Crowded-field photometry from HST-imaging*
Institut for Fysik og Astronomi, Aarhus Universitet, Ny Munkegade, DK-8000 Århus C, Denmark
Accepted: 9 May 1997
We present a thorough investigation of stellar photometry based on HST imaging of crowded fields at and 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 . 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 (, ) are found too bright by several tens of a magnitude. For the field at 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.
Key words: methods: data analysis / galaxies: elliptical / galaxies: individual: M 32 / galaxies: stellar content
© European Southern Observatory (ESO), 1998