Hα + [N II] observations of the H II regions in M81

In a first of a series of studies of the Hα + [N II] emission from nearby spiral galaxies, we present measurements of Hα + [N II] emission from H II 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) 0.6/0.9 m f/3 Schmidt telescope at the Xinglong Observing Station, using a multicolor filter set. Spectra of 10 of the brightest H II regions are obtained using the NAOC 2.16 m telescope with a Tek 1024 × 1024 CCD. The continua of the spectra are calibrated by flux-calibrated images taken from the Schmidt observations. We determine the continuum component of our Hα + [N II] 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 Hα + [N II] emission from the spectra to normalize this interpolated, continuum-subtracted Hα + [N II] image. We estimate the zero-point uncertainty of the measured Hα + [N II] emission flux to be ∼8%. A catalog of Hα + [N II] fluxes for 456 H II regions is provided, with those fluxes being on a more consistent linear scale than previously available. The logarithmically binned Hα + [N II] luminosity function of H II regions is found to have slope α = -0.70, consistent with previous results (which allowed α = -0.5 to -0.8). From the overall Hα + [N II] luminosity of the H II regions, the star formation rate of M81 is found to be ∼0.68 M_⊙ yr^-1, modulo uncertainty with extinction corrections.