THE GALACTIC PLANE INFRARED POLARIZATION SURVEY (GPIPS): DATA RELEASE 1

The Galactic Plane Infrared Polarization Survey (GPIPS) covers 76 deg(2) of the first Galactic quadrant midplane, 18 degrees <= l <= 56 degrees and -1 degrees <= b <= + 1 degrees, in H-band (1.6 mu m) linear polarimetry to reveal the plane-of-the-sky orientation of the magnetic field in diffuse and denser atomic and molecular clouds. The Survey consists of 3237 overlapping 10 x 10 arcmin fields observed using the Mimir instrument on the 1.8 m Perkins telescope. Here, the first community release of GPIPS data for 559 fields (17% of the Survey) is announced and basic characteristics are described. Data products consist of H-band stellar photometry and polarimetry as well as combined images. The formats and contents of the products are described and quality cuts are explored to provide insight into opportunities and limitations of the data. The Survey probes to distances as far as the Galactic bulge, revealing magnetic field properties that correlate with spiral arms and also show significant small-scale structure. The polarizations are classified into three "usage" samples, based on stellar brightness and polarimetric uncertainty. The brightest, lowest uncertainty polarizations are suitable for individual use and direct magnetic field mapping. The next two fainter samples are useful, once averaged, for probing magnetic fields to greater distances, albeit with lower resolution. Based on this release, the full GPIPS data set will number about 5.6 million stars, with more than 1 million in the high-quality sample. This increases, by many orders of magnitude, the number of polarimetric probes of the Milky Way's magnetic field.