Automatic Determination of Unbiased Luminosity Functions for H II Regions. I. The Method

Details Automatic Determination of Unbiased Luminosity Functions for H II Regions. I. The Method Automatic Determination of Unbiased Luminosity Functions for H II Regions. I. The Method

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112.1096m&link_type=abstract

Astronomical Journal v.112, p.1096

Astronomy and Astrophysics

Astronomy

12

Hii Regions, Galaxies: Ism

Scientific paper

A new method has been developed for measuring luminosity functions of H II regions in galaxies. For each H II region, the integration threshold for determining the flux in a Balmer line is set to be a fixed percentage of the peak surface brightness of the H II region. Unlike photometry where the threshold is set at a fixed surface brightness level, the resulting distribution of luminosities should be linked directly to the distribution of the rate of production of ionizing photons, regardless of the choice of percentage, because the fraction of the total flux measured for each H II region is independent of surface brightness. Another advantage is that crowded regions can be resolved objectively and automatically. The technique has been applied to continuum-free Hα images of the grand-design spiral NGC 3938 and the flocculent spiral NGC 4414. The shapes of the resultant cumulative luminosity functions are not very sensitive either to the value of the percentage adopted to define integration boundaries, to the accuracy of background subtraction, or to the image quality. For these two galaxies at least, luminosity functions created using a fixed surface brightness threshold overall agree in shape with those derived by the new technique. Even though surface brightnesses do vary systematically with luminosity, the scatter at any given luminosity appears to be large enough to hide any systematic differences between the methods.

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