The Turbulent Interstellar Medium

1. Prospects for Studying Interstellar Magnetic Fields with a Far-Infrared Polarimeter for CALISTO/SAFIR C. Darren Dowell (Jet Propulsion Laboratory, California Institute of Technology), David T. Chuss (NASA/Goddard), Jessie L. Dotson (NASA/Ames) Abstract: Polarimetry at mid-infrared through millimeter wavelengths using airborne and ground-based telescopes has revealed magnetic structures in dense molecular clouds in the interstellar medium, primarily in regions of star formation. Furthermore, spectropolarimetry has offered clues about the composition of the dust grains and the mechanism by which they are aligned with respect to the local magnetic field. The sensitivity of the observations to date has been limited by the emission from the atmosphere and warm telescopes. A factor of 1000 in sensitivity can be gained by using instead a cold space telescope. With 5 arcminute resolution, Planck will make the first submillimeter polarization survey of the full Galaxy early in the next decade. We discuss the science case for and basic design of a far-infrared polarimeter on the CALISTO/SAFIR space telescope, which offers resolution in the few arcsecond range and wavelength selection of cold and warm dust components. Key science themes include the formation and evolution of molecular clouds in nearby spiral galaxies, the magnetic structure of the Galactic center, and interstellar turbulence. Dust Polarimetry as a Tracer of Magnetic Fields Asymmetric dust grains are aligned with respect to interstellar magnetic fields. Maps of their polarized emission and absorption are therefore maps of magnetic field structure, which provide a unique diagnostic of the energetics of and structure formation within molecular clouds. The Turbulent Interstellar Medium Magnetic field strengths can be estimated from far-IR polarization angle dispersion using the Chandrasekhar-Fermi method[9,10,11]. These MHD simulations[11] illustrate: (left) strong field/small dispersion; (right) weak field/large dispersion. With as many as one million polarization measurements output from CALISTO, it will be possible to compute polarization power spectra to compare to models[11]. Other Galaxies The sensitivity from space will allow for the first time the mapping over large areas of far-IR polarization from “ordinary” galaxies. 350 m polarization map of Orion from the ground-based Caltech Submm. Observatory[1,2], confirming three features noted previously[3]: 1) Magnetic fields (red vectors and green lines) perpendicular to the Orion ridge, 2) Hourglass morphology of the field lines, consistent with gravitational contraction, 3) Central concentration of the polarized flux (color scale), implying aligned grains deep in the cloud. Far-IR/submm polarimetry of Galactic center clouds from suborbital telescopes[4,2]. (Color scale: 850 m[5], contours: 90 cm[6].) The magnetic field in the dense ISM shows complex structure, seemingly opposed to that in the ionized gas[6]. Space far-IR polarimetry will allow this entire region to be mapped in ~1 hour. Simulated CALISTO 100 m image and magnetic field map of the Andromeda Galaxy, resulting from 50 hours of observing. The color scale is simply the Spitzer 24 m image[12], which has the same 6˝ angular resolution as CALISTO at 120 m. Magnetic field vectors are placed where the 100 m flux (interpolated from Spitzer maps[12]) is ≥20 MJy/sr, which corresponds to AV ≥ 0.5 and for which CALISTO achieves (P) ≤ 0.3% in this survey. The simulated magnetic field is a simple azimuthal model plus “white noise” dispersion. The angular resolution is sufficient to discern individual Giant Molecular Clouds. • CALISTO and Polarimetry • CALISTO: The CALISTO concept for the Single-Aperture Far-InfraRed mission is described by Lester et al. (poster 136.06). CALISTO is a ~5 m astronomical background limited far-IR telescope, ~1000 times more sensitive than Spitzer and Herschel, which could be launched in ~2020. A 50-500 m imaging polarimeter is under consideration for CALISTO’s instrument suite. • Context for CALISTO: • Planck will have mapped 850 m polarization of the entire Galaxy (where AV > 4) with 300˝ resolution*. • ALMA will make very high-resolution 450 m polarization maps -- over 20˝ fields of view -- of disks, envelopes, and cloud cores where AV > 200*. • SOFIA will reopen the far-IR band to polarimetry, mapping magnetic fields in arcminute-sized clouds with AV>4. • SPICA will begin a new era of far-IR astronomy with cold space telescopes, offering imaging and spectroscopy. • CALISTO polarimetry: • CALISTO can measure the polarization of very low column densities in 1 hour: AV > 0.002. • Alternatively, CALISTO can survey polarization over 1 square degree where AV > 1 in 4 hours (3232 detector). • Measuring linear far-IR polarization is straightforward and is usually accomplished with a polarization-sensitive detector and a polarization modulator such as a quartz half-wave plate[7] or reflective variable phase-delay modulator[8]. • *basis for Planck calculation: Tcmb/Tcmb = 3.010-5 in Q (Planck Scientific Programme, p. 4), Tdust = 20 K, AV/(850 m) = 22000, (P) ≤ 0.3% • *basis for ALMA calculation: 0.1 mJy rms in 1 hr (ALMA web site), 1” beam, Tdust = 20 K, AV/(450 m) = 6000, (P) ≤ 0.3% Magnetic field maps of nearby spiral galaxies such as M31 will allow tests of GMC formation processes such as swing amplification[13] (left) and magneto-Jeans instability[14] (right). Far-IR Spectropolarimetry and CMB Foregrounds A CALISTO polarimeter will provide the unique opportunity to study the polarization spectrum of the entire sky, including the tenuous Galactic cirrus. The polarization spectrum can be used to constrain dust grain physics and to probe the environs of molecular clouds[15]. CALISTO will provide short-wavelength complementarity to the Inflation Probe. Understanding the polarization of the Galactic cirrus is critical to extracting the small polarized signature of Inflation from the Cosmic Microwave Background. Concluding Remark / References The authors are continuing a study of the science and technical case for far-infrared polarimetry with CALISTO; your input is sought: cdd@submm.caltech.edu . [1]M. Houde, et al. 2004, ApJ, 604, 717 [9]E. Ostriker, et al. 2001, ApJ, 546, 980 [2]J. L. Dotson, et al. 2008, ApJS, submitted [10]F. Heitsch, et al. 2001, ApJ, 561, 800 [3]D. A. Schleuning. 1998, ApJ, 493, 811 [11]D. Falceta-Gonçalves, et al. 2008, ApJ, submitted [4]D. T. Chuss, et al. 2003, ApJ, 599, 1116 [12]K. D. Gordon, et al. 2006, ApJ, 638, L87 [5]D. Pierce-Price, et al. 2000, ApJ, 545, L121 [13]W.-T. Kim & E. C. Ostriker 2001, ApJ, 559, 70 [6]T. N. LaRosa, et al. 2000, ApJ, 119, 207 [14]W.-T. Kim & E. C. Ostriker 2006, ApJ, 646, 213 [7]G. Novak, et al. 1989, PASP, 101, 215 [15]J. E. Vaillancourt 2002, ApJS, 142, 53 [8]D. T. Chuss, et al. 2006, Applied Optics, 45, 5107 AAS Poster 136.05, January 2008