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A Near-Infrared Survey of the Inner Galactic Plane for Wolf-Rayet Stars III. New Methods: Faintest WR Stars

A new method of image subtraction is applied to images from a J, K, and narrow-band imaging survey of 300 square degrees of the plane of the Galaxy, searching for new Wolf-Rayet stars. Our survey spans 150 degrees in Galactic longitude and reaches 1 degree above and below the Galactic plane. The survey has a useful limiting magnitude of K =15 over most of the observed Galactic plane, and K =14 (due to severe crowding) within a few degrees of the Galactic center. The new image subtraction method described here (better than aperture or even point-spread-function photometry in very crowded fields) detected several thousand emission-line candidates. In June and July 2011 and 2012, we spectroscopically followed up on 333 candidates with MDM-TIFKAM and IRTF-SPEX, discovering 89 emission-line sources. These include 49 Wolf-Rayet stars, 43 of them previously unidentified, including the most distant known Galactic WR stars, more than doubling the number on the far side of the Milky Way. We also demonstrate our survey's ability to detect very faint PNe and other NIR emission objects.