Gemini Spectra of Star Clusters in the Spiral Galaxy M101

TL;DR

This study analyzes spectra of 41 star clusters in M101 to determine their ages, metallicities, and kinematics, revealing a diverse cluster population and insights into the galaxy's star formation history.

Contribution

First low-resolution spectral analysis of M101 star clusters combining age, metallicity, and velocity measurements to understand galaxy evolution.

Findings

25 young massive clusters with ages of hundreds of Myr

16 older globular clusters with ages up to 10 Gyr

YMCs rotate with HI gas, GCs do not

Abstract

We present low resolution, visible light spectra of 41 star clusters in the spiral galaxy M101, taken with the Gemini/GMOS instrument. We measure Lick indices for each cluster and compare with BaSTI models to estimate their ages and metallicities. We also measure the line-of-sight velocities. We find that 25 of the clusters are fairly young massive clusters (YMCs) with ages of hundreds of Myr, and 16 appear to be older, globular clusters (GCs). There are at least four GCs with best fit ages of approximately 1-3 Gyr and eight with best fit ages of approximately 5-10 Gyr. The mean metallicity of the YMCs is [Fe/H] of approximately -0.1 and for the GCs is [Fe/H] of approximately -0.9. We find a near-continuous spread in both age and metallicity for our sample, which may indicate that M101 had a more-or-less continuous history of cluster and star formation. From the kinematics, we find that the YMCs rotate with the HI gas fairly well, while the GCs do not. We cannot definitively say whether the GCs sampled here lie in an inner halo, thick disk, or bulge/psuedobulge component, although given the very small bulge in M101, the last seems unlikely. The kinematics and ages of the YMCs suggest that M101 may have undergone heating of its disk or possibly a continuous merger/accretion history for the galaxy.