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An upper limit to polarized submillimetre emission in Arp 220




We report the results of pointed observations of the prototypical ultraluminous infrared galaxy (ULIRG) Arp 220 at 850 μm using the polarimeter on the Submillimetre Common User Bolometer Array instrument on the James Clerk Maxwell Telescope. We find a Bayesian 99 per cent confidence upper limit on the polarized emission for Arp 220 of 1.54 per cent, averaged over the 15-arcsec beam-size. Arp 220 can serve as a proxy for other, more distant such galaxies. This upper limit constrains the magnetic field geometry in Arp 220 and also provides evidence that polarized ULIRGs will not be a major contaminant for next-generation cosmic microwave background polarization measurements.