HST Paschen-alpha Survey of the Galactic Center: Data Reduction and Products

Our HST/NICMOS Paschen-� survey of the Galactic center, first introduced in Wang et al. (2010), provides a uniform, panoramic, high-resolution map of stars and ionized diffuse gas in the central 416 arcmin 2 of the Galaxy. This survey was carried out with 144 HST orbits using two narrow-band filters at 1.87 µm and 1.90 µm in NICMOS Camera 3. In this paper, we describe in detail the data reduction and mosaicking procedures followed, including background level matching and astrometric corrections. We have detected � 570,000 nearIR sources using the ‘Starfinder’ software and are able to quantify photometric uncertainties of the detections. The source detection limit varies across the survey field but the typical 50% completion limit is � 17th mag (Vega System) in the 1.90 µm band. A comparison with the expected stellar magnitude distribution shows that these sources are primarily Main-Sequence massive stars (& 7M⊙) and evolved lower mass stars at the distance of the Galactic center. In particular, the observed source magnitude distribution exhibits a prominent peak, which could represent the Red Clump (RC) stars within the Galactic center. The observed magnitude and color of these RC stars support a steep extinction curve in the near-IR toward the Galactic center (Nishiyama et al. 2009). The flux ratios of our detected sources in the two bands also allow for an adaptive and