The young stellar clusters in M51 and the impact of GMC encounters

Abstract

We investigate the young stellar cluster population of M51 and how it is affected by encounters with giant molecular clouds (GMCs). We combine a galactic model with $N$-body simulations of 5000 unique clusters in the mass range [600 - 24000] $M_{\odot}$. We simulate each cluster twice: with ($C_R$) and without ($C_N$) tidal perturbations from the GMCs. We are able to reproduce the majority of the observed mass- and age functions. However, for the age function we see a large discrepancy for clusters with masses $\sim$ 5000 $M_{\odot}$, which is likely related to incompleteness in the observations. We find that old low-mass clusters, located close to the galactic centre, are most likely to be disrupted. The effect of GMC encounters causes a decrease in survivability by 8 per cent points for the oldest clusters with initial masses below 6000 $M_{\odot}$. For 15 clusters we find that the presence of GMCs can protect the $C_R$ clusters from the rest of the galactic tidal field and thereby cause them to retain a significantly higher fraction of stars compared to the $C N$ clusters. For clusters that are super-virialized we find that the $C_R$ clusters have a higher virial ratio compared to the $C_N$ clusters, which is a result of interactions with GMCs. We see no significant difference between the $C_R$ and $C_N$ populations, indicating that over a time period of 200 Myr the effect of the GMCs cannot be detected.