On the nature of the brightest globular cluster in M81

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 ([Alpha/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 10 million solar masses, the cluster is comparable in mass to M31-G1 and is four times more massive than Omega 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.

Wide-field HST/ACS images of M81: The Population of Compact Star Clusters

We study the population of compact stellar clusters (CSCs) in M81, using the HST/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 FWHM 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. Based on the 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. 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 (M_I=-9.8 mag), which would correspond to a mass-limit of 4x10^4 M_solar if the brightest clusters are younger than 10 Myr. The red clusters have photometric masses between 10^5 to 2x10^7 M_solar for the adopted age of 5 Gyr and their luminosity function resembles very much the globular cluster luminosity function in the Milky Way. The brightest GC in M81 has M_B^0=-10.3 mag, which is ~0.9 mag brighter than w-Cen, the most massive GC in the Milky Way.