H+[Nii] Observations of the HiiRegions in M81

In a first of a series of studies of the H-alpha + [NII] emission from nearbyspiral galaxies, we present measurements of H-alpha + [NII] emission from HIIregions 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. TheCCD images are taken with the NAOC 0.6/0.9m f/3 Schmidt telescope at theXinglong Observing Station, using a multicolor filter set. Spectra of 10 of thebrightest HII regions are obtained using the NAOC 2.16m telescope with a Tek1024 X 1024 CCD. The continua of the spectra are calibrated by flux-calibratedimages taken from the Schmidt observations. We determine the continuumcomponent of our H-alpha + [NII] image via interpolation from the moreaccurately-measured backgrounds (M81 starlight) obtained from the twoneighboring (in wavelength) BATC filter images. We use the calibrated fluxes ofH-alpha + [NII] emission from the spectra to normalize this interpolated,continuum-subtracted H-alpha + [NII] image. We estimate the zero pointuncertainty of the measured H-alpha + [NII] emission flux to be $\sim$ 8%. Acatalogue of H-alpha + [NII] fluxes for 456 HII regions is provided, with thosefluxes are on a more consistent linear scale than previously available. Thelogarithmically-binned H-alpha + [NII] luminosity function of HII regions isfound to have slope $\alpha$ = -0.70, consistent with previous results (whichallowed $\alpha=-0.5 \sim -0.8$). From the overall H-alpha + [NII] luminosityof the HII regions, the star formation rate of M81 is found to be $\sim 0.68M_{\odot} {\rm yr}^{-1}$, modulo uncertainty with extinction corrections.Comment: 18 pages, 7 figures, accepted for publication in the Astronomical Journa