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A&A Supplement series, Vol. 127, January II 1998, 327-333

Received January 31; accepted May 9, 1997

Crowded-field photometry from HST-imaginggif

M. Sodemann and B. Thomsen

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Institut for Fysik og Astronomi, Aarhus Universitet, Ny Munkegade, DK-8000 Århus C, Denmark


We present a thorough investigation of stellar photometry based on HST imaging of crowded fields at tex2html_wrap_inline1084 and tex2html_wrap_inline1086 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 tex2html_wrap_inline1084. 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 (tex2html_wrap_inline1090, tex2html_wrap_inline1092) are found too bright by several tens of a magnitude. For the field at tex2html_wrap_inline1086 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.

keywords: methods: data analysis -- galaxies: elliptical -- galaxies: individual: M 32 -- galaxies: stellar content

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