Crowded-field photometry from HST-imaging

Abstract

We present a thorough investigation of stellar photometry based on HST imaging of crowded fields at 85arcsec and 10arcsec from the centre of the high-surface brightness elliptical galaxy M 32. The Principal Investigators of the present archive data have elsewhere presented an impressive colour-magnitude diagram of the field at 85arcsec . Based on the same data we enlarge on their photometric analysis and supplement with error estimators that more clearly show the implications of severe image crowding on the stellar photometry. We show that the faintest stars (I ga 25.0, V ga 26.0) are found too bright by several tens of a magnitude. For the field at 10arcsec we conclude that it is not possible to obtain reliable stellar photometry, standard deviations being larger than 0.4 mag. Artificial-star experiments show that only very few of the brightest stars of the luminosity function can be expected to represent single objects, the majority being either spurious or not as bright as measured. The paper as such introduces and demonstrates basic guide lines which may be used when dealing with stellar photometry of severely crowded fields. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under the NASA contract NAS 5-26555.