Mortality and dust expulsion in early phases of stellar clusters Evidence from NIR photometry of nearby, spiral galaxies

Context. It is often argued that young stellar clusters suffer a significant infant mortality that is partly related to the expulsion of dust and gas in their early phases caused by radiation pressure from hot stars and supernovae. Near-infrared (J - K)- M-K diagrams of young stellar clusters in nearby spiral galaxies show a bi-modal distribution that is consistent with a fast decline of their intrinsic extinction at an early epoch. Aims. The distinct features in the color-magnitude diagrams (CMD) and the fast change of colors for the youngest clusters allow us to place constraints on their early evolutionary phases, including the time scale for the decreasing extinction caused for instance by gas and dust expulsion. Methods. Monte Carlo simulations of cluster populations were performed using the power-law distribution function g(M, tau) proportional to M-alpha tau(gamma). Integrated colors were computed from Starburst99 models. The simulated near-infrared CMD were compared with those observed for six grand-design, spiral galaxies using statistical goodness-of-fit tests. Results. The CMDs indicate a significant mortality of young, massive clusters with gamma = -1.4 +/- 0.5. High initial extinction A(V) = 8-11(m) and strong nebular emission are required to reproduce the bi-modal color distributions of the clusters. An extended star formation phase of longer than 5 Myr is suggested. The reduction of the internal extinction of the clusters starts during their active star formation and lasts for a period of 5-10 Myr.