Kinematics of star clusters in M101

Abstract

We have identified a few thousand star clusters in the nearby, late-type spiral galaxy M101, including ≈ 90 candidate ancient globular clusters (GCs), from multi-band Hubble Space Telescope (HST) images. We obtained follow-up low-resolution (R ≈ 2000) optical spectroscopy from Gemini-GMOS for 41 total clusters, of which 16 are old GCs and 25 are young massive clusters (YMCs). We measure radial velocities for these clusters and find that, as expected, the YMCs rotate with the HI disk. The old GCs do not show any obvious evidence for rotation and have a much higher velocity dispersion than the YMCs, suggesting that the GCs in M101 are likely part of a stellar halo or thick disk.