We analyse the photometric, chemical, star formation history and structural properties of the brightest globular cluster (GC) in M81, referred as GC1 in this work, with the intention of establishing its nature and origin. We find that it is a metal-rich ([Fe/H]=−0.60 ± 0.10), alpha-enhanced ([α/Fe] ∼ 0.20 ± 0.05), core-collapsed (core radius r c = 1.2 pc, tidal radius r t = 76r c ), old (> 13 Gyr) cluster. It has an ultraviolet excess equivalent of ∼ 2500 blue horizontal branch stars. It is detected in X-rays indicative of the presence of low-mass binaries. With a mass of 1.0 × 10 7 M ⊙ , the cluster is comparable in mass to M31-G1 and is four times more massive than ω Cen. The values of r c , absolute magnitude and mean surface brightness of GC1 suggest that it could be, like massive GCs in other giant galaxies, the left-over nucleus of a dissolved dwarf galaxy.

We study the population of compact star clusters (CSCs) in M81, using the Hubble Space Telescope/Advanced Camera for Surveys (ACS) images in the filters F435W, F606W and F814W covering, for the first time, the entire optical extent of the galaxy. Our sample contains 435 clusters of full width at half‐maximum less than 10 ACS pixels (9 pc). The sample shows the presence of two cluster populations, a blue group of 263 objects brighter than B=22 mag, and a red group of 172 objects, brighter than B=24 mag. On the basis of analysis of colour–magnitude diagrams and making use of simple stellar population models, we find the blue clusters are younger than 300 Myr with some clusters as young as few Myr, and the red clusters are as old as globular clusters (GCs). The luminosity function of the blue group follows a power‐law distribution with an index of 2.0, typical value for young CSCs in other galaxies. The power law shows unmistakable signs of truncation at I=18.0 mag (MI=−9.8 mag), which would correspond to a mass limit of if the brightest clusters are younger than 10 Myr. The red clusters have photometric masses between 105 and for the adopted age of 5 Gyr and their luminosity function resembles very much the GC luminosity function in the Milky Way. The brightest GC in M81 has M0B=−10.3 mag, which is ∼0.9 mag brighter than  Cen, the most massive GC in the Milky Way.

Abstract.We study the population of compact stellar clusters in M 81 using the HST/ACS images in the filters F435W, F606W and F814W that cover a total field of view of approximately 340 square arcmin, the largest area covered to date for this galaxy. We present details about the selection criteria, which were based on both morphological and photometrical features. The extracted sample of compact stellar clusters shows the presence of two cluster populations, a blue cluster group (young) with more than 300 objects, and a red cluster group (old) containing 138 objects. Surprisingly, the young group lacks clusters more massive than 10 000 M which are present in large numbers in its neighbor M 82. The luminosity function of the young group follows a power-law distribution with an index of 2.0. The luminosity function of the red group closely resembles that of the globular clusters in the Milky Way. Assuming an age of 5 Gyr, these red clusters have masses between 0.1 and 10 × 10 6 M .