H II regions as extragalactic distance indicators. I - Observations and structural considerations

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Astrometry, H Alpha Line, H Ii Regions, Nebulae, Astronomical Photography, Astronomical Photometry, Hydrogen Ions

Scientific paper

H-alpha photographic and photometric data have been obtained for 40 of the late-type galaxies measured by Sandage and Tammann (1974) in their calibration of the extragalactic distance scale. Microphotometric maps derived for the largest three H II regions in each galaxy are used to examine the physical reality and consistency of their subjective core and halo H II region diameters. While the structure of many nebulae is consistent with the visual data, a majority exhibit a more complex morphology which precludes the definition of meaningful metric diameters, visually or otherwise. Microphotometric metric diameters are found to be very susceptible to distance-dependent systematic errors, but carefully chosen and seeing-corrected isophotal diameters appear to offer a superior alternative as standard yardsticks.

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