technique. Because the degree of polarization of dust continuum emission is low (typically 5% - e.g. Matthews et al. 2001), a systematic polarimetric study of nearby star-forming filaments and pre-stellar cores requires the large improvement in mapping speed foreseen for NIKA2 as compared to MAMBO-2. While Planck will provide polarization information on large scales (> 5’ or > 0.2 pc in the nearest clouds) between 0.85 mm and 3 mm, only a higher resolution instrument such as the proposed camera can probe the critical 0.01-0.05 pc scales ( 10’’-60’’ in the nearest clouds) at which magnetic field lines may channel the matter of interstellar filaments into growing dense cores. Likewise, BLAST- Pol, the Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry (Fissel et al. 2010), can probe linearly polarized dust emission from Galactic giant molecular clouds at 250, 350, and 500 μm, but only at modest resolution (30’’-1’).

Another science case is to map the structure of the magnetic field in nearby face-on galaxies. Such observations will complement existing studies based on radio observations of synchrotron polarization (e.g. Fletcher et al 2011, MNRAS 412, 2396). While synchrotron polarization images the field structure over the full volume of galaxies including their halo, dust traces specifically the field within the thin disk where interstellar matter is concentrated and star formation occurs.

Mis à jour le 25 octobre 2023