Transcript Document

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
The Turbulent Interstellar Medium
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.
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
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).
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.
Far-IR Spectropolarimetry and CMB Foregrounds
Context for CALISTO:
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.
• 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:
Concluding Remark / References
• CALISTO can measure the polarization of very low column densities in 1 hour: AV > 0.002.
The authors are continuing a study of the science and technical case for far-infrared polarimetry with CALISTO;
your input is sought: [email protected] .
• Alternatively, CALISTO can survey polarization over 1 square degree where AV > 1 in 4 hours (3232 detector).
[1]M.
• 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%
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AAS Poster 136.05, January 2008