Astronomy and Astrophysics – Astrophysics
Scientific paper
Feb 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987a%26as...67..447b&link_type=abstract
Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), vol. 67, no. 3, Feb. 1987, p. 447-481. CNR-supported research.
Astronomy and Astrophysics
Astrophysics
139
Andromeda Galaxy, Globular Clusters, Astronomical Catalogs, Galactic Structure, Spatial Distribution, Ubv Spectra
Scientific paper
The results of a new independent search for cluster candidates over an area of about 3 x 3 degrees centred on M31 are here reported together with a complete revision of all the previous lists. The new general catalog of clusters in M31 includes 353 candidates classified into two classes, A (254 objects) and B (99 objects), differing only in level of confidence : A very high, B high. Several (faint) clusters might also be present amongst the 152 objects listed in class C, which includes plausible candidates unable to achieve the A, B rank. Miscellaneous non-stellar objects (with an expected percentage of actual clusters of the order of a few percent) are finally listed in class D. The degree of homogeneity, contamination and completeness of the new sample has been widely checked (also with some spectroscopic observations) and discussed in comparison with the previous major surveys. The homogeneity should be good enough to guarantee that the level of completeness is the same over the whole surveyed area. The degree of contamination, hard to quantify without a complete spectroscopic survey, should not exceed 20 % of the total sample. It is likely that the nature of contaminating objects and their incidence are functions of the galactocentric distance. The degree of completeness is estimated to be not less than 90 % of the maximum level which can be reached with our morphological and photometric approach down to V = 18 (Mν = -6.5). Very compact clusters should represent the most important source of incompleteness in this magnitude range. The comparison with the latest major lists of candidates has given the following results: (i) Rejection of a significant number of candidates from the sample of Crampton et al. (1985). In particular, over 109 new candidates, only 2 attain our A, B rank and 27 are included in the C class. This may affect their conclusion on the absence of a gradient in the mean colors (and thus metallicity) in the cluster population with increasing distance in M31. (ii) 50 objects listed by Sargent et al. (1977) and present in our field (18 objects are external), have not been included in our classes A and B and 66 added. Most of the rejections are due to the galaxy-nature of the candidates, whilst part of the additions come from an area not covered by their survey. (iii) No conclusion can be drawn with our technique on the density of clusters within 1.5 Kpc from the nucleus (Wirth et al., 1985). In conclusion, the sample may still be affected by some systematic losses depending on cluster intrinsic structure and galactocentric distance. It should, anyway, suffer of a very limited contamination and achieve a high level of homogeneity and completeness.
Battistini P.
Bònoli Fabrizio
Borngen F.
Braccesi A.
Federici Luciana
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