Searching for Compact Star Clusters in M81 Using HST/ACS Images

TL;DR

This study identifies and characterizes two distinct populations of stellar clusters in M81 using HST/ACS images, revealing differences in their ages, masses, and luminosity distributions.

Contribution

It provides the first detailed analysis of M81's cluster populations, distinguishing young and old groups based on morphology and photometry.

Findings

Two cluster populations: young (560 objects) and old (120 objects).

Young clusters lack masses over 10,000 solar masses, unlike M82.

Luminosity functions follow power-law and globular cluster distributions.

Abstract

We study the stellar cluster population in M81, using the HST/ACS images in the filters F435W, F606W and F814W that cover a total field of view of approximately 140 square arcmin. We present details about the selection criteria, which were based both on morphological and photometrical features. The extracted sample of stellar clusters shows the presence of two cluster populations, a blue cluster group (young) with 560 objects, and a red cluster group (old) with 120 objects. The young group lacks clusters more massive than 10000 solar masses, that are present in large numbers in its neighbor M82. The luminosity distribution function of the young group follows a power-law distribution with an index of 2.0, whereas that for the red group resembles very much the globular cluster luminosity function in the Milky Way. Assuming an age of 5 Gyr, these red clusters have masses between 0.1 to 10 million solar masses.