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 ∼ 10 3 to ∼ 10 7 M ⊙ , peaking at ∼ 10 4.3 and ∼ 10 5.7 M ⊙ for young (< 1 Gyr) and old (> 1 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 GCs -one of the oldest ages with all values of metallicity down to ∼ −2 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.

Metallicities of Outer Halo M31 Globular Clusters from Integrated Light Calcium-II Triplet Spectroscopy

“…From resolved HST photometry,Larsen et al (2021a) find that EXT 8 is consistent with being as or more metal-poor than the Milky Way GC M15 (whichSakari et al 2013 find to have [Fe/H] = − 2.3, from high-resolution IL spectroscopy). The metallicities for EXT 8 from the other papers range from [Fe/H] = − 1.8(Wang et al 2021) to [Fe/H] = − 2.3(Chen et al 2016). The value from this paper, [Fe/H] = − 2.27, is higher than the high-resolution value fromLarsen et al (2020), but is generally consistent with the other IL studies.…”

“…This analysis finds PA-36 to be more metal-rich than found by Chen et al (2016) and Wang et al (2019), as shown in Figure 10. PA-36's CaT-based metallicity is also slightly higher than predicted from its (V − I)0 color (Figure 9).…”

An extremely metal-deficient globular cluster in the Andromeda Galaxy

“…Its integrated light color with respect to the u-band (u -g = 1.11) is less red than most of the other GCs, suggesting a low metallicity. Previous low-resolution spectroscopy yielded an age ≥8 Gyr and [Fe/H] between -2.8 and -2.0 (12,13).…”

Globular clusters in the stellar stream surrounding the Milky Way analogue NGC 5907

“…While our GC selection here is based on the assumption that the GC system of NGC 5907 is similar to that of the Galaxy, we note that we obtain a similar result if we define our selection region using the GC system of M31 (e.g., Chen et al 2016). We provide a complete summary of all the various classes of point-sources detected in this work in Table 2.…”

“…However they have very blue colors (g − i < 0.3, r − i < 0.1) and occupy the same region in color-color space as the young star clusters (YC) identified in M31 (e.g. Peacock et al 2009;Chen et al 2016). We identify 34 such point-sources as YC candidates, with 14 of them spatially aligned along the outskirt of the galaxy disk as shown in Figure 5.…”