The galactic plane infrared polarization survey (GPIPS)

The scientific motivation, data collection strategy, data reduction, and analysis methods are presented for the Galactic Plane Infrared Polarization Survey (GPIPS). The chief goal for the Survey was to reveal the nature of the magnetic field threading the Galactic disk, in particular through regions of low to moderate extinction (1-20mag of A _V ) and star formation in the cool interstellar medium. The Survey region spans 76deg ² of the northern Milky Way disk, from ℓ = 18° to 56° and b =-1° to +1°. Linear polarimetric imaging observations began in 2006 in the near-infrared H band (1.6 μm) using the Mimir instrument on the 1.8m Perkins telescope, located outside Flagstaff, AZ. Mimir used a cold, fixed wire grid and a rotateable cold, compound half-wave plate to obtain "step-and- integrate" polarimetry over its full 10 × 10 arcmin field of view. The GPIPS bright and faint polarimetric limits are approximately 7th and 15thmag, respectively, set by saturation and photon noise. Polarimetric uncertainties track with stellar magnitude, from about 0.1% to 25%, on average, from the brightest to faintest stars. Across the 3237 field GPIPS region, approximately 0.5 million stars are estimated to show detectable linear polarization (P/σ _P > 3); most of these have m _H < 12. This represents many orders of magnitude improvement in the number of polarization measurements across this region. GPIPS observations are more than 90% complete and should finish in 2012.