HIIphot: Automated photometry of HII regions applied to M51

We have developed a robust, automated method, hereafter designated HIIphot, which enables accurate photometric characterization of H II regions while permitting genuine adaptivity to irregular source morphology. HIIphot utilizes object recognition techniques to make a first guess at the shapes of all sources and then allows for departure from such idealized "seeds" through an iterative growing procedure. Photometric corrections for spatially coincident diffuse emission are derived from a low-order surface fit to the background after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 H II regions are detected above the 5 sigma level. A simple, weighted power-law fit to the measured Ha luminosity function (H II LF) above log L-H alpha = 37.6 gives alpha = -1.75 +/- 0.06, despite a conspicuous break in the H II LF observed near L-H alpha = 10(38.9). Our best-fit slope is marginally steeper than measured by Rand, perhaps reflecting our increased sensitivity at low luminosities and to notably diffuse objects. H II regions located in interarm gaps are preferentially less luminous than counterparts which constitute M51's grand design spiral arms and are best fitted with a power-law slope of alpha = -1.96 +/- 0.15. We assign arm/interarm status for H II regions based upon the varying surface brightness of diffuse emission as a function of position throughout the image. Using our measurement of the integrated flux contributed by resolved H rr regions in M51, we estimate the diffuse fraction to be approximately 0.45-in agreement with the determination of Greenawalt et al. Automated processing of degraded narrowband data sets is undertaken in order to gauge (distance-related) systematic effects associated with limiting spatial resolution and sensitivity.