Mapping Galactic Clouds with the Balloon-Borne Large ApertureSubmillimeter Telescope (BLAST)

Williams, Paul Andrew

doi:https://doi.org/10.21985/n2-7jc0-cq19

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Mapping Galactic Clouds with the Balloon-Borne Large ApertureSubmillimeter Telescope (BLAST)

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One of the most pressing open questions in star formation is the role of magnetic fields relative to turbulence and gravity, and how these three processes set the stellar initial mass function and star formation efficiency. In this work, we present the state of BLAST-TNG (the Balloon-borne Large Aperture Submillimeter Telescope - The Next Generation) a polarimeter built to measure magnetic fields in molecular clouds, and new results obtained by comparing polarimetric observations from BLASTPol (The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry), PolKa (English name: Polarimeter for bolometer cameras), and ALMA (Atacama Large Millimeter/submillimeter Array). The role of magnetic fields in star formation remains an open question because magnetic field strength is difficult to measure directly. Measurements of Zeeman splitting are the only method to measure magnetic field strength directly in molecular clouds, but these measurements are very difficult to make and have yielded only a few tens of detections (Crutcher, 2012). Indirect measurements, which only measure magnetic field direction, are significantly simpler. These measurements have been made for entire molecular clouds at moderate resolution (Fissel et al., 2016), and for the entire sky at low resolution by Planck (Lamarre et al., 2010; Planck Collaboration et al., 2014, 2015). Magnetic field direction can be mapped using dust emission polarimetry. Aspherical dust grains in the interstellar medium tend to align with their long axes perpendicular to the magnetic field, and emit light that is polarized along their long axis. Therefore, to measure magnetic field direction, we can rotate measured polarization vectors by 90 degrees. These dust grains are cold (15-50 K), therefore this emission peaks in the submillimeter. The BLAST Collaboration has built several instruments over the past two decades, two of which (BLASTPol, Galitzki et al. (2014) and BLAST-TNG, Lourie et al. (2018a)) were polarimeters built to study the role of magnetic fields in star formation. This work describes the flight history of the BLAST instruments, the technological advancements that made BLAST-TNG possible, the pointing system (of which key components were developed at Northwestern), and several analyses completed after the flight. We also describe some recent work which compares magnetic field measurements across three orders of magnitude in spatial scale, which required observations from BLASTPol, PolKa, and ALMA. We find that the densest part of Vela C South Ridge contains a star forming core threaded by magnetic fields that have a consistent direction across all spatial scales measured. This indicates that the magnetic field has energy density at least as great as that in turbulent gas motion (Ostriker et al., 2001).

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