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Description
In a first of a series of studies of the H{alpha}+[NII] emission from nearby spiral galaxies, we present measurements of H{alpha}+[NII] emission from HII regions in M81. Our method uses large-field CCD images and long-slit spectra and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey (the BATC survey). The CCD images are taken with the National Astronomical Observatories of China (NAOC) 0.6/0.9m f/3 Schmidt telescope at the Xinglong Observing Station, using a multicolor filter set. The observations reported here were taken on 30 individual nights over the time period 1995 February 5 to 1997 February 19. Spectra of 10 of the brightest HII regions are obtained using the NAOC 2.16m telescope with a Tek 1024x1024 CCD, between 1997 April 9 and 11. The continua of the spectra are calibrated by flux-calibrated images taken from the Schmidt observations. We determine the continuum component of our H{alpha}+[NII] image via interpolation from the more accurately measured backgrounds (M81 starlight) obtained from the two neighboring (in wavelength) BATC filter images. We use the calibrated fluxes of H{alpha}+[NII] emission from the spectra to normalize this interpolated, continuum-subtracted H{alpha}+[NII] image. We estimate the zero-point uncertainty of the measured H{alpha}+[NII] emission flux to be 8%. A catalog of H{alpha}+[NII] fluxes for 456 HII regions is provided, with those fluxes being on a more consistent linear scale than previously available. The logarithmically binned H{alpha}+[NII] luminosity function of HII regions is found to have slope {alpha}=-0.70, consistent with previous results (which allowed {alpha}=-0.5 to -0.8). From the overall H{alpha}+[NII] luminosity of the HII regions, the star formation rate of M81 is found to be ~0.68M_{sun}_/yr, modulo uncertainty with extinction corrections.
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