An updated survey of globular clusters in M 31 - III. A spectroscopic metallicity scale for the Revised Bologna Catalog

摘要

Aims. We present a new homogeneous set of metallicityestimates based on Lick indices for the old globular clusters of theM31 galaxy. The final aim is to add homogeneous spectroscopicmetallicities to as many entries as possible of the Revised BolognaCatalog of M31 clusters, by reporting Lick index measurements from any source (literature, new observations,etc.) on the same scale. Methods. New empirical relations of [Fe/H] as a function of[MgFe] and Mg2indices are based on the well-studied galacticglobular clusters, complemented with theoretical model predictions for .Lick indices for M31 clusters from various literature sources(225clusters) and from new observations by our team(71clusters) have been transformed into the Trager etal.system, yielding new metallicity estimates for 245globularclusters ofM31. Results. Our values are in good agreement with recent estimatesbased on detailed spectral fitting and with those obtained from colormagnitude diagrams of clusters imaged with the Hubble Space Telescope.The typical uncertainty on individual estimates is dex, as resulted from the comparison with metallicities derived from color magnitude diagrams of individual clusters. Conclusions. The metallicity distribution of M31 globularcluster is briefly discussed and compared with that of theMilkyWay. Simple parametric statistical tests suggest that thedistribution is probably not unimodal. The strong correlation betweenmetallicity and kinematics found in previous studies is confirmed. Themost metal-rich GCs tend to be packed into the center of the system andto cluster tightly around the galactic rotation curve defined by theHIdisk, while the velocity dispersion about the curve increaseswith decreasing metallicity. However, also the clusters with display a clear rotation pattern, at odds with their MilkyWay counterparts. Key words: galaxies: individual: M31 - galaxies: star clusters - catalogs - Local Group