Two Populations of Old Star Clusters in the Spiral Galaxy M101 Based on HST/ACS Observations

TL;DR

This study identifies two distinct populations of old star clusters in M101, revealing differences in luminosity functions, sizes, and spatial distributions, indicating varied origins and evolutionary histories within the galaxy.

Contribution

The paper provides the first detailed analysis of the luminosity functions and spatial properties of star clusters in M101, distinguishing between halo and disk populations based on HST observations.

Findings

Bright clusters have a peaked luminosity function similar to the Milky Way.

Faint clusters follow a power-law luminosity function with alpha ≈ -2.6.

Faint clusters are larger and more evenly distributed, likely associated with the disk.

Abstract

We present a new photometric catalog of 326 candidate globular clusters (GCs) in the nearby spiral galaxy M101, selected from B, V, and I Hubble Space Telescope Advanced Camera for Surveys images. The luminosity function (LF) of these clusters has an unusually large number of faint sources compared with GCLFs in many other spiral galaxies. Accordingly, we separate and compare the properties of "bright" (M_V < -6.5) versus "faint" (M_V > -6.5; one magnitude fainter than the expected GC peak) clusters within our sample. The LF of the bright clusters is well fit by a peaked distribution similar to those observed in the Milky Way (MW) and other galaxies. These bright clusters also have similar size (r_{eff}) and spatial distributions as MW GCs. The LF of the faint clusters, on the other hand, is well described by a power law, dN(L_V)/dL_V proportional to L_V^alpha with alpha = -2.6 plus or minus 0.3, similar to those observed for young and intermediate-age cluster systems in star forming galaxies. We find that the faint clusters have larger typical r_{eff} than the bright clusters, and have a flatter surface density profile, being more evenly distributed, as we would expect for clusters associated with the disk. We use the shape of the LF and predictions for mass-loss driven by two-body relaxation to constrain the ages of the faint clusters. Our results are consistent with two populations of old star clusters in M101: a bright population of halo clusters and a fainter, possibly younger, population of old disk clusters.