HIIPHOT: Automated Photometry of H II 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 σ level. A simple, weighted power-law fit to the measured Hα luminosity function (H II LF) above logL_Hα=37.6 gives α=-1.75+/-0.06, despite a conspicuous break in the H II LF observed near L_Hα=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 α=-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 II 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.