Spectroscopic Study of Globular Clusters in the Halo of M31 with
  Xinglong 2.16m Telescope
  We present the spectroscopic observations for 11 confirmed globular clusters
of M31 with the OMR spectrograph on 2.16m telescope at 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 the radial
velocities. It is noted that most GCs of our sample are distinct from the HI
rotation curve of M31 galaxy, especially for B514, MCGC5, H12 and B517,
suggesting that most of our sample clusters do not have kinematic association
with the star forming young disk of the galaxy. We fitted the absorption line
indices with the updated stellar population model Thomas et al. (2010) with two
different tracks of Cassisi and Padova, separately, by applying the
$\chi^2-$minimization method. The fitting results show that all our sample
clusters are older than 10 Gyr, and metal-poor ($-0.91 \le $ [Fe/H] $\le -2.38$
dex). 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
for all the confirmed clusters exists with a slope of $-0.028\pm0.001$ dex
kpc$^{-1}$. However, the slope turns to be $-0.018\pm0.001$ dex kpc$^{-1}$ for
all the halo clusters, which is much shallower. If we only consider the outer
halo clusters with $r_{\rm p}>25$ kpc, the slope becomes $-0.010\pm0.002$ dex
kpc$^{-1}$ and if one cluster G001 is excluded from the outer halo sample, the
slope is $-0.004\pm0.002$ dex kpc$^{-1}$. Thus we conclude that metallicity
gradient for M31 outer halo clusters is not significant, which agrees well with
the previous findings.
