Probing the role of the galactic environment in the formation of stellar clusters, using M83 as a test bench

Abstract

We present a study of the M83 cluster population, covering the disc of the galaxy between radii of 0.45 and 4.5 kpc. We aim to probe the properties of the cluster population as a function of distance from the galactic centre. We observe a net decline in cluster formation efficiency ($\Gamma$, i.e. the amount of star formation happening in bound clusters) from about 26 % in the inner region to 8 % in the outer part of the galaxy. The recovered $\Gamma$ values within different regions of M83 follow the same $\Gamma$ versus star formation rate density relation observed for entire galaxies. We also probe the initial cluster mass function (ICMF) as a function of galactocentric distance. We observe a significant steepening of the ICMF in the outer regions (from $-1.90\pm0.11$ to $-2.70\pm0.14$) and for the whole galactic cluster population (slope of $-2.18\pm0.07$) of M83. We show that this change of slope reflects a more fundamental change of the 'truncation mass' at the high-mass end of the distribution. This can be modelled as a Schechter function of slope $-2$ with an exponential cut-off mass ($M_{\rm c}$) that decreases significantly from the inner to the outer regions (from 4.00 to $0.25\times 10^5$ M$_\odot$) while the galactic $M_{\rm c}$ is $\approx1.60\times10^5$ M$_\odot$. The trends in \Gamma and ICMF are consistent with the observed radial decrease of the $\Sigma({\rm H}_2)$, hence in gas pressure. As gas pressure declines cluster formation becomes less efficient. We conclude that the host galaxy environment appears to regulate 1) the fraction of stars locked in clusters; 2) the upper mass limit of the ICMF, consistently described by a near-universal slope $-2$ truncated at the high-mass end.