The Wisconsin Hα Mapper Northern Sky Survey

Abstract

The Wisconsin Hα Mapper (WHAM) has surveyed the distribution and kinematics of ionized gas in the Galaxy above declination -30°. The WHAM Northern Sky Survey (WHAM-NSS) has an angular resolution of 1° and provides the first absolutely calibrated, kinematically resolved map of the Hα emission from the warm ionized medium (WIM) within ~+/-100 km s-1 of the local standard of rest. Leveraging WHAM's 12 km s-1 spectral resolution, we have modeled and removed atmospheric emission and zodiacal absorption features from each of the 37,565 spectra. The resulting Hα profiles reveal ionized gas detected in nearly every direction on the sky with a sensitivity of 0.15 R (3 σ). Complex distributions of ionized gas are revealed in the nearby spiral arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the WHAM-NSS provides information about the WIM out to the tangent point down to a few degrees from the plane. Ionized gas is also detected toward many intermediate velocity clouds at high latitudes. Several new H II regions are revealed around early B stars and evolved stellar cores (sdB/O). This work presents the details of the instrument, the survey, and the data reduction techniques. The WHAM-NSS is also presented and analyzed for its gross properties. Finally, some general conclusions are presented about the nature of the WIM as revealed by the WHAM-NSS.