Spectroscopic study of globular clusters in the halo of M31 with the Xinglong 2.16 m telescope II: dynamics, metallicity and age

Abstract

In our Paper I, we performed the spectroscopic observations of 11 confirmed GCs in M31 with the Xinglong 2.16m telescope and we mainly focus on the fits method and the metallicity gradient for the M31 GC sample. In this paper, we analyzed and discussed more about the dynamics, metallicity and age, and their distributions as well as the relationships between these parameters. In our work, eight more confirmed GCs in the halo of M31 were observed, most of which lack the spectroscopic information before. These star clusters are located far from the galactic center at a projected radius of ∼ 14 to ∼ 117 kpc, which are more spatially extended than that in the previous work. The Lick absorption-line indices and the radial velocities have been measured primarily. Then the ages, metallicities [Fe/H] and [α/Fe] have been fitted by comparing the observed spectral feature indices and the SSP model of Thomas et al. in the Cassisi and Padova stellar evolutionary tracks, respectively. Our results show that most of the star clusters of our sample are older than 10 Gyr except B290∼ 5.5 Gyr, and most of them are metal-poor with the metallicity [Fe/H] < −1, suggesting that these clusters were born at the early stage of the galaxy's formation. We find that the metallicity gradient for the outer halo clusters with r p > 25 kpc may not exist with a slope of −0.005 ± 0.005 dex kpc −1 and if the outliers G001 and H11 are excluded, the slope dose not change significantly with a value of −0.002 ± 0.003 dex kpc −1 . We also find that the metallicity is not a function of age for the GCs with age < 7 Gyr while for the old GCs with age > 7 Gyr there seems to be a trend that the older ones have lower metallicity. Besides, We plot metallicity distributions with the largest sample of M31 GCs so far and it shows the bimodality is not significant and the number of the metal-poor and metal-rich groups becomes comparable. The spatial distributions shows that the metal-rich group is more centrally concentrated while the metal-poor group is occupy a more extended halo and the young population is centrally concentrated while the old populaiton is more extended spatially to the outer halo.