Kinematics of Star Clusters in M101

TL;DR

This study identifies and characterizes star clusters in M101, revealing that young clusters rotate with the galaxy disk while old globular clusters likely belong to a halo or thick disk, based on spectroscopic data.

Contribution

It provides the first spectroscopic analysis of star clusters in M101, distinguishing between young and old populations and their kinematic behaviors.

Findings

YMCs rotate with the HI disk

Old GCs show high velocity dispersion

Old GCs likely part of a stellar halo or thick disk

Abstract

We have identified a few thousand star clusters in the nearby, late-type spiral galaxy M101, including approximately 90 candidate ancient globular clusters (GCs), from multi-band Hubble Space Telescope (HST) images. We obtained follow-up low-resolution (R approximately 2000) optical spectroscopy from Gemini-GMOS for 43 total clusters, of which 18 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.