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 Msolar yr-1, modulo uncertainty with extinction corrections.
The Astronomical Journal
- Pub Date:
- September 2003
- Galaxies: Individual: Messier Number: M81;
- Galaxies: Photometry;
- ISM: H II Regions;
- 18 pages, 7 figures, accepted for publication in the Astronomical Journal