Institution(s): 1. Korea Astronomy and Space Science Institute
We present photometric and spectroscopic studies of halo star clusters in a dwarf irregular galaxy NGC 6822. The spectra of these halo clusters show that they are old (>=8 Gyr) and metal poor ([Fe/H] <=−1.5), and their luminosities indicate that these clusters are as massive as ~105 M⊙, which makes them old massive star clusters (Hwang et al. 2014). The massive star clusters are not uncommon in dwarf galaxies. However, these massive clusters in NGC 6822 are unique in terms that they have extended structure with half-light radii Rh ≈ 7.5 –14.0 pc, and that they are widely distributed, ranging from 10.′7 (≈1.5 kpc) to 77′ (≈11 kpc) from NGC 6822 center, which is almost perpendicular to the HI gas disk-like structure with young stellar components (Hwang et al. 2011). Interestingly, we have found out that the radial velocities of the massive clusters do not conform to the systematic rotation displayed by the HI structure nor the intermediate age carbon stars. There appears to be no consistent systematics among the velocities of these massive clusters, either. This may imply that these massive clusters have accreted into the halo of NGC 6822, not formed on-site. We are going to discuss the implication of these results regarding the formation of massive star clusters and the evolution of dwarf galaxies.