BLAST: The Balloon-Borne Large Aperture Submillimeter Telescope

BLAST Science

Stars are the basic units of the Universe, and yet, after decades of research, much about the physical processes regulating star formation remains poorly understood. Large-scale observations of star forming regions provide counts of the number of dense clouds, each of which will eventually evolve into tens to hundreds of stars. However, when simple models of gravitational collapse are applied to the clouds they yield a Galactic star formation rate (SFR) many times larger than what is actually observed. Some process or combination of processes must be slowing the collapse of the clouds. The two prevailing theories involve turbulence, which prevents the effective dissipation of energy and Galactic magnetic fields, which are captured and squeezed by the collapsing cloud providing a mechanism for mechanical support. Distinguishing between the many different theories of STRs requires measurement of the magnetic fields in these clouds.

The new Polarized Balloon-borne Large Aperture Submillimeter Telescope (Super BLASTPol) will make groundbreaking measurements of polarized dust emission in galactic and extragalactic star forming regions. Numerical simulations have shown that magnetic fields critically affect star formation efficiency in addition to the lifetime of molecular clouds. Observationally however, the strength and morphology in these clouds is poorly constrained. A valuable tracer of magnetic fields in star forming regions is polarization. By mapping polarization from dust grains aligned with respect to their local magnetic field, the field orientation can be traced. Molecular clouds typically have temperatures of several tens of Kelvin with emission peaking in the submillimeter. Most previous submillimeter polarimetry observations have generally been restricted to bright targets (>1 Jy) and small map sizes (<0.05 deg2).

Above: Preliminary results from the BLAST-Pol 2010 flight. The lines indicate the inferred magnetic field direction, with the length of the line proportional to the percentage of the dust emission which is polarized. Left: A map made from 3 hours of BLASTPol data on the Carina Nebula. The color map is the 500 μm total intensity, white vectors are BLAST-Pol measurements at 500μm (smoothed to 30' resolution), and black vectors are measurements from the SPARO polarimeter at 450μm (Li et al., 2006a). Right: A 1.4 deg2 map of the nearby Vela C molecular cloud at 250μm (blue), 350μm (green) and 500μm (red), plotted over a Herschel SPIRE 350μm intensity map (Hill et al., 2011).

BLASTPol and its successor, Super BLASTPol, are the first instruments to combine the sensitivity and mapping speed necessary to trace magnetic fields across entire clouds with the resolution to trace fields down into dense substructures, including cores and filaments. BLASTPol maps polarization at 250, 350 and 500 μm, with a diffraction-limited beam FWHM of 30" at 250 μm. Because of this, BLASTPol provides the critical link between the PLANCK all-sky polarization maps with 5' resolution and ALMA's ultra-high resolution, but only a 20" field of view.

Super BLASTPol will use the PLANCK data to refine its target selection. ALMA will then be able to utilize Super BLASTPol maps to "zero in" on areas of particular interest. Together, these three instruments will be able to probe the inner workings of star formation with previously unreachable resolution, sensitivity and scope.

The above left is a Galactic-scale Planck image followed by the BLAST observation of Vela (Netterfeld et al., 2009) and the magnetic eld map for a protobinary in Perseus acquired using the Submillimeter Array (a precursor to ALMA) (Girart et al., 2006).