Ultraviolet imaging polarimetry of the Large Magellanic Cloud. I. Observations

We have used the rocketborne Wide-Field Imaging Survey Polarimeter (WISP) to image a 1°.5 × 4°.8 area of the western side of the Large Magellanic Cloud (LMC) at a wavelength of λ = 2150 Å and a resolution of 1′ × 1′.5. These are the first wide-field ultraviolet polarimetric images in astronomy. We find the UV background light of the LMC to be linearly polarized at levels ranging from our sensitivity limit of 4% to as high as ≈40%. In general, the polarization in a pixel increases as the flux decreases; the weighted mean value of polarization across the WISP field is 12.6% ± 2.3%. The LMC's diffuse UV background, in uncrowded areas, rises from a minimum of (5.6 ± 3.1) × 10^-8 ergs s^-1 cm^-2 Å^-1 sr^-1 (23.6 ± 0.5 mag arcsec^-2) to (9.3 ± 1.1) × 10^-8 ergs s^-1 cm^-2 Å^-1 sr^-1 (23.1 ± 0.2 mag arcsec^-2) in regions near the bright associations. We use our polarization maps to investigate the geometry of the interstellar medium in the LMC and to search for evidence of a significant contribution of scattered light from OB associations to the diffuse galactic light of the LMC. Through a statistical analysis of our polarization map, we identify nine regions of intense UV emission which may be giving rise to scattering halos in our image. We find that starlight from the N11 complex and the LH 15 association are the strongest contributors to the scattered light component of the LMC's diffuse galactic light. This region of the northwestern LMC can be thought of as a kiloparsec-scale reflection nebula in which OB stars illuminate distant dust grains that scatter the light into our sight line. In contrast, the polarization map does not support the scattering of light from the large B2 complex in the southern WISP field; this effect may be astrophysical, or it may be the result of bias in our analysis.