The Impact of Resolution on Observed HII Region Properties from WFPC2   Observations of M101
  Two continuum subtracted H-alpha HST frames of M101 are used to determine the positions, angular sizes and absolute fluxes of 237 HII regions using a semi-automated technique. From these we have constructed the luminosity and 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 substantial 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 roughly 40 pc FWHM. From the ground such resolutions are currently only obtainable for the nearest galaxies.   We find evidence for two regimes of clustering of the HII regions and diffuse emission, suggesting two different regimes of star formation in late type spiral galaxies. This property in combination with the blending that occurs at ground based resolutions might be responsible for the break in the luminosity function which is usually found at log L=38.6 erg/s.   From the high resolution HST data we find a luminosity function slope of alpha=-1.74+/-0.08. We also observe a flattening at luminosities log L<36.7 erg/s. For the diameter distribution we find a characteristic scale of D0=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. 
P.O. Pleuss (1), C.H. Heller (2), K.J. Fricke (1) ((1)   Universitaetssternwarte, Goettingen, Germany, (2) Georgia Southern ...