Globular Clusters in M31 with the Hubble Space Telescope

Abstract

The study of the globular clusters in the Galaxy has provided a wealth of essential information on stellar evolution and dynamics, as well as on the formation and early evolutionary stages of the Galaxy itself. With the use of the Hubble Space Telescope for the first time the same type of studies can be applied to a globular cluster system beyond the halo of our Galaxy, namely to the closest giant spiral galaxy M31. Because of the HST optical aberration, however, accurate photometry of individual stars for a Colour-Magnitude Diagram has not been possible so far, and only surface brightness profiles could be obtained. We have observed thirteen globular clusters in M31 with the HST FOC/96 and F430W (B band) filter. Excellent surface brightness photometry was obtained. Accurate profiles and structural parameters were derived using three independent PSF deconvolution methods (i.e. Regularized Multi-Gaussian, Richardson-Lucy and Maximum Entropy). Our conclusions are: 1. The three deconvolution methods yield perfectly consistent results as far as the surface brightness profiles are concerned. This opens the possibility of morphological studies of M31 globular clusters comparable to the ground-based studies of globulars in the Galaxy. 2. Care must be taken for the conservation of flux when absolute photometry is performed. 3. One of our M31 clusters shows a clear PCC morphology: this is the first such detection outside the immediate vicinity of the Galaxy, and was made possible by the superior HST space resolution (see Bendinelli et al. 1993 for more details). 4. The structural parameters (i.e. core radius, half-light radius and central surface brightness) derived for the 13 globulars in M31 have been compared with: a) the results obtained from ground-based observations. The ground-based core radii tend to be systematically larger by ~ 30% or more, unless accurate seeing deconvolution is performed; b) the parameters obtained for Galactic globular clusters. They all agree very well, suggesting that the two families of clusters consist of very similar objects. The complete results and detailed analysis and discussion are presented in a forthcoming paper (Fusi Pecci et al. 1993).

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