Hubble Space Telescope Photometry of Globular Clusters in M81

TL;DR

This study uses Hubble Space Telescope imaging to identify and analyze the properties of 221 globular clusters in M81, providing insights into their sizes, distribution, and luminosity function.

Contribution

It presents a new catalog of globular clusters in M81, including both spectroscopically confirmed and candidate clusters, with detailed photometric and size measurements.

Findings

Luminosity function peak at V ~ 20.26 mag, indicating a distance of ~3.7 Mpc.

Red clusters are slightly larger than blue clusters in median half-light radius.

Cluster size increases with galactocentric distance.

Abstract

We perform aperture photometry and profile fitting on 419 globular cluster (GC) candidates with mV \leq 23 mag identified in Hubble Space Telescope Advanced Camera for Surveys BVI imaging, and estimate the effective radii of the clusters. We identify 85 previously known spectroscopically-confirmed clusters, and newly identify 136 objects as good cluster candidates within the 3{\sigma} color and size ranges defined by the spectroscopically confirmed clusters, yielding a total of 221 probable GCs. The luminosity function peak for the 221 probable GCs with estimated total dereddening applied is V ~(20.26 \pm 0.13) mag, corresponding to a distance of ~3.7\pm0.3 Mpc. The blue and red GC candidates, and the metal-rich (MR) and metal-poor (MP) spectroscopically confirmed clusters, are similar in half-light radius, respectively. Red confirmed clusters are about 6% larger in median half-light radius than blue confirmed clusters, and red and blue good GC candidates are nearly identical in half-light radius. The total population of confirmed and "good" candidates shows an increase in half-light radius as a function of galactocentric distance.