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

Abstract

Two continuum subtracted Hα HST frames of M101 are used to determine the positions, angular sizes and absolute fluxes of 237 H Ii 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. Further support for the dominant role played by the seeing is provided by simple analytical models. We also study the clustering properties of H Ii regions and their effect on the luminosity function by construction of a Minimal Spanning Tree (MST). We find evidence for two regimes of clustering of the H Ii regions and diffuse emission. These intrinsic clustering properties in combination with the spatial resolution typically obtainable from ground based observations might be responsible for the break in the H Ii region luminosity function which is usually found at log L_Hα=38.6 erg/s, suggesting two different regimes of star formation in late type spiral galaxies. 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 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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained from the data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.