THE LAMOST SPECTROSCOPIC SURVEY OF STAR CLUSTERS IN M31. II. METALLICITIES, AGES, AND MASSES

Abstract

We select from Paper I a sample of 306 massive star clusters observed with the Large Sky Area Multi–Object Fibre Spectroscopic Telescope (LAMOST) in the vicinity fields of M31 and M33, and determine their metallicities, ages, and masses. Metallicities and ages are estimated by fitting the observed integrated spectra with stellar synthesis population (SSP) models with a pixel–to–pixel spectral fitting technique. Ages for most young clusters are also derived by fitting the multi–band photometric measurements with model spectral energy distributions (SEDs). The estimated cluster ages span a wide range, from several million years to the age of the universe. The numbers of clusters younger and older than 1 Gyr are, respectively, 46 and 260. With ages and metallicities determined, cluster masses are then estimated by comparing the multi–band photometric measurements with SSP model SEDs. The derived masses range from to M ⊙ , peaking at and M ⊙ for young ( Gyr) and old ( Gyr) clusters, respectively. Our estimated metallicities, ages, and masses are in good agreement with available literature values. Old clusters richer than [Fe/H] ∼ −0.7 dex have a wide range of ages. Those poorer than [Fe/H] ∼ −0.7 dex seem to be composed of two groups, as previously found for Galactic globular clusters—one of the oldest ages with all values of metallicity down to dex and another with metallicity increasing with decreasing age. The old clusters in the inner disk of M 31 (0–30 kpc) show a clear metallicity gradient measured at −0.038 ± 0.023 dex kpc −1 .