J.-Ph. Bernard CESR, Toulouse and a few others (having looked at polarized Planck data)

1. Galactic Science with B-pol ? J.-Ph. Bernard CESR, Toulouseand a few others (having looked at polarized Planck data) J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

2. 1 Dust Polarization • Elongated grains rotate and relax to rotation // to magnetic field B • polarized extinction // to B, polarized emission orth to B • The same grains are reponsible for polarized extinction and emission • p=polarization degree (p~few %)Y=polarization angle ([-90°, 90°]) • Models predict mild to no variations of p with l in the Submm • This is because only large grains rotate (and therefore align). Transition a~0.1 mm • p(l) in submm is sensitive to grain model: shape, composition size distribution • Comparison with extinction polarization in the Visible promizing (same as for emission) Same b/a SphericalGraphite Draine & Fraisse 2009 Polarization is a new dimension to the study of dust and will get us information not accessible via intensity: dust shape, composition, alignment, magnetic properties, …) J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

3. Planck limitations 3 5’ resolution • Simulations by Pelkonen et al. 2007, based on MHD simulations + dust model • Noise only (no systematic effects) • At 353 GHz (Planck polarized channel most sensitive to dust), the accuracy is limited to Dp/p=12% (Dc=10°) at 15’ resolution for Av=4 mag (NH>8 1021 H/cm2) • Diffuse clouds (Av<1 mag) have Dp/p > 40 % (Dc > 30°) at 15’ resolution Pelkonen et al. 2007 353 GHz, 15’ resolution 15’ resolution J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

4. Planck limitations 4 • The Planck data will therefore not allow a study of dust polarization in diffuse clouds below a resolution of a few degrees. • This is true also in the most covered regions of the Planck sky (deep fields) • This is confirmed by the preliminary analysis of the actual Planck data • This is even more true at lower frequencies, preventing to study variation with wavelengths • For comparison, visible studies reach Dp/p=10% for Av=0.05 mag Therefore, apart from statistical studies, no detailed study of the dust polarization and B field structure will be possible in the diffuse clouds (Av < 1 mag) Pelkonen et al. 2007 353 GHz, 15’ resolution This is strictly due to noise, so the exquisite B-pol sensitivity could greatly improve the situation VIS J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

5. Why is diffuse ISM important ? 5 Diffuse is simpler: • Diffuse ISM offers better defined dust excitation conditions than bright regions: - Intensity: diffuse UV radiation field is known - Polarization: de-polarization due to tangled B-field is less • For this simple reason, the study of dust emission has always been easier in the diffuse ISM • It is likely that the same will be true in polarization IRAS color variationsin the diffuse ISM Diffuse allows extinction measurements: Extinction polarizationin the diffuse ISM • There is a lot to learn about dust structure (e.g. elongation) and composition (e.g. metallic inclusion favoring alignment with B-field) from the comparison between polarized visible extinction and polarized FIR/mm emission. • Polarization in absorption is difficult toward dense regions and is becoming sensitive to low Av Diffuse is central to B-mode component separation: • For the detection of B-modes, component separation in diffuse regions is likely to be important, and very little will be known until the B-pol measurements themselves J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

6. Gal. Science with B-pol ? 6 • YES, there is useful Galactic Science to be performed with B-pol • The gain in sensitivity with respect to Planck will allow for the first time to study dust polarization and magnetic field orientation at the B-pol resolution (whatever it’ll be) in the diffuse ISM • The gain with respect to Planck will be the highest for the highest angular resolution of B-pol (so we strongly prefer 10’) • Dust Physics: evidence possible variations of dust polarization with wavelengths, and spatial variations due to column densities and/or grain properties, … • Magnetic field structure: reveal the magnetic structure of the solar neighborhood (Local bubble, loops, galactic fountains, …) • The above will only be possible statistically with Planck • There is probably a similar argument for Synchrotron • There is probably a similar argument for spinning dust polarization • These aspects are strongly linked to component separation for B-pol, since dust polarization at high latitude will be a major foreground Maybe the best proof of this is that we are constructing balloonexperiments to do just that … J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

7. PILOT PILOT 7 http://pilot.cesr.fr • A stratospheric balloon experiment • Science Objective: measure linear polarization of dust emission in the FIR • Reveal the structure of the magnetic field • Geometric and magnetic properties of dust grains- Understand Polarized foreground • Complement Planck observations at l<850 mm- Reach better accuracy: up to 10-50 times better than Planck for the same dust column density- Reach slightly higher angular resolution • Observations:Galactic plane (|b|<20°) and diffuse MIS (cirrus). • Caracteristics:l=240 & 550 mm, resolution: 3’ • Weight: ~ 1 ton • Altitude: ~ 40 km • Status: Financed by CNES. Currently in assembling phase • Schedule: First flight end 2011 J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

8. PILOT Sensitivity Planck: polarized Planck: unpolarized PILOT 8 • Pilot sensitivity to a given dust column density can be 10-50 times better than Planck, thanks to: • Higher frequency (more dust emission) • More detectors (although less sensitive) • Scanning strategy allowing deep fields • And despite: • Much less observing time • Less sensitive bolometers Note: Planck+Pilot will tell how dust polarization varies with frequency.This will be important to define where to put the foreground channels of B-pol(higher frequency=smaller =cheaper) and how to extrapolate to CMB frequencies J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010

9. Conclusions 9 • Polarization is currently bringing a new dimension to the study of dust, allowing to measure quantities not accessible from intensity only (like dust grain elongation, magnetic properties, …) • Dust polarization allows to trace the ISM magnetic field structure • It is likely (from similarity with intensity) that the most useful information on dust will come from the diffuse ISM • Planck will be limited to dust clouds with Av>4 mag (NH>8 1021 H/cm2) at 15’ resolution at 353 GHz. Worse at lower frequencies. What will it look likein polarization ? Galactic Science with B-pol ?: YES !The B-pol sensitivity will offer the possibility to study for the first time dust polarizationand B-field distribution in diffuse dust clouds (low Av) This galactic science will be most relevant for B-pol component separation This could be complementary to current and future polarization experiments in the FIR This science does not need all sky coverage (MW plane will not benefit) but would benefit from the highest angular resolution and reasonable frequency coverage Maybe it calls for a truly high frequency channel … Possible difficulties (for a proposal): - Hard to predict what will come out from Planck about dust polarization - What will be able to show in a proposal about the Planck polarization ?(this question will probably apply to other aspects too). J.-Ph. Bernard, B-pol meeting, Paris, July 29th 2010