Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope

Abstract

We present spectroscopic observations for 11 confirmed globular clusters (GCs) of M31 with the OMR spectrograph on the 2.16 m telescope at the Xinglong site of National Astronomical Observatories, Chinese Academy of Sciences. Nine of our sample clusters are located in the halo of M31 and the most remote one is out to a projected radius of 78.75 kpc from the galactic center. For all our sample clusters, we measured the Lick absorption-line indices and radial velocities. It is noted that most GCs in our sample are distinct from the HI rotation curve of galaxy M31, especially for B514, MCGC5, H12 and B517, suggesting that most of our sample clusters do not have a kinematic association with the star-forming young disk of the galaxy. We separately fitted the absorption line indices from the updated stellar population model of Thomas et al. with two different tracks of Cassisi and Padova, by applying the χ2—minimization method. The fitting results show that all our sample clusters are older than 10Gyr, and metal-poor (−2.38 ≤ [Fe/H] ≤ −0.91dex). After merging the spectroscopic metallicity of our work with the previously published ones, we extended the cluster sample out to a projected radius of 117 kpc from the galaxy's center. We found the metallicity gradient exists for all the confirmed clusters with a slope of −0.028 ± 0.001 dex kpc−1. However, the slope turns out to be −0.018 ± 0.001 dex kpc−1 for all the halo clusters, which is much shallower. If we only consider the outer halo clusters with rp > 25 kpc, the slope becomes −0.010 ± 0.002 dex kpc−1 and if one cluster G001 is excluded from the outer halo sample, the slope is −0.004 ± 0.002 dex kpc−1. Thus, we conclude that the metallicity gradient for M31's outer halo clusters is not significant, which agrees well with previous findings.