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 utili zes object recognition techniques to make a first guess at the shapes of al l sources and then allows for departure from such idealized "seeds" through an iterative growing procedure. Photometric corrections for spatially coin cident diffuse emission are derived from a low-order surface fit to the bac kground after exclusion of all detected sources. We present results for the well-studied, nearby spiral M51 in which 1229 H II regions are detected ab ove 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 = -1.7 5 +/- 0.06, despite a conspicuous break in the H II LF observed near L-H al pha = 10(38.9). Our best-fit slope is marginally steeper than measured by R and, perhaps reflecting our increased sensitivity at low luminosities and t o notably diffuse objects. H II regions located in interarm gaps are prefer entially less luminous than counterparts which constitute M51's grand desig n 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 vary ing surface brightness of diffuse emission as a function of position throug hout the image. Using our measurement of the integrated flux contributed by resolved H rr regions in M51, we estimate the diffuse fraction to be appro ximately 0.45-in agreement with the determination of Greenawalt et al. Auto mated processing of degraded narrowband data sets is undertaken in order to gauge (distance-related) systematic effects associated with limiting spati al resolution and sensitivity.