The impact of resolution on observed HII region properties from WFPC2 observations of M 101

Two continuum subtracted H alpha HST frames of M101 are used to determine t he positions, angular sizes and absolute fluxes of 237 H II regions using a semi-automated technique. From these we have constructed the luminosity an d diameter distribution functions. We repeat this process on the images after artificially reducing the linear resolution to that typically obtained with ground based imaging. We find s ubstantial differences in the luminosity function and diameter distribution . The measured internal properties, such as central surface brightness and radial gradient are dominated by the PSF at linear resolutions less than ro ughly 40 pc FWHM. From the ground such resolutions are currently only obtai nable for the nearest galaxies. Further support for the dominant role played by the seeing is provided by s imple analytical models. We also study the clustering properties of H II re gions and their effect on the luminosity function by construction of a Mini mal Spanning Tree (MST). We find evidence for two regimes of clustering of the H II regions and diffuse emission. These intrinsic clustering propertie s in combination with the spatial resolution typically obtainable from grou nd based observations might be responsible for the break in the H II region luminosity function which is usually found at logL(H alpha) = 38.6 erg/s, suggesting two different regimes bf star formation in late type spiral gala xies. From the high resolution HST data we find a luminosity function slope of al pha = -1.74 +/- 0.08. We also observe a flattening at luminosities logL < 3 6.7 erg/s. For the diameter distribution we find a characteristic scale of D-0 = 29.2 pc from an exponential fit. However, a scale free power law with index <beta> = -2.84 +/- 0.16 provides a better fit to the data.